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Atlas of Sedimentary Rocks Under the Microscope-Longman (1984)


A. E. ADAMS W. S. MACKENZIE C. GUILFORD

The English Language Book Society is funded by the Overseas Development Administration of the British Government. It makes available low? priced, unabridged editions of British publishers' textbooks to students in developing countries. Below is a list of some other books on earth sciences published under the ELBS imprint.

Blyth and de Freitas
A Geology for Engineers

Edward Arnold Deer, Howie and Zussman
An Introduction to the Rock-Forming Minerals

Longman Evans

An Introduction to Ore Geology

Blackwell Scientific Hall
Igneous Petrology

Longman Holmes; Holmes (reviser)
Holmes: Principles of Physical Geology

Van Nostrand Reinhold (UK) Kearey and Brooks
An Introduction to Geophysical Exploration

Blackwell Scientific MacKenzie, Donaldson and Guilford
Atlas of Igneous Rocks and Their Textures

Longman MacKenzie and Guilford
Atlas of Rock-Forming Minerals

in Thin Section

Longman Read and Watson
Introduction to Geology Vols

1 and 2

Macmillan Tucker
Sedimentary Petrology

Blackwell Scientific Watson
Geology and Man

Unwin Hyman

Atlas of Sedimentary Rocks under the Microscope
A. E. ADAMS W. S. MACKENZIE C. GUILFORD

[IEL[BSI[
English Language Book Society/Longman

Longman Scientfi i c & Technical Longman Group UK Ltd, Longman House, Burnt Mill, Harlow, Essex CM20 2JE, England Associated com panies throughout the world ?Longman Group UK Ltd 1984

All rights reserved; no part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the Publishers. First published 1984 Reprinted 1987 ELBS edition first published 1988 ISBN 0 582 02701 2 Printed and bound in Great Britain by William Clowes Ltd, Beccles and London

Contents

Preface
Acknowledgements

vi vii

Part I

Terrigenous clastic rocks

2

Part 2

Carbonate rocks

33

Part 3

Other sedimentary rocks

75

Appendix I Appendix 2 Appendix 3

Preparation of a thin section of a rock Staining a thin section of a limestone Preparation of a stained acetate peel of a limestone

97 99 100

References
Index

101 102

Preface

The study of rock s

using thin sections and a petrographic m i cr oscope was initiated by llcnry Clifton Sor by in the middle of the nineteenth century and the first rocks he described were silicified limestones from the Jurassic in Yorkshire. This work was published in 1851. His presi dential address to the Geological Society of London in 1879 was entitled 'On the struct ure an d origi n of limestones' and Sorby had a series of plates made from camera Iucida drawings, reproduced for private circulation with copies of the text of his a dd ress. These
.

illustrated the microscopic characteristics o f l imestones from through?

Bri tish geological record and amoun ted to the first petro? atlas. Despite the pertinence of Sorby's work, much of which is still valid today, few people recogni;ed its importance at the time. While the petrographic study of igneous and metamorphic rocks became increasingly important. that ofsedimentary rocks languished until well into the present century. Since about 1950. with much geological research directed toward? the search for oil and gas trapped in the pore-spaces of sedimentary rocks, sedimentary petrography has become one of the most important fields of geology and forms a key part of most undergraduate courses. The aim of this book is therefore similar to that of the previously published Atlas <>l1'Kneo11s rocks and their texwres, in that it is designed to be a laboratory handbook for the student beginning a study of sedimentary rocks in thin section, whether he or she is an amateur or an undergraduate. Only a basic knowledge of mineralog y a nd palaeon tology is assumed. While we make no claim that the book is comprehensive, we ha v e tried to include photogra ph s of most of the compon ents of sedimen tary rocks encountered in thin sections during an undergraduate cou r se in geol ogy. The book is in three pa rts. Part I det?ls with the terrigenous clastic rocks and concentrates on sandstones. since the petro graphic micro? scope is most u sefully employed with rocks of this grain size. We have a \lcm ptcd to show the common del rita I com ponents of sandstones and the range of rock types occur ring wi th out becom ing involved in details o f the ma ny clas si fications which exist. Part 2 deals with the carbonate rocks and is the longest section in the book. This is because to the newcomer to carbonate petrology, limestones contain a bewildering variety of grain types. The bioclasts in particular show such variation in shape and structure that it has been difficult to know what to leave out. We have attempted to show the range of common bioclast types while realizing that this section of the book cannot be comprehensive within the limits of the n umber of photographs which we arc able to reproduce. Most ofthe photographs of limestones arc from stained thin sections and acetate peels. The staining aids identification of minerals and textures and also makes limestones more attractive to study. The reader examining a collection of unstained section? of carbonate rocks should still find the ph oto graph? and text useful 111 ide ntifying gra in types and textures. Photograp hs of unstained limestone sections arc included th rougho ut to remind the reader what untreated ma terial looks l ike. Part ? illustrntcs ironston e? cherts. eva po rites, ph ospho rites and
out the graphic
? . ? .

carbonaceous rocks in thin section. We hope the section on evaporites will be of pa rt icu lar interest, as published colour ph otomicrographs or some mi nera ls arc rare. Three a ppen dices arc included. Ap pe nd ix I is a sligh tl y modified form oft he a ppcn dix in the !It/as a/igneous rocks and their texll/res and describes how a thin section may be made. Appendix 2 describes a me thod of staining thin sections of limestones and Appendi x 3 contains instruc t ions on how to make acetate peels. Throu ghout the book we have tried to keep the text descriptive and to avoid dctai Is of intcrprcta tion. However. it has proved impossible to omit discussion in some cases, particularly with the carbonate rocks where identification of gra ins and textures goes hand in hand with an interpretation of their origin. We ha ve allcmpted to show typical material rather than particularly good examples of any feature illustrated. Extcn!-.IVC cross-referencing is given to help the reader in finding other photographs of similar phenomena. Inevitably the bulk of the illustrated material comes from the British Isles: we believe howc,cr that it is representative of sedimentary rocks the world over. Finally, we mu?t repeat the cautionary note in the preface to Atlasof igneous rock.\and their texture.\. This book is a laboratory handbook to a.uist in the ?tudy of scdimentt?ry rocks in thin section. There is no substitute for the student examining material under the microscope for him- or herself and we hop e this book will encourage students to make their own petrogra phic observations.

Acknowledgements

Although this book is based on thin sections and acetate peels held in the teaching collections of the Department of Geology, University of Manchester, it would not have been possible without the generous loan of material from the research collections of many colleagues. We are particularly indebted to Professor Sir Frederick Stewart who loaned much of the material for the evaporites section. We are grateful to Drs. J. M. Anketell, P. Gutteridgc. J. Kantorowicz. J. E. Pollard. A. T. S. Ramsay, K. Schofield, Mr R. D. Vaughan and Professor E. K. Walton, all ofwhom loaned material and made suggestions or comments on the manuscript. We would also like to thank Professor J. B. Dawson for permission to include a photograph of one of Sorby's thin sections from the collection held at Sheffield University. We wish to thank Patricta Crook for her patient typing of various versions of the text and Phil Stubley for drafting the originals of the diagrams. Finally we wish to acknowledge the help given to us by all the staff of the Longman Group. We acknowledge permission from Springer Verlag and Professor F. J. Pettijohn to reproduce Figs. A and D, and the American Association of Petroleum Geologists for Figs. E and F and Tables 3 and 4.

Part 1 Terrigenous clastic rocks

Terrigenous clastic rocks

Introduction

Terrigenous clastic sediments arc made up of transported fragments derived from the weathering of pre-existing igneous, sedimentary or metamorphic rocks. These rocks are classified initially according to grain size, using the Udden-Wentworth scale (Table 1). the It is those terrigenous sediments of intermediate grain size coarser siltstones, sandstones and finer conglomerates and breccias? that arc most usefully studied using the petrographic microscope, since the grain types can be identified by this means. The principal component grain types are quartz, feldspar and rock fragments. The matrix of such sediments may be the fine-grained weathering products of the source rocks, such as clay minerals, or it may be a secondary cement. Clays and shales arc too fine-grained for study using the petro? graphic microscope and must be examined by electron microscopy or X-ray diffraction. The components of coarser conglomerates and breccias can usually be identified with the aid of only a hand lens. The shape and roundness of the components of terrigenous clastic rocks arc important in describing sedimentary textures. Categories of roundness for grains of high and low sphericity are shown in Fig. A. Sedimentary textures arc discussed on p. 24.

Tabl e I.

Grain-size classification of sedime nts
Class term

Size in mm

of class

Grain size terms for rock

boundary
boulders

256 cobbles 64 pebbles

4
2

rudite rudaceous rock conglomerate i b recca

granules
very coarse sand coarse sand medium sand fine sand very fine sand coarse silt medium silt fine silt very fine silt clay claystone siltstone argillite argillaceous rock mudstone mudrock shale arenite arenaceous rock sandstone

I

0.5(!) 0.25(.!)
0.125(A) 0.0625(??) o.o312Ul> O.Ol56{.f4) 0.0078( 1 !s> 0.0039(lt6)

5. Well-rounded

1. Angular

0.

0
. .
.

.

Fig. A

0
.

Categories of roundness for grains of low and high sphericity (after Pettijohn e ta!., 1973)

3

Terrigenous clastic rocks

I, 2

Qua rtz

The most abundant grain type in sandstones and con? glomerates is quarll. In addition to the size and shape of individual quart? grains. the following features should be ob?erv..:d since they may provide clues to the provenance of the sediment: Whether the quartz grains arc single crystals (mono? or arc made up of a number of crystab an diiTerent orientation!> (polycrystallinc). 2 Whether extinction is uniform (the grain extingui?hc? in one position on rotation of the stage) or undulose (the grain cxtingui?hcs O\'cr a range of at least s· on rotation of the stage). 3. The presence or ab?ence of inclusions. 4. I n the ea?c of polycrystallinc grains. whether the crystal boundaries arc straight or sutured.
I. cry stalltnc) 1 and 2 ?how subroundcd quartz grains which arc single cryst;ds. taken with plane-polarized light (PPL) and with crossed polars (XPL). The matrix between the sand grains contains opaque iron oxide and some calcite. The Iauer shows high-order pink and green interference colours.

I {{//(/ 2: Red MouJIIain l·imnarirm. Silurian. Birmin?f111111. !flahr111w. USA: nw?nification x38; I PPL. 2 XPl..
4

3,

4, 5

Terrigenous clastic rocks

Quartz
(continued)

The three rounded grains in the ccntrc of3 and 4 are made up of a number of quart; c1·y?tals in dillcrcnt orientations and arc thu? colllfiO.Iil<' or polrl'rystal/in(' quartz. The compo\ile nature or the grain? is clear only in the view taken 111th polars cros,cd. Note that the boundaries hct11ecn the Cl')\tal\ arc sulllrcd. Thi? is characteristic of quart; from a metamorphic source. Composite quart7 from igncou? '>Ourcc? usuall)' has straighter crystal houndanc\. The much liner '>cdimcnt surrounding the compo,?tc quart; grain'> contam\ monocrystalline quart?: and hnl\\ ni'h cla\b of fine-grained material which are prohahl) 'hale or 'late fragmcnh. 5 sh011., a composite quartl grain viewed under crossed polar,.m '' h1ch not only arc the crystal boundaries within the grain sutured. hut aho the crystal' arc elongated in a prcl'e?red direct ion. Such grains are called sheared qu ar lt or .1/rc?tc fwd 1111'/tllllorphic· quart;. In this type of quartz. indi1·idual cry-.tals n orm a lly show undulosc extinction as a re?ul t of strain. Evidence l'or this in the example shown comes from the non-unil'orm interference colours shown hy many or the crystal:,.

3 wul 4. TrichmJ! I:Jedv. Silurian. Pomarllechau, D.1Jed. ll'ale.1; IIIIIJ!IIi/imtion x 16:3 PPL. 4 XPL. 5: Carho11i/i'rous. Angle'sey. Wales: magnification x 41 . .\'Pl.. 5

Terrigenous clastic rocks

6, 7, 8

Q u a rtz
(continued)

The quart7 grain in the centre of the field of view in 6 appears to be a single homogeneous crystal. In 7 however, where the same field of view is seen under crossed polars, the quartz grain is clearly made up of pans of two crystals. One, cromprising the upper left portion of the grain is showing a mid-grey interference colour, whereas the rest of the grain comprises a crystal with areas ?ho" ing :.lightly different interference colours. The left? and right-hand sides are in extinction and interference colour!> become progressively paler towards the centre of the gram. Such a grain would show sweeping extinction "hen rotated. This phenomenon. known as undulose ntinction. i!> a result of strain and is found in quartz gram? from both igneous and metamorphic sources. Quartt cry:;tals may sometimes incorporate mineral mclusions and identification of the minerals may yield information about the provenance of the sediment. The quartz grain in the centre of 6 and 7 has a number of needle-shaped inclusions, although they are too small for the mineral to be identified at the magnification shown. Inclusions of the fluid present at the time of crystallization arc common in quartz crystals and are known as fluid inclusions or vacuoles. 8 shows a quartz grain with abundant vacuoles. These appear as dark specks, and in the sample illustrated, many are concentrated in lines running at a low angle to the length of the picture. Quartz with abundant vacuoles is usually derived from a source of low-temperature origin, such as a hydrothermal vein, and appears milky-white in a hand specimen. the photo? graph also shows a green mineral in the matrix around the quartl' grain, which is chlorite.

6 and 7: locality and age unknoll'n; magnification x 72: 6
PPL. 7 XPL. 8: Coal Measures. Upper Carboniferous. Lancashire. Englmul: magn(fication x 72. PPL. Undulose extinction can also be seen in 5. 6

9, 10

Terrigenous clastic rocks

Feldspa r

rapid, producing micas and clay minerals. Therefore f eldspars arc most abundant and best preserved in rocks derived from mechanical weathering. The identification of feldspars in thin section is straightforward in the case of multiple-twinned grains of plagioclase or microcline. or where pcrthitic textures arc present. Distinguishing be? tween untwinned orthoclase and quartt can be difficult. The f oll owi ng f eatures may help:
I.

Feldspa rs arc a major constituent of many sandstones and congl om era tes Alkali feldspars are more common than calcic plagi oc lase pa rt ly because t hey arc more resistant to chcmicul we a thering and partly because th e ultimate soun:e of many t errigenou? rock s is gra nit e or gneiss rocks in which the feldspars arc mainly the alkali varieties. The chemical weathering of feldspars may be
. , . .

2.

clear. Refractive Indcx

Al terat ion because orthoclase is more su sceptible to chemical wea t hering than quartl. it i? often clou dy or brown-coloured in PPL. whereas quartz is usually the index of quartz is very close to.

but higher than that of Canada balsam. whereas the index of orthoclase is always lower than balsam. J. I n terferenc e llgure orthoclase is biaxial with a moderate 2 V. quart;. is unia xial unless strained.
9 and 10 sh ow a large p lagiocla se grain which is easily identified by the twi n ning in the photograph with polars crossed. The grain ?bows a combination of two types of twins which arc probably Carlsbad (simple twin) and albite (multiple twinning). Th e cloudiness seen in PPL is caused h> pa tchy altcrat10n of the feldspar. The highly birefringen t. nne grai ned alteration product is probably
-

sericite. a m?ca.

9 and /(}: Cohan Conglomera/e. Wah's: nw?nificraion
x

Silurian.

Rlwyader.

15: 9 ??L, 10 XPL.

7

Terrigenous clastic rocks

II, 12, 13

Fe l d s p a r
(continued)

II and 12 ?how a p..:bble-si7ed rragrnent composed almost

or m?croclinc. Microclinc can be identified easily hy the cross-hatched twinning which it invariably shows. Although the microclinc shows little alteration. feldsp<?r grams in the upper left. including multiple-t,?mned plagiocla?e. arc browni!>h coloured as a result of alter? all?m. ln contrast. the quartz in the upper right is rclat?,·cl? clear and unaltered. Graul!> ?hO\\ ing perthitic intergrO\\ ths. comprising hlchs or lamellae of sodium-rich feldspar in potassium? rich feldspar. arc not uncommon in sediments. 13 and 14 ?hO\\ a very coarse sand-sized fragment of perthite. Most of the other sediment grains are quartz and the matrix contains highly birefringent mineral grains too small to identify at the magnification shown. 15 and 16 show grains of orthoclase and quart; 1 he feldspar can he identified in the PPL view by ib cloudy appearance due to alteration. The quart7 is clear and unalt..:rcd ln the photograph taken with polars cross..:d. it can he ?cen t ha t one feldspar exhibits a simple Carlsbad twin (upper right of photograph). but most of the grains ar..: not twinned. Two multiple-twinned plagioclase cry?tals arc also visible in the lield of view.
entirely
. .

8

14, 15, 1 6

Terrigenous clastic rocks

Fel d s p a r
(continued)

I orridonian. Precamhrian. Isle of Skye. Scotland: nwgni/ic{f{ion x 16; II PPL. 12 XPL. /3 and I4: Torridonian, Precamhrian. Torridon, Scotland; magn(/icationx J!S: IJ PPL. 14 XPL. 15 and /6: Torridonitm, Precambrian. Sc01/and; nwgn{/ic? .. 16 XPI ationx 72: 15 PPI Feldspars are also shown in 54, 55, 58, 59, 64 and 65.
?.

II am/ 12·

Terrigenous clastic rocks

17. 18, 19

Rock f rag ments

Rod fragments. i n particular metamorphic rock frag? ment?. arc important contributors to many detrital ?edimcnts. 17 and 18 show a sediment with many rock fragments.

The two fragments in the centre of the photograph above the large quart/. grain are made up of fine-grained material which cannot be resolved at this magnification. They arc fragments ofshale or slate, and the character istic platy shape is a result ofderivation from a cleaved source rock containing abundant platy minerals. The sediment is very poorly-sorted. containing many small rock frgmcnts, quart/ grains and at least one twinned feldspar (in the centre. ncar the top). as well as the large quartz grain, part of which is seen at the base of the photograph. Fragments of coarser-grained metamorphic rocks arc often schistose. 19 and 20 show a fragment of muscovite? bearing quartt-rich rock. The mica flakes show a prc? tcrcntial alignment resulting in a schistose texture. Such fragments arc sometimes classified as schisrose quarr: rather than metamorphic rock fragments. Sedimentary rock fragments, other than chert, arc relat ive ly uncommon in terrigenous sedimentary rocks because they usually break down fairly easily into their comronent grains. 21 and 22 show a large sandstone fragment. Note that although the component particles arc all quartz. they arc clearly distinguishable even with PPL. This contrasts with the composite quart/. grain \hnwn in 3 and 4, where individual crystals arc not visible in PPL. The photograph taken with XPL shows that the individual quartz grains are separated by a cement with bright interference colours. This is likely to be clay.

20. 21,22

Tcrrigenou!> clastic rocks

Rock frag ments
(contmued)

17 and 18: Caban Conglomerate. Silu ria n, Rhayader. Wales: magnific ation x 27: 17 PPL, 18 XPL. 19 and 20: C a ban Conglomerate, Silurian, Rhayader. Wale ,\; IIWKnification x 28; 19 PPL, 20 XPL. 21 and 22: Arenig Co nglomerate, Rhosneigr, Anglese y, Wales: ma xn!fication X 16; 21 PPL, 22 XPL. 11

-

Terrigenous clastic rocks

23, 24,25

Rock f rag ments
(continued)

The variety of igneous roc k fragments found i n sediments
" few examples. Reference to the A tlas ()(igneous rocks and their te\turl.'.llllay as?ist in the identification of fragments. The ferro-magnesian minerals which arc common in basic igneous roch arc relatively unstable in earth surface conduion? and often show alteration. making identifica? uon or fragments difficult. 23 and 24 \hO\ \ a \olcaniC rock fragment in the centre or the field of' IC\\. I t con?ist? of plagioclase Iat h' set in an altered groum.lmass '' hich ?? too finc-gntined f or it!> con\lltucnts 10 be identified at the magnification shown. 1\ 'econd rod fragment to the right of centre i? composed of quart/ Cr)Stals SCI i n a birefringent matrix probabl) of cht) m1ncrals. The sedimem also contains separate feld? spar grains, some of which arc multiple-twinned plagio? dasc. and both monocrystalline and polycrystallinc qu;u·v. The m at ri x or the whole rock contains hirefring? ent day or mien minerals. 2S and 26 s how two diOercnt igneous rock fragments. To t he lclt ' and above the centre of the field of view is a fi ne- grai ne d. probably volcanic. basi?: rock. It con sists of microphcnocrysts of plagioclase feldspar set in a gro und? mn-.s or fcldspar. very small pyroxene crystaIs and opaque-.. Pale green chlorite occurs. possibly tilling ori gi nal \CSich.:s. Thi!> chlorite is black in the XPL view l)Wing to it-. \Cry low birefringence. The lower part of the held of viC\\ i? mostly occupied by a coarse-grained pluwnic rock fragment consisting mainly of pl agioc lase fcld,par and pyroxene. The simplc-t\\ inned fcld!>par? ma) be ;?lk<tli feldspar. although in this case no difference in rcfra?o:ti\C 1ndcx between them and the muhiplc-l\\inncd pklglocla'c feld!>par? could be detected. This illustrate? the dlflicuh) 111 prCCI\C Identification of igneou:. rod .. fragment-. In adduion to ?maller rock fragment\ the ,edimcnt cont<un? ,ubangular quart? grains and on the nght hand edge a single cr) stal of a ferro-magnesian mmcral. probabl) amphibole. !>hO\\ing an orange inter? ference colour 1n the XPL \icw. The relatively fresh 1gneou? rock fragmenb and ferro-magnesian ouneral grain' suggest that thi!> sediment underwent lillie tran>? pon al'ter cros10n from source rocks. Chert fragml:nts arc quit e common i n sedimentary rock-. \illl.:l: c hert is stable and resistant to weathering. Plate? 27 and 28 show a thin section of a conglomerate in which the largl: rounded fragments arc chert. The view

??·as great'" the variety of igneous rocks themselves and

lad or ?pace prcvcnb the inclusion of more than

12

26,27,28

Terrigenous clastic rocks

Rock f rag ments
(contmued)

tal-.?.:n 1\llh cro?s?.:d polars shows th at the fragments arc made up of ve ry tine-grained quart; (micro-quartz, p. 82). Small fragments of chert can he d illicult to d istinguish I rom line-gra111cd a cid 1 olea me rm:l-. ? all hough the Ia Iter ma) 'ho\\ porphyritic texture?. In the sample illustrated the matn\ comains subangular to suhrounded quart7 grains and ?mall chert fragments set 111 an iron oxide-rich cement (brown in PPL).

23 wttl 24 Cill'.lllldnmll (irit. Ordtii'U·iull. CloK?ryn. G??y? tll'tltl ll'ah·'· lllagn{ t /callo/1 x !5. 23 1'1'1 . . 24 XPL. 25 and 26 (ifell lpp Conl{folllcmtc. Ordol'ician. Ayrshire. \t·otlmul. magni/ll'ation x II. 25 PPI 26 X Pl 27 a11d 28. 1/cnf(mfl·hirc Puddlllgltlln<'. l'ertiurr _ . Chiltem 1/i/lv. f:'n?/uncl; 11/l/f<ll(/il'lltion x 13. 27 PPL. 28 XPL. Other rock .fi'agments are shou·11 in 3, 4, 33. 34. 56--61. 66, 67.
.. ?.

13

Terrigenous clastic rocks

29,30

Micas

Micas rarely form more than a few percent of any terrigenous sediment. even though they may be con? ?picuous in hand specimens. Muscovite is more common than biotite since it is more resistant to weathering. The source is usually a granitic or schistose rock. 29 and 30 show u typical micaceous sandstone. Note the parallel alignment of the muscovite flakes and the concentration at a particular level. This indicates the bedding. The thin section has been positioned to show the second-order interference colours of the muscovite. If the thin section was shown with the bedding horizontal, the muscovite Oakes would be at extinction.

29 and 30: Tifestones. Silurian. Uangadog. D.1fed. Wah?.>: 1/III.J.!IIi/icotioll x 16: 29 PPL. 30 XPL. Other 111icas are sholl'n in 68 and 69.

14

Terrigenous clastic rocks

C l a y m i nerals

Clay minerals form a significant fraction of sandstones and arc the major constituents of argillaceous rocks. They may be detrital or authigenic. However. since they cannot be readily identified using an ordinary light microscope, but arc studied by usc of the electron microscope and by X-ray diffraction. they are not considered in detail here. Clay minerals can be seen in plates 22-24, 45, 46, 62?7.

15

Terrigenous clastic rocks

31, 32

C h l o r ite

The sheet si lica te mineral. chlorite, is abundant i n sedi? dcnvcd from l ow-grade metamorphic rocks, as an alter? ation product. especially of volcanic rock fragments or as
-

mentary rocks. It may occur as detrital flakes, usually

an a ut h igeni c mineral filling pore-spaces. Plates 31 and 32 show a fine grai ned sedimentary rock in which many ?mall fr<tgmcnt!> arc vi!>ible. but are less than l mm across at th1' magnification. The rock is therefore a siltstone. T he larger rounded grai ns which are colourless in PPL. and !>hO\\ ,lightly anomalous bluish-grey interference colours m XPL. arc chlorite. In this case chlorite has gro" n in the rock a!> a result of the breakdown of small rocl.. fragments and fine-grained matrix during low-grade mctamorphbm.

31 and 32 Ordol'ician. L/angranog, 1111/KIIi/icatioll x 72,31 PPL, 32 XPL. Chlorite is shou?n also in 8. 58 and 59 16

Dy(ed,

Wales: '

33,34,35

Terrigenous clastic rocks

G l a ucon ite

Glauconite i s a hyd ro us pot assium iron al umino-silicate m i nera l which forms excl usi vely in marine en vi ronments usu ally in fair ly shnllow waters. It common ly occurs as rounded pelle ts which arc aggregates of many small crysta ls 33 nnd 34 show a number of glauconite pellets in a coarse <;and?aonc. The gla uconite is easily iden ti fiab le in the photograph taken in plane-polarised light by its green or b rownish green colour. The grain in upper centre part of the field of view i ncorporates a number of silt-sized quart7 grams. Glauconite has mod erate bi refringe nce but as the p i cture taken with crossed polars shows, interference colours arc masked by the natural colour of the mineral. The remainder of the sediment consists of monocrystallinc quart..: gra ins and in the lower r ight portion of the field of view. a sedimentary rock fragment. The cement, which is showi ng high order interference colours. is calcite. 35 shows a sandstone rich in gl aucon i te and containing ?ubrounded quart..: grains (low relief) and car bonate grains and cement (high relief). Note that many of the bright green gla u con ite pellets have brown margins. These are limonite and result from the oxidation of the ferrous iron in glauconite.
, . .

?

33 and 34: !.ower Greensand. Lower Cretaceous. Folkes10ne, J::nf(laml: magnification x 22; 33 PPL. 34 XPL. 35: !.ower Creraceous. Co. Antrim. Nortllem Ireland; magnijimtio11 x 22. PPL. Glauconite is shown also in2l4 and215. 17

Terrigenous clastic rocks

36, 37,38

Sa ndstones- Matrix and cement

On deposition. many sandstones contain lillie sediment matrix between the component grains. Some terrigenous mud may be deposited with the grains and those sedi? ments with more than 15% clay matrix are classified as greywackes (62--67). A few sandstones have a matrix of carbonate mud. 36 and 37 show a sediment containing large. rounded quartz grains together with smaller, 'ubangular to subrounded grains in a fine-grained matrix ha,ing high relief. In the XPL photograph. high-order uHerference colours. characteristic of calcite, can be seen. Thts sample is a sandstone with a carbonate mud matrix, '' hich was probably deposited at the same time as the grain?. rather than being introduced later as a cement. Cementation is the principal process leading to poros? ity reduction i n sandstones, the most common cements bctng quart;. calcite and clay minerals. Clay mineral coatings on component grain surfaces arc important in the dtagenesis of sediments. in that they may inhibit t he growth of pore-filling quartz or calcite cements. Such textures require the

usc of the electron microscope for

detailed study. 38 and 39 show a highly porous sandstone with rounded quart7 grains. The speckled areas which appear black in the XPL photograph are po res filled with the mounting medium. Although comprising a loose fabric of
grains . the sandstone is well-cemented by secondary

(authigenic) quartz in the form of overgrowths on the detrital grains. The surfaces of the original grains arc pick ed out by a thin red-brown rim of iron oxide. Since both the overgrowth and the detrital cores of each grain show untform interference colours, it is clear that the overgrowths grew in optical continuity with the grains on "htch they nucleated. otc that where overgrowths are wcll-de,eloped. the overall shape of the grains has changed from rounded to subhedral. A good example of cuhedral cry5tal terminations can be seen near the top of the photograph on the right-hand side. Calcite cements in sandstones arc usually fairly coarse?

grained (sparite p. 34). Occasionally they are so coarse tlwt one cement crystal envelopes many detrital grains, resulting in a poikiliric texture. 40 and 41 show a sandstone in which the detrital grains arc subangular to subrounded quartz. The cement is calcite of such a grain si7e that there are only a few crystals in the field of view

shown. Individual cement crystals can be distinguished in the XPL photograph by their slightly different inter? ference colours (high-o rde r grey and pink). 18

39, 40, 41

Terrigenous clastic rocks

San dstones - Mat rix and cement
(contmued)

16 and 17· localily and age unkncm·n: magn(ficmion x 16: 36 PPL, 17 XPL. 38 and 19: Penrilh Sand s/One, Permian, Penrilh, Cumbria,

40 and 41: Middle Jurassic. Bearreraig Bay, Isle o f Skye. Scorltmd; magn(ficarion x 20; 40 PPL. 41 XPL. 19

England; magn(ficalion x 27: 18 PPL, 39 XPL.

Terrigenous clastic rocks

42,43,44

Cements
(continued}

42

shows a high magnification view of a fine sandstone which contains both quartz and calcite cements. The quart/ cement is in the form of overgrowths on detrital grains. Evidence for this is the euhedral terminations seen on ?ome grains (good examples can be seen just above the centre of the photograph). Unlike the quartz in the

sample in 38 and 39, the shape of the original detrital particles is not visible where overgrowths are present. Calcite cement postdates the quartz overgrowths and infills pores. The thin section has been stained with Alitarin RedS and potassium f crricyanidc (sec p. 34) and the calcite is a very pale mauve colour because it contains some iron. 43 and 44 show a fine sandstone which is cemented by gypsum. Gypsum has approximately the same birefrin? gence as quart.?: and so it does not show up well in the XPL photograph. In the PPL view. the higher relief of the gypsum and its cleav<tge help to distinguish it from quartz. Some of the gypsum cement crystals enclose several detrital grains. One showing pale grey interference colours in the XPL view occupies the upper left part oft he photograph.

42: ivliddle Jurassic, Yorkshire. England: magnific? o/ion x 72, PPL. 43 and 44: Cretaceous. Tunisia; magnification x 24; 43 PPL, 44 XPL. 20

45,46

Terrigenous clastic rocks

Ceme nts
(continued)

45 and 46 show a quartz sandstone at high magnification. Note the mica flake in the centre of the photograph. In the field of view shown many of the intergranular pores are unfilled (e.g. lower left) and arc thus black in the XPL view. However, the quartt. grains and mica Aake in the centre· of the v1cw arc surroUI)dcd by numerous small crystals with low relief and showing first order grey

interference colour?. These are clay minerals in the form of a cement. Usually an electron microscope is needed to demonstrate the shapes of clay mineral crystals and techniques such a? X-ray diffraction to determine the exact identity of the minerals. In the example shown. the crystab arc largeenough for the typical low birefringence of kaolinite to be seen, together with the 'book' texture which develops as a result of the characteristic form of a series of stacked platy crystals. This is best seen Im? mediately above and to the right of the mica Aake.

45 and 46 Lmrer Carboni f erous, Fif eshire, Sc01lwul: magni fication >< 90; 45 PPL, 46 XPL. 21

Terrigenous clastic rocks

47, 48, 49

C o m pact i o n- P ressure? solution

Sandstones which are not cemented early i n diagenesis usually show signs of compaction. Since most of the grains in sandstone are rigid, there is usually little evidence for grain fracture and breakage (cf. limestone compaction. p. 58). Thus, apart from grain repacking during early compaction, the most important process of compaction is pressure-solution. This is the process whereby a sediment under load is subject to selective solution. 47 shows a sandstone with high intergranular porosity (the pores. now filled with the mounting medium, are the grey areas). Most of the quartz grains are coated with a thin brown rim of hematite cement. At many of the contacts between grains, one quartz grain has undergone solution leading to the penetration of one grain by another (concavo-convex contacts). Good examples can be seen in the upper left part of the photograph. This is the first stage of pressure-solution. Where pressure-solution is more intense, the contacts between grains become sutured. 48 and 49 show a sandstone in which grain contacts are irregular and wavy because of pressure-solution. Silica dissolved during the process may be precipitated as cement away from grain contacts, leading to the destruction (occlusion) of poros? ity. As can be seen, the result is a texture in which the original grain boundaries can no longer be identified. The sample illustrated is particularly unusual in that a thin 1.0nc ofclay or mica separates the sutured quartz grains. It has a higher relief than the quartz and is clearly visible in the photograph taken with PPL. This thin zone of material together with the sutured contacts enables the quan.r grains to move slightly relative to their neighbours. This property impans Acxibility to the sandstone, demon? ?trablc in hand specimens. Sandstones of this type arc known as flexible sandstones or itaco/umites and are extremely rare.

stone. Triassic. Cheshire. England: 47: Nell' Red Sand nl(lgn!fimtion x 43. PPL. 4,y and 49: lraco!wnite. Brazil: magn{/ication x 31: 48
PPL. 49 XPL.

22

so, 51, 52

Terrigenous clastic rocks

Grain solution and rep l acement

50 shows a porous sandstone. In this example the mounting medium has been impregnated with a dye so that the pores appear mauve-coloured. Note that the margins of some of the quartz grains arc cmbayed. This has occurred as a result of corrosion of the quartz during diagenesis and has led to enhancement of the porosity. The common porosity types in sediments are illustrated in Fig. F (sec p. 65) and in 1 5 1-160, with examples from limestones. Many of the terms are equally applicable to sandstones. 51 and 52 show a sandstone cemented by a few large calcite crystals. Note the typical high-order interference colours displayed by the calcite. seen in the XPL photo? graph. The detrital grains, including both monocry? stallinc and polycrystalline quartz, arc coated with a thin brown rim of iron oxide. The texture of the rock is unusual, in that it is apparently not grain-supported. being about 30% quartz grains and 70% calcite. Replace? ment of original detrital grains by calcite is partly responsible for this appearance, areas of calcite outlined by iron oxide being interpreted as the original grains. A good example can be seen in the centre.

50:

Saltll'ick

Formation ,

Middle

Jurassic,

Eskdale.

Yorkshire. England; magnification x 132, PPL. 5/ all(l52: New Red Sand stone, Tria?·sic, Bri.r:lwm, Devon. En?rla11d; magnification x 43; 51 PPL, 52 XPL. 23

Terri genou s clastic rocks

Sandstone classification
Modern sandstone classifications require the estimation of the propor? tions of the principal grain types and thus thin section study is required. Of the different sandstone classifications proposed. we present a widely-used example, that of Folk ( 1974). Figure B shows the classification of those rocks containing less than 1 5 % fine-grained matrix in terms of the three principal components: quartz, feldspar (plus granite and gneiss fragments) and other rock fragments. Those sandstones containing more than 1 5 % fine-grained matrix are the greywackcs, and arc subdivided according to Fig. C. We refer readers to Pettijohn ( 1975) for dctai Is of other sandstone classifications and for the classification of conglomerates and mudrocks, where studies using the petrographic microscope arc less important.

Sediment maturity
There arc two type? of sediment maturity mineralogical and textural. Mineralogically mature sediments are those containing a high propor?

tion of the most chemically stable and most physically resistant minerals such as quartL, chert and ultrastable heavy minerals, such as Lircon and tourmaline. Mineralogically immature sediments contain the less stable grains. such as feldspars. and those rock fragments not consisting principally of quarll. The textural maturity of a sediment depends on the content of fine? grained material. the sorting and the roundness of the grains. A scale of textural ma tu rity proposed by Folk ( 1951) is presented below. lmmnture stage
Submnturc stn,::c Sediment contains > 5% clay matrix. Grains poo rly-sorted and not well-rounded. sediment contains < 5% cla y matrix. Grains poorly-sorted and not wel l-roun ded. sediment contains lillie or no clay. Grains well? sorted. hut 110t well-rounded. sediment con tains no clay. Grains well-sorted and

Mature stage -

(All quartz. but not chert)

Q Sub litharenite

Supcrmnturc stugc

well-rounded.
Diagrams illustrating visual estimation of sorting sediments using thin sections arc shown in Fig. D.

F (Feldspar + granite
+ gneiss frag ments)

1:1

(All other
rock fragments) Very well-sorted

Wel l-so rted

Sedimentary RF

arenite

Phyllarenite

Vo1can1c RF

Fig. B Cla.lsific(l/icm c1/ swui.IIOIIC'.\. The upper lriangle shows a sands!one c/assi f iccllinnf or sedimenls ll'ith less than 15% fine-grained ma!rix. C/assi /imlion illl'oll'e.\ the remora/ c ?(malrix, cemenl, micas etc. and reca/cu/(1/ion f!{ compmwnt.\' to 10 0%. The lower 1riangle shmn luJII litlwrenite.\ IIIli\' be /itrther classified. (From Folk . 1974)

Moderately-sorted

Poor ly-so rted

Fig. D

Soning

Illin sections (a{ter Peuijolln et al.. 1973)

95o

Quartz

Ouartzwacke

Feldspath1c
greywacke

Lithic greywacke

Fel dspar

1:1

Rock fragmen ts

marrix ( gn'\'u·ac/..c?., J

F ig.

C

Cla.H(fication of'swulstone will/ more than 15% f ine-grained

24

53,54,55

Terrigenous clastic rocks

Quartz a r e n i te, a rkose

53 shows a sandstone whi<.:h <.:onsists almost entirely of quart1 and is thus clas?ificd as a quartz arenite. Such sandstones were <.:ailed quart;itcs in older classifications. although it is perhaps bcucr to restrict the term quartzite to metamorphic rocks. Since they contain more than 95% quartt. quart; arcntics are always mineralogically mature. The example ?hown is texturally submature to mature. lackmg clay and being reasonably well-sorted. Rounding of the grain? is difficult to assess because the effects of comract1on and cementation have obscured the shape of the original grain!>. 54 and 55 ?how a ?cdimcnt in which more than 50% of the grams arc feldspar. easily Identifiable in PPL by the broY. n colour rcsuhmg from their alteration (see p. 7) and in cros?cd polars by the remnants of multiple \\\inning in many grains. Most of the other grains in the sediment arc clear quart;. so the sample is an arkose. A sediment with such a high proportion of relatively unstable f eldspar grains is mineralogically immature. The matrix coni<! ins abundant opaque iron oxide.

53: Millstone Grit. Upp<'r Carhon(/ <'rous. Craig-_r-/)inas. Soutlt Wale.\: 11/Uf!lli /ication x 17. XPL 54 and 55: Torridonian. Premmhrian. 5)1'otlmlfl: magnific? ation x 10:54 PPL. 55 XPl-.
25

Terrigenous clastic rocks

56, 57,58

L i t h a re n i tes

Litharcnitcs arc sandstones with less than 95% quart? and more rock fragments than feldspar. They may be classified according to whether the rock fragments arc predominantly sedimentary, volcanic or metamorphic (Fig. B p. 24). 56 and 57 show a sedarcnitc, in which the fragments arc from carbonate rocks. The fine-grained fragment just abo\'C the centre is from dolomite rock. Examples of hme?tone fragmentl> can be seen in the lower r ight-hand quadrant. The sediment also contains monocrystallinc quart/ and echinoderm plates. The latter are the speckled grains with uniform interference colours (see p. 44). In this example the echinoderms are reworked from an older limestone and are not fragments of fossils living at the time of linal deposition of the sediment. Thus they arc classified as sedimentary rock fragments rather than as f os,il material. 58 and 59 show a mineralogically immature sediment consisting mainly of igneous rock fragments cemented by pale brown ch loritc. The clcar areas in the view taken with PPL show high-order interference colours under crossed polars and arc carbonate. A variety of grain types is present and all show some signs of alteration. Many of the rock Cragmenls contain partially-altered phenocrysts of plagioclase in a groundmass of plagioclase laths and another mineral too fine-grained to determine, but which may he chlorite. Individual plagioclase crystals arc also present and vary from euhedral laths to subhcdral grains. The porphyritic texture of the igneous rocks suggests a volcanic source rock and thus the sediment is a volcanic arenite. Such an immature sediment would be very clo\c to its source rock and it is, perhaps, a locally? reworked pyroclastic rock. 60 and 6 1 sho'' a sediment which is more than 75% quart/. The remaining grains are rock fragments and hence the sediment may be classified as a sublitharcnitc. The rock fragments arc of fine-grained sedimentary and metasedimentary rocks.

26 /

59, 60, 61

Terrigenous claslic rocks

L i t h a ren ites
(continued)

56 and 57: Brockram. Permian. Applehy. Cumhria. England: magn{(ication x 16: 56 PPL. 57 XPL. 58 and 59: Ordovician. Builth Wells. POII')'S. Wales; magnificcuion x 12: 58 PPL. 59 XPL. 60mu161: Coal Measures. Lancashire. En111and: magni fic? tllion x 14:60 PPL. 61 XP/,.

Terrigenous clastic rocks

62, 63, 64

Greywackes

Greywackes are t hose sandstones containing more than 15°,'o fine-grained matrix. Their classification is shown in Fig. C (see p. 24). 62 and 63 show a typical greywacke, b eing poorly? sorted and containing abundant tine-grained matrix (almol>t opaque in the view taken with plane-polarised light). The fragments are predominantly monocrystalline and polycrystalline quartz grains, but a small percentage of rock fragments (cloudy particles of fine-grained material) make this a lithic grcywacke. 64 and 65 illustrate a sediment with about 15% matrix and contain1ng quartz and abundant feldspar grains. Feldspars include plagioclase with multiple twinning and perthitic alkali feldspar. The sediment is thus classified as eldspathic greywackc. af 66 and 67 show a grcywacke in which quartz, feldspar and rock fragments arc visible. Quart.? grains are clear in the PPL view. whereas the f eldspars arc broy, nish owing to alteration. The XPL view shows that some feldspar grains arc multiple-twinned plagioclas e whereas others are microcline showing typical cross-hatch twinning (e.g. right of'field of view: about halfway up). The grain in the centre of the field of view is an igneo us rock fragment consisting of plagioclase and amphibole. The amphibole can be recognized by its green absorption colour and its two cleavages at 1 20?. Smaller fine-grained rock frag? ments and indi' idual ferro-magnesian mineral grains are also present.

65,66,67

Terrigenous claslic rocks

Greywackes
(continued)

62 ant/63: Asll?illian. Dy( ed. Walc?s: lllllf(ni ficalion x 16: 62 PPI.; 63 XPl.. 64 and 65: Loclllily and a?c unknown: magnijic(l{ion x 16: 64 PPL. 65 XPL. 66 and 67: Sil11rian. Pcehlesliire. Scotland: magni fic? a tion x 43: 66 PPI 67 XPl
?. ?.

29

Terrigenous clastic rocks

68,69

Si ltstones

Siltstones are those terrigenous sediments in which the majority of the grains are between 16 and 2 ? 6 mm in diameter (Table I , see p. 3). 68 and 69 show a coarse siltstone (notice that the magnification is higher than that of most of the previous photographs) with abundant quart? grains and thin mica flakes. The micas include both muscovite (colourless) and biotite (yellow or brown). Muscovite is more abundant and shows second? order interference colours. seen in XPL. The sediment is cemented by calcite. showing high relief in PPL and high? order interference colours with polars crossed.

68 and 69: Axe llllll locality unknown: magn(/ication x 71: fi8 PPI., 69 XPL.
30
--- --------

70, 71

Terrigenous clastic rocks

Si ltsto n es
(continued)

Many siltstones show small-scale sedimentary structures.
70 shows a laminated siltstone. the laminae being defined

by changes in grain size. The dark layers, seen near the base of the photograph, arc composed almost entirely of clay-sized material, whereas the band just above the centre is composed of clear fine sand-sized quartz. The photograph also shows graded layers (below and above the coarse band). The fining-upwards grading is shown by a decrease in the clear quartz material and an increase in the dark-coloured clay. 71 shows a siltstone 111 which ripple cross-lamination can be seen. indicating a flow from right to left, as seen in the picture. The ripple structure is picked out by the alternation of dark clay-nch and pale clay-poor laminae.

70: Coal Measures. Upper Carhoni{erous. Lancashire. Engla111/; magnifica/ion x 15, PPL 71: Ashgillirm. arion x 9. PPL. Uangrano[(. Dy(ed, Wales: magn(fic?

Part 2

Carbonate rocks

33

Carbonate rocks

Introduction

Unlike terrigenous sediments. carbonate rocks comprise material formed mostly at or ncar I he site of final accumulation of the sediment. Much of the material is produced by biological processes. Two carbonate minentls arc common in older limestones - cah·ife. CaCO,. and d olomite, CaMg(C01}2. In recent shallow marine carbonate sediments the mineral aragonite, also CaC03? is abundant. However, it is metastable under the normal conditions prevailing in sediments and i? u?uall? dl-,,ol\cd once a limc'>tonc is exposed to circulating meteoric waters. Altcrnall ,cJy 11 may 1nvcrt directly to calcite. Dolomite is normally a secondar) mineral replacing calcium carbonate, although it ma) form m \Ctllments very soon after their deposition. Both calcite and dolomite may contain some ferrous iron. in which case the prefix / erroan is used before the mmcral name. The opllcal propertic'> of calcite and dolomite are similar and therefore they can be difficult to distingubh optically. Simple chemical ?taining technique? arc often employed by carbonate sedimentologists to distinguish calcite from dolomite and to distinguish ferroan from non-fcrroan minerals. The dye Ali:arin !?ed S is used to diiTercntiatc calcite and dolomite, whereaspotassiwn /('rricranide is used to di fferentiate ferroan and non? f crroan minerals. The dyes arc dissolved in a weak a c id solution. This also helps to distinguish dolomite from calcite. as dolomite does not react with cold dilute ucid whereas calcite docs. producing a contrast in
Tahle l. Elchint: and stlliniiiK
I Mineral
Effect of

relief between the two minerals. Results of the etching and staining process arc shown in Table 2. Details of the procedure are given in Appendix 2. The intensity of the stain colour is partly related to the intensity of the etching with acid. Fine-grained crystal fabrics with many crystal boundurie? etch more rapidly and t h us show deeper stain colours than coarse crystal fabric? with few crystal boundaries. Stain colours arc particularly weJI-iJlustrated in 100, 124. 131, 161 and 165. Other stams h;ne been used to distinguish between aragonite and calcite and to idcntif) magnesian calcites: details are given in books on technique? in sedimentary petrology such as that of Carver (1971). Carbonate roc"s may also be examined using acetate peels. These record an i rn pre?sion of an etched rock surface (which also may be stained) on a thm sheet of acetate fi lm . Acetate peels have the advantage of being cheap and ca::.y to make. but because they arc isotropic. minerals cannot be identified by optical properties, such as relief and birefringence. Details of the procedure for making acetate peels are given in Appendix 3. Components The three most important components of carbonate rocks arc al/ochemical components, microcrystalline calcile. and sparry calcite. I . Allochcmical components or allochems. are organized aggregates of carbonate sediment which have formed within the basin of deposition. They include ooids. bioclasls peloids. intraclasts and oncoids and arc considered in detail in the f ollowing section (72 to
.

charaeleristics ol carbonate minerals
Stain Combined result

St:lin colour with

etthin,::

colour with
I>Ota??iurn

Alitarin
RedS

ferric) anide

Calcllc ( nonf crroan) Calcttc (fcrroan)

I I

able (rdu:f reduced! Con\ldCr· able (rchcf re duc ed)

COihltkr-

I l

rink to rcd-bnm n Pmk to rcd-bnm n

I I

None

'

Pink to red? brown

<kcp blue dcpcndmg on
tron content ?one

Pale

10

MaU\e to
blue

Dolomnc Ncghg1ble (non-fcrroan) (rehct m;unlaincd)
Dolomite (ferroan) 1\icghgtblc (rdicf maintatncd)

None

'

Colourless

None

Very pale blue

Very pale blue (appea rs turquoise or greenish in thin scctio?

Microcr}stallinc calc1tc or micrite is carbonate sediment in the form of grains less than S11m in diameter. Much of it forms in the basin of deposition. either as a prcc1pitatc from seawater or from the disintegration of the hard parts of organisms. such as green algae. The term ·carbonate mud· is also used for this fine sediment. although stnctly mud mcludes material of clay- and silt size (up to 6:2Jim). Micrite 1s illustrated in 84. 89. 1 1 1 and 157. J. Sparry calcite. lflarite or 1par refers to crystals of 5 pm or more in diameter. Much of it is coarse. with crystals commonly up to I mm in ?iLe. It i'> usually a pore-filling cement and thus may form in a rod a long time after deposition of the or iginal allochems and micrite. Sparite is illustrated in 73. 82. 124 and 131.
.

.,

120).

The clas?itkation of limestones involves the identification of al lochems and estimation of the proportions of micrite and spa rite (sec p. 62).

34

72,73,74

Carbonate rocks

Ooids

Ooids o r ooliths arc spherical o r ellipsoidal grains, less than 2 mm in diameter, having regular concentric laminae developed around a nucleus. Ancient ooids often show both the concentric laminae and a radial structure. It is not always certain whether the radial structure is primary. or f ormed during the inversion of aragonite to calcite. 72 shows ooids with well-developed radial and con? centric structures. The nuclei arc micritic carbonate grains. The sample shows a range of ooids. from those with a small nucleus and thick cortex (the oolitic coating). to those with a large nucleus and a single oolitic lamina. The latter are called superficial ooids. The matrix between the ooids is a mixture ofcarbonate mud and sparry calcite cement. 73 illustrates ooids with a rather poorly-preserved concentric structure. The structure may have been partly lost by micritization (p. 54). The speckled plates with thin micrite coatings are echinoderms (an example can be seen half way up the right-hand edge). The pink-stained cement is non-ferroan sparry calcite. The unstained

grains with low relief are secondary (authigenic) quartz replacing calcite.

74 shows ooids with relatively thin cortices coating detrital quartz nuclei. Note how the early ooid laminae fill in depressions on the surface of quartz grains and arc absent from angular corners. The cement is pink-stained non-f erroan sparry calcite.

72: Stained thin section, Upper Jurassic, Cap Rhir, Morocco; magnification x 31. PPL. 73: Stained thin section. 1/unt ' s Bay Oolite. Lower Carbo? niferous, South Wales: magnification x 43. PPL. 7tl: Stained thin section, Carboniferous Lmestone. i Llan? gollen, Clll'yd. Wales; nwgnification x 27, PPL. Ooids can also be seen in 125, 127. 137, 146. 1 4 7 and 155. 35

Carbonate rocks

75, 76, 77

Peloids and I nt ra c l asts

A large proportion of the allochems in limestones arc grains composed partly or entirely of micrite, but having no concentric laminae in their outermost zones. Various terms have been used to classify these grains and most depend on an interpretation of their origin. Those g rains composed of micrite and lacking any

rccognitable internal structure are called pe loids. 15 ?hows a limestone in which the allochems arc mainly peloids. circular to elliptical in cross-section and averag? ing about 0.1 mm in diameter. Such peloids arc generally interpreted as faecal in origin and are called pellets. The photograph shows pellets at the lower end of the size range for typical pellets. which extends up to 0.5 mm. 76 shows larger. less regular peloids, some of which have a trace of internal structure although its nature can? not be identified. In the lower part of the photograph arc speckled echinoderm plates, and midway up the right hand edge arc segments of the dasycladaccan alga

Koninckopora (sec 1 1 3). Both echinoderms and algae ?how signs of replacement by micrite around their margins (micritization. p. 54). It is probable that the p?:loids were formed by intense micritization of bioclasts, thus accounting for their vague relict structures.

lmracla.\tS arc sediment which was once incorporated on the sca-Aoor of the basin of deposition and was later reworked to form new sediment grains. 77 shows a large grain which might be described as a 'coated bioclast'. I t comprises a nucleus, which i s a fragment of a brachiopod ?hell, surrounded by a coating of microcrystalline calcite. The coating is not laminated. so the grain cannot be called

an oncotd (see p. 38); it is external to the shell and has a ?harp contact with it so the coating was not formed by mtcri!lntion (sec p. 54). It is therefore likely that it is a fragment of locally-reworked sediment, the brachiopod o,hcll having once been incorporated in a fine-grained '>Cdtmcnt which was later eroded to produce intraclasts.

Stamc>cl thin section. Upper Jurassic. Cap Rhir, Morocco: 1111/KIIi/ication x 33, PPL. 76. Lmtained thin section, Woo Dale Limestone, Lower Carhoni/(orous. Lo11g Dale, Derbyshire, England; magnific? ation x 1 1, PPL. 77: Stained thin section. Urswick Limestone, Lower ' row. Trowbarrmr. Cumbria. England; nwgllijiC? Carlwn(fe alion x 15. PPL Peloids are also shown in 86, 123, 130. 134, 147, !58 and
75: 162.

36

78,79,80

Carbonate rocks

Agg regate g ra i ns a n d l i t h oc l asts

78 and 79 show aggregate gmim. These are made up of

irregular aggregates of a small number of recognizable particles cemented together by micrite or fine sparite. 78 shows the botryoidal form typical of these aggregates. The component particles include ooids (the grain right of centre) a'> well as peloids and a few bioclasts. These aggregates arc similar to the grapestones of modern sedimentary environments, where particles become cemented on the sea-floor in areas of low sedimentation r ate. The opaque material in the top centre is bitumen (sec
160). 79 shows large aggregate grains with a smooth. rather

than botryoidal, external form. The micritic material binding the part1clc together completely envelopes them, and is more important volumetrically than the cementing material of the grains shown in 78. It is unlikely that the aggregation occurred by cementation on the sca-Aoor, but the particles arc probably reworked grains and thus could also be described as intraclasts. The matrix is micrite with a lillie sparitc and some bioclasts. Uthoclasts or extraclasls are eroded fragments of lithificd sediment which have been transported and redeposited. 80 shows lithoclasts which are made up of ooids and bioclasts cemented by very pale pink-stained non-ferroan sparry calcite. Note the truncation of both particles and cement at the lithoclast margins. indicating reworking of lithified sediment. The equant sparite ce? ment within the lithoclasts is typical of precipitation from meteoric waters (p. 55), so these fragments arc or a limestone which was not cemented in either the original environment of depo?ition of the component particles in the lithoclasts, nor in the final environment of deposition of the cla?ts themselves. They arc. in fact. fragments of a Carboniferous Lime?tonc reworked during the Jurassic. The final cement is lilac-\tamcd. coarse fcrroan calcite.

78. Umtained thin veuion. Bee Low Limestone. Lo11·er Carbonif erous. Windy Knoll, Derhyshire. England: mag? nifimtiml x 27, PPL. 79: Swined r!tin section, 011anamane F ormation. Middle Jurmsic. Air Chehrid. Wesrem High Atlas, Morocco: 1iwgni{ication x 14, PP!.. 80: Stained thin section, S11flon Stone. Lower Jurassic, Oxmore-hy-Sea. South Wales: magnif ication x 28. P PL.

37

Carbonate rocks

81 , 82, 83

P i s o i ds a n d O n c o i d s

The nomenclature of carbonate grains which arc larger than 2 mm in diameter and have an outer layer with concentric laminae, depends to a certain extent on an interpretation of their or igin. Thus the term pisoid or pi.wlith usually refers to grains presumed to have formed inorganically, usually in a subaerial environment. On the other hand, oncoids. or oncoliflrs, are presumed to be biogenic. blue-green algae on the grain surfaces, trapping and binding fine sediment particles. 81 is a photograph of a polished rock surface showing oncoids. Note the size of the grains, the asymmetrical growth and the wavy nature of many of the laminae, all features characteristic of oncoids. The bluish-grey areas arc sparry calcite and the orange-brown areas arc stained with iron oxides. 82 and 83 show concentrically-laminated grains whose origins are more difficult to interpret. 82 shows grains which arc about 2 mm in diameter. The outer surfaces arc not as smooth as most ooids, although the concentric lamination is very regular. Grains in the upper right show irregular outer coats of micrite and some particles have apparently grown together to form compound grains (e.g. lower left). This latter feature is unlikely to occur in ooids, where precipitated carbonate laminae are formed while the grain is held in suspension. These grains are therefore interpreted as oncoids. The cement is sparite. This photograph is of a thin section made by Sorby in 1849 and illustrates the high quality of his sections. 83 shows grains with a regular. well-defined concentric layering. in grains up to 5 mm in diameter. This is typical of inorganic growth and these grains may be pisoids. Pisoid? are commonly fractured or broken. Broken pieces can be seen towards the top right of the photograph.

81 : Polis/red swf ace. L/anel?r Formation. Lower Carbo? niferous. Blaen Onneu, South Wales: magnification x I .8. 82: Unstai11ed thin section. Wenlock Limestone. Silurian, Mafl?em Hills. En?land: magni fication x 13. PPL. 83: Stai11ed tlli11 section, L01rer Jurassic. Greece: magn(/ic? atioll x I I . PPI?.
38

Carbonate rocks

Skeletal particles (Bioclasts)
Introduction Skeletal particles, or bioclasts, are the remains, complete or frag? mented. of the hard parts of carbonate-secreting organisms. There is such a variety in the mineralogy. structure and shape of skeletal material that several books could be written on this subject alone. When trying to identify bioclasts, the following features should be considered: I . The overall shape and size or the particle. 2. The internal wall structure of the particle. Many structures are more easily visible with polars crossed than in plane polarized light. It is important to distinguish those bioclasts which were originally calcite and have well-preserved wall structures from those which were originally aragonite and have had their wall structure modified or replaced during the alteration to .calcite. In this section we have attempted to show the diversity of skeletal structures present in ancient limestones, concentrating on examples from groups which arc particularly common or occur over a wide stratigraphic range. For more detailed descriptions and illustrations of skeletal particles readers are referred to Majewske ( 1 969), Horowitz and Potter ( 1 971), Bathurst ( 1 975) and Scholle ( 1 978).

39

Carbonale rocks

84, 85,86

B i ocl asts
Molluscs

Bivalves a nd gastropods are common components of limestones. Most were made of aragonite, so although there arc a diversity of structures. these arc not seen in ancient li mestone:.. Most originally aragonitic molluscs arc preserved as casts that is the aragonite dissolved out during diagenesis leaving a mould which later became filled with a spa rite cement. There arc, however. import· ant molluscan groups which had a calcite shell, especially among the bivalves. and these have well-preserved wall \t ruct urcs. 84 shows a limestone with abundant molluscan casts. In thi? casc shell moulds have been in filled with a few large calcllc crystals. Gastropods can be seen. both i n long \ectton (lov.cr right) and transverse section (lower left). The long straight shells a rc bivalve fragments. Careful 111?pection shows that the long valves in the upper left h<I\C a two-layer structure- a thick layer ofcoarse sparite and a t hin layer with a different structure. This Iauer layer may have been calci te originally, indicating that the organism had a mixed aragonite/calcite skeleton. The rock matrix is micritic sediment. 85 shows a limestone made up almost entirely of rounded bivalve fragments preserved as casts. The shape or the fragments is shown by the thin micrite rims on the m argins of the shells. These arc micrite envelopes and formed by micritization by endolithic algae (p. 54). The cement infilling the bivalves and between the shells is a line sparite, initially pink-stained non-ferroan calcite, but crroan towards the centres of pore-spaces as becoming f indicated by the bluish staining. R6 illustrates a section through a large thick-shelled ga?tropod. again preserved as a cast. The outer margin of the 'hell is picked out by a thin calcite layer, not more than 0.5 mm thick at this magnification. but the inner margm is only clear where sediment has partially filled the 1ntcrnal cavity. The sediment around the shell contains abundant ?mall peloids.

84. "iwined thin .I?'Ction. Eyam Limestone. Lower Carbo?
.

n i / e r ou.\, Ricf.. low Quarry. tllion x 13. PPI-.

Derbyshire, England: magnific?

85: Stained 1/iin section. Upper Jurassic. D orset, England:
ma?ni/iwtion x 14. PPL. 86: S111 ined ace/ale peel. Man in P PI ? .

Limes/One, Lmrer Carbo? niferous. Mil/om. Cumbria, England: magn(/ication x 7.

40

87,88,89

Carbonate rocks

B i ocl asts
Molluscs

(continued)

The photographs on this page illustrate bivalves which were entirely or partly calcite. The oysters arc one of the most important groups of calcitic bivalves. 87 shows two large pink-stained oyster fragments, each having a foliated internal structure. Fragments of oysters may be difficult to distinguish from brachiopods, although their thick shells with a rath?r irregular foliated structure arc characteristic. Note also how the left-hand end of the upper fragment is upturned and splitting. The rest of the sediment comprises broken? up bioelasts set in a blue-stained f erroan calcite cement. The white areas are holes in the section. Some bivalves have a thick prismatic layer. the prisms being elongated at right angles to the shell wall. 88 shows a fragment of the common Mesozoic bivalve Inoceramus (nght). The shell is sectioned more or less parallel to its length and hence the prisms arc seen in cross-section. Individual prismatic crystals break away easily from the ?hell and in this example most of the sediment is composed of these crystals. seen in various sections. 89 shows examples of thin bivalve shells known as filaments. These are the valves of planktonic bivalves and an: common in Mesozoic pelagic limestones. The micritic sediment between the shells contains small circular areas ofsparite. These are probably calcite casts of the siliceous microfossils. radiolaria (p. 82).

J\1iddle Jurassic. l.t?ck/1{/mpton Hill. Gloucevtershire. Hnglwul: 11Wf(n[/ ic? lltion x 8 . PPL. 88 Swmed thin section, Upper Crellln'ou:., Stmtlwird.

erior Oolite. 87 Swim'd t!tin section. In/

89

Stained thin section. a/loll x 16. PPL.

Sht'. Scotland: magnification x 14. PPI.. Triassic .

Greece: magnific?

Other molluscs are shown in 105. 1 2 4 , 135. 136. 1 43,

156 (Ill(/ 159.

153,

41

Carbonate rocks

90, 91, 92

B i oc l asts
B rachiopods

The articulate brachiopods are important constituents of Palaeozoic and Mesozoic limestones. They were origin? ally calcite and so their shell structures are weB-preserved. Typically, brachiopods have a thick inner layer of calcite fibres aligned with their length at a low angle to the shell wall. A thin outer prismatic layer may be preserved. 90 shows a broken brachiopod of which parts of both valves arc present and surrounded by a micrite envelope (p. 54). The fibrous structure is clearly visible, as are tine tubes at right angles to the shell wall. filled with blue? ?taincd ferroan calcite cement. These are endopunctae and they characterize some groups of brachiopods. The sample also shows a good example of coarse. blue-stained fcrroan calcite cement. 91 shows two large fragments of pseudopunctate brachiopods. In these. the fibrous wa11 structure is inter? rupted. not by open tubes but by calcite rods. The left? hand fragment shows the pseudopunctae sectioned par? allel to their length. Note the wavy nature of the fibres adjacent to the pseudopunctae. The right-hand fragment is a section of a shell showing the pseudopunctae in cross? section. 92 illustrates a brachiopod fragment with its outer prismatic layer preserved. The foliated nature of the inner part of the wall is also well shown. The shape of the fragment suggests that it is part of a ribbed shell. I t is also illlfllllltlllte. lacking either endopunctac or pscudopunc? tae. These factors in an Upper Jurassic brachiopod indicate that it is part of a rhynchonellid. The fine-grained calcite matrix contains abundant colourless fine sand? and silt-size quartz.

90: Swi11ed tlti11 section. Inf erior Oolite. Middle Jurassic. l.ec? hampton Hill. Glm1c?srersltire. England: magnific? ation x 32. PPL. 91: Stained tltin section. Monsal Dale Limestone. Lmrer ' rous. Cressbrook Dale. Derbyshire. England: Carhon(fe lllliKII(/ication x 16. PPL. 92: Stained tltin section. Upper Jurassic. Jehel Amsilf<'ll. Morocco: magnification x 40. PPL.
42

93,94, 95

Carbonate rocks

B i oclasts
Brachiopods (continued)

93 shows a number of small impunctate brachiopods with the large pedicle valve and smaller brachial valve complete. The roughly elliptical fragment in the lower centre is a section transverse to the length of the fibres making up the shell wall, and shows a characteristic fine net-like structure. Some spiriferoid and pentameroid brachiopod shells have an inner layer composed of calcite prisms aligned at right angles to the length of the shell. In this case the outer f oliated and prismatic layers arc much reduced in thick? ness. 94 shows two large shell fragments with a thick inner pr ismatic layer. One is below the centre of the field of view, lying parallel to the bottom of the photograph, the other is near the left-hand edge. The sediment also contains brachiopod fragments with the more normal, f oliated structure in a matrix of fine sparite, probably of neomorphic origin (see p. 60) containing grey-coloured crystals of replacement dolomite. Some pseudopunctate brachiopods possess hollow spines. 95 shows transverse sections through several spines. They have a structure similar to the brachiopod valve with a f oliated inner layer and an occasionally preserved outer prismatic layer. The section through the large spine in the upper left of the picture shows part of the outer prismatic layer preserved. Note how the shape ofthe spine gives the foliated layer a concentric structure. A longitudinal section through a brachiopod spine can be seen in the upper left-hand corner of 94.

93: Stained acewte peel. /:. )·am Limestone. Lower Carbo? nif erous, Rickl011' Quarry. Derbyshire. England: magnific? ation x 20. PPL. 9tl: Stained acetate peel. ?yam Limestone. Lower Carbo? niferous, llead stone Cwtitq?. Derbyshire, En?land; magnif? ication x 27, PPL. 95: Stained thin section, l:.)?am Limestone, Lower Carbo? n!j' erous, Rick/ow Quarry, Derbyshire. Enf(land; magnific? ation x 28, PPL. Other brachiopods and brt1chiopod spines are shown in 77. 103. 106, 1 20 , 123 and 183.

43

Carbonate rocks

96,97,98

B i oc l asts
Echinoderms

F.chinodcrms. particularly echinoids and crinoids, arc major contributors to the allochcmical fraction of mar.inc limestones. They arc easy to identify because they break down into plates which, although they may exhibit a wide variety of shapes, arc single calcite crystals with uniform extinction. They usually have a speckled or dusty ap? pearance as the result of infilling of the fine pores which permeate the plates. 96 and 97 show a crinoidal limestone in which the ?ed?mcnt is 75% crinoids. ote the speckled appearance of the plates. most of which have uniform interference colours and arc thus single crystals. although the ossicle in the upper left comprises two crystals. one showing a greenish colour and one a red colour under crossed polars. The clear spar ·surrounding some of the crinoid fragments is a cement. The XPL photograph shows that the interference colour of this cement is the same as the adjacent crinoid fragment. Hence it is probable that the cement is in optical continuity with the crinoid. Such cements arc common in echinoderm-bearing sediments and arc called S)'lllaxial rim cements (p. 57). The remain? der or the sample comprises micritic sediment and fragments of fenestrate bryozoans (e.g. lower right hand corner). Echinoid spines arc widespread. particularly in Meso;.oic and Cenozoic limestones. 98 shows one complete transverse section of a spine (lower right of field). together with a smaller broken fragment. Echinoid spines arc circular or elliptical in cross-section and show a vanety of radial structures. Like other echinoderm frag? ments. they arc single crystals.

Eyam Limestone. Lower Quarry. Derhysltire. F:ng/wul: magnification x 13: 96 PPL. 97 XPL. 98: Srained tltin section. Quaremary. Cap Rltir. Morocco; nwgni/imtion x 31. PPL. f:'cllinoderm .fi'(lf4/III!IIIS are also shown in 73, 76. 78. 132.
133. 139. 148. 154. 178. 183 and 184.
44

96 all(/ 97: Stained rhin section. Carhon{/' e rous. Once-a-u·eek

99, 100

Carbonate rocks

B i o c l asts
Corals

Corals arc best identified by their overall morphology. The rugose and tabulate Palacotoic corals were calcite. thu? their microstructures are well-preserved. The walls arc usually fibrous and small fragments which lack evidence oft he characteristic coral form can be difficult to 1dcnufy. 99 shows a transverse section and parts of two longi? tudi nal <;cctions of the colonial rugose corn I U!hostrolion. Note the thick outer wall and septa seen in the transverse section. The columella and th in tabulae arc clearly visible in t he longitudinal section. Parts of' the coral walls have been silicified (brownish colour). The pore-filling material is mainly sparite cement with some micritic sediment between the corallites. 100 shows a section through a tabulate coral. otc the corallitc walls and thin tabulae but absenc e of other internal structures. The infill is sparitc cement, initially non-fcrroan calcite (pink-stained). but finally ferroan {blue-stained). The Mcso.wic and Ccnowic sclcractinian corals arc composed of aragonite and hence their microstructure is not well-preserved in limestones. Sclcr<lCtinian corals arc shown in 126, 144 and 145.

99· Swined 1hin sec1ion, Monml Dale Umeswne. Loll'er Carbonif erous. Coombs Dale. Derb1'.1hire. EnKiand: maK? ni/IC'fllion x 16. PPL. 100: Swined 1hin section. Torquay Umes1one. Devonian . Brixl/(/111, Devon. EnKiand; maKni/icalion x 16. PPL.
45

Carbonate rocks

101, 102, 103

B i ocl asts
Bryozoans

Bryo?:oans arc widespread in marine limestones and arc particularly common in Palaeozoic reef complexes. Most bryo?:oans had calcite hard parts and a laminated wall structure is preserved. Among the most characteristic bryozoans are the frond-like fenestrate types, examples of which are seen in tOt. Note the thick wall of laminated calcite surrounding cement-filled pores (zooccia). Most of the fragments are tranwcrsc sections but the large piece to the lower left of centre is a longitudinal section. 102 is a transverse section of a stick-like bryozoan colony. showing the overall rounded shape of the ·stem' and of the ?:ooecia within. Some of these have been infilled with fine sediment (upper right of fragment) but most have a blue-stained, ferroan calcite cement inti II. I n 103. the two circular. concentr ically-laminated grains stained red-brown are brachiopod spines. These arc encrusted by a bryozoan. Note the thick calcite wall of the bryo;oan and the pores of different sizes within the skeleton, filled with pink-stained non-ferroan calcite cement. Some fr<tgmcnts of fenestrate bryozoans can be seen along the left-hand side of the photograph.

/Of · Swined thin section. ?yam Limestone. Lower Carho? ni/ermn . Ricklmt· Quarry. Derbyshire. England: magn(fic? ation x 16. PPL. /(}2: Swined thin section. Ouanamane Formation. Middle .!ura.uic, West em High Atlas. Morocco: magn(fic? ation x 27. PPI .. 103: .S'tained thin section. Red Hill Oolite. ?/liscale.l
QuanT. Dalton-in-Fumes.\, Cumbria . England: magni/ic? utioll x 20. PPL Or/wr hr l'fl:oons art' sholl'n in 96. 97. 132. 133 and 178.
46

104, 105, 106

Carbonate rocks

B i o c l asts
Arthropods

Ostracods

These photographs show examples of the arthropod microfossils, the ostracods, which are widespread parti? cularly in sediments deposited in brackish or hypersaline conditions. Ostracods have thin valves with a finely prismatic or granular microstructure. 104 shows a group of complete two-valved shells, some filled with sparite cement, some with micr itic sediment and some with both . Note the overlap of valves seen in some sections - a characteristic feature of many ostracods. 105 shows disarticulated ostracod valves (thin curved shells) associated with longer straight lengths of shell, which arc fragments of a calcitic non-marine bivalve.

lOti: Swi11ed thi11 sectio11, Red Hill Oolite, Lower Carbo? umess, Cumbria, niferous, Elliscales Quarry, Dalto11-i11-F England: mag11ijication x 40, PPL.
105: U11stai11ed thin section, Upper Carbo11i { erous. Cob? ridge Brickll'orks. Hanley. Staff ord rhire. E11gla11d; mag? nif u·ation x 16, PPL. Ostracods are shown also i11 1 17, 1 19 and 136.

Trilobites

Trilobite hard parts were originally calcite and a finely granular microstructure is preserved. Each crystal is in a similar but not identical orientation to its neighbours. leading to sweeping extinction when the sample is rotated with the polars crossed (not illustrated here). 106 shows a cross-section of a trilobite (centre) and part of a brachiopod shell (base). Note the hooked shape seen at the left-hand end of the trilobite fragmen t produced by incurving of the skeleton at its margin. A vein of blue? stained fcrroan calcite follows the edge of the skeleton along part of its length. Note that the trilobite is stained mauve and hence consists of slightly fcrroan calcite. This contrasts with the brachiopod fragment which is non? f erroan calcite. In some rocks it is thought that bioclasts originally comprising high magnesium calcite m ay be replaced by ferroan calcite. whereas tho?c of low mag? nesiUm calcite rem ain unaff ected.
,

106: Srained thin section, We11lock Limestone, Silurian. Engla11d; magnification x 21, PPL.

47

Carbonate rocks

107, 108, 109

B i oc lasts
Foramin ifera

Foraminifera arc widespread in marine limestones. Most arc calcite but they show a variety of shapes and wall structures. A selection of examples showing some of the variation amongst the f oraminifera, is shown here. The large!tt and perhaps the best-known f oraminifera arc the nummulites of the Lower Tertiary, examples of which arc shown in 107. Note the thick walls which have a radml fibrou\ structure, the fibres being aligned at right angle? to the test wall. The matrix is mainly micritic ?cdiment \\ ith a little blue-stained fcrroan calcite cement. 108 'how., discocyclinids, a type of foraminifer with many shall chambers. The matrix is micrite with many fragmented bioclasts. 109 shows a foraminiferal limestone in which the organisms are micrite-walled miliolids. The cement is tine 'paritc although unfilled pore-spaces remain (e.g. centre of field of view). Partly-tilled moulds of bivalves can be ?ecn outlined by thin micrite envelopes. These arc the elongate curved grains seen on the right-hand side of the photograph.

107: Stained thin section, Eocene, San Salvador, Ma jorca; magnification x 15. PPL. 108: Stained thin section, Eocene, Greece; magnific?
otion x 115. PPL. 109: Stained thin section, Upper Miocene, Ca/a Pi, Ma jor('a; magnification x 27, PPL. 48

1 10, I l l

Carbonate rocks

B i o c l asts
Foraminifera (continued)

110 shows a number of species of micrite-walled forams (cndothyracids). Note how the different sections show different arrangements of chambers. Much of the sedi? ment comprises fragmented bioclasts in a pink-stained. non-ferroan calc ite matrix. Foraminifera are important contributors to pelagic sediments. 1 1 1 shows numerous pelagic foraminifera in an almost opaque micritic sediment. The large keeled f orms arc globorotalids, and smaller types include rounded globcriginids.

Carboniferous. Dam Dale. Derbyshire. Eng/am/; magnific? ation x 19. PPL. Ill: Swined lhin section. Upper Cretrwenus. Pindos Znne.
Central Greel'e: magni fication x 35. PPL. Other.f oraminif era are shown i n 1 16, 120 and 157.

1/0 Swmed thin section. Woo Dale Limestone. Lou er

49

Carbonate rocks

112, 113, 1 14

B i oc l asts
Algae

Those algae in which all or part of the skeleton becomes calcified arc known as the skeletal calcareous algae. They arc important contributors to carbonate sediments throughout the Phanerozoic and exhibit a wide diversity of f orms. Green algae arc one of the most important groups and the photographs on this page illustrate three examples. one from each of the major groups, the Codiaceae, Dasycladaceae and Charophyceae. For de· tails of calcareous algae see Johnson (1961), Wray (1977) and t=ltigel ( 1982). 1 1 2 shows segments of one of the common forms of codiacean alga, Halimeda. which still occurs toda) Living examples contain organic filaments embedded in aragonite. The example shown is from a poorl)· consolidated Quaternary sediment which had to be impregnated with resin before a peel could be made. The grey areas between the algal segments and in the holes originally occupied by the filaments are the i mprcgnatmg medium. In this sample the Halimeda segments arc sull aragonite, although the wall structure cannot be seen at this magnification. Halimeda fragments are often poorly? preserved because of the loss of microstructure during the replacement of aragonite by calcite. 1 1 3 shows two types of algae. The large fragment with the honeycomb structure and walls of fine-grained calcite is the common Carboniferous dasycladacean alga Koninckopora. Several algal segments of a different type can be seen below the Koninckopora. At the level of viewing here they have no discernible wall structure and there is a slight resemblance to echinoderm fragments. In fact they have a fine fibrous wall structure and hence arc not !>inglc crystals. They may show branching and an example of Y-branching can be seen in the lower-right part of the photograph. These belong to a problematic group. often referred to as ancestral coralline algae, but sometimes classified with the foraminifera or stromato· poroids. The third group of green algae arc the charophyte1. although these are sometimes classified separately. ThC) arc frc!>hwater plants. occurring in the Mesozoic and Cenozoic. and usually only the reproductive parts (oogo? n?a) arc calcified. These are small egg-shaped bodies 11ith vanou!> ornaments. 1 14 shows three oogonia in cr?. section.
magni f tcation x 13. PPL. 113: Stained thin section. Chee Tor Rock, Lmrer Carbo· ni/erou.\ , Tunstead Quarry, Derbyshire. England: magm( 1 12: Swinecl ace/ate peel. Quaternary. Mombasa. Kenya,

ication x 17. PPL. 114: Swined thin section. lggui el Behar Formarion. Upptr Jurassic, Wesrem arion x 56. PPL. High A rlas, Morocco:

magn(fi(

115, JJ6, 1 1 7

Carbonate rocks

Biocl asts
Algae (continued)

Many algae possess a central stem encased with calcium carbonate, through which filaments pass to the outside. liS shows numerous sections of such an alga in a brown? stained micrite matrix. Both longiLUdinal and transverse sections are present. The transverse sections are roughly c1rcular or elliptical and the centres arc infilled with micrite sediment. Around the margins of the wall can be seen the holes formerly occupied by the filaments, now occupied by micrite sediment. Details ofthe wall structure have not been preserved so the alga was probably aragonite. The longitudinal sections show that the centre of the stem contains poorly-preserved casts of the algal filaments. The red algae arc important skeletal calcareous algae, and one group, the coralline algae, are major con? tributors to sediments, including reefs, during the Cenozoic. 1 1 6 shows a fragment of a coralline alga, with 1ts characteristic reticulate appearance caused by thin micrite walls separating small, more or less rectangular, cells. The spar-filled holes within the skeleton, called conceptaclcs, arc also characteristic. To the left of the coralline alga can be seen part of a nummulitid foramini? fera, with its thick radial-fibrous wall. The blue-green algae occur typically as long narrow filaments and only a few species become calcified. Girvan? e/la, illustrated in 1 1 7 is widespread and occurs over a long stratigraphic range. It is made up of bundles of narrow tubes about a millimetre in diameter at this magnification, with a thin micrite wall. They can be seen in longitudinal section (e.g. upper part of the photograph) and transverse section (e.g. lower right). The remainder of the sediment compr ises a few bioclasts (e.g. an ostracod, lower left) and a mixture of carbonate mud sediment and sparitc cement, the latter being partly pink-stained, non? fcrroan calcite and partly bluish ferroan calcite.

115: Stained thin section, Upper Cretaceous, Tunsia; i magnification x 19, PPL. 116: Stained thin section, Eocene, Greece; magnific? ation x 23. PPL. 117: Stained thin section, Clzatburn Limestone, Lower Carbonif erous, Chatburn, Lancashire, England; magnific? ation x 37. PPL. Other algae are shown in 76, 128, 130 and 150.

61

Carbonate rocks

118, 119, 120

B i oc l asts
Calcispheres and Worm Tubes

Worm tubes

Although rarely abundant, calcareous worm tubes are widespread in shallow marine and freshwater limestones. 1 1 8 shows a bioclastic limestone with a large fragment comprising numerous worm tubes, seen in cross-section. Most of the tubes are filled with micrite. The associated fauna include a bryozoan (top centre}, molluscan casts (top r ight}, an echinoderm fragmen t (top right) and brachiopods (bottom). The sediment is cemented by blue. stained ferroan calcite. 1 1 9 shows sections through the coiled calcareous worm tube Spirorbi s. Sparry calcite c ement fills the chambers and the surrounding sediment is micrite with a few thin-valved ostracods (e.g. upper left).

Inf erior Oolite, Middle Jurassir. 1-ecklwmplon Hill, Glouceslershire, England; magnific· tlliOII X 13, PPL. 119: Uns1t1ined lhin section, Ardwick Limestone. Upper Carboni f erous, Manches1er, England; magnifica/ion x /7, PPL. Worm wbes are also shown in 214 and 215.
Calcispheres

118: Swined lhi n seclion,

)

Calcispheres arc small hollow spherical bodies of calcite, usually with a micritic wall. They are particularly com· mon in Upper Palaeozoic limestones and may be the calcified reproductive parts of dasycladaccan algae. 120 shows numerous calcispheres - the circular objects l\1th deep, red-brown-stained walls - associated with micrite· walled cndothyracid foraminifera and a foliated brachio? pod shell which extends right across the field of view.

120: Swined ace/ate peel, Woo Dale Limes/one, Loll'lt f erous, Long Dale, Derbyshire, England; magnific· Carboni ation x 2 1 , PPL.
52

111 I /IOf/

121, 122

Carbonate rock?

Non - s ke l et a l a l g a e
Stroma to I ites

Stromatolites arc laminated rocks interpreted as fos? ?ilized algal mats. The mats are formed of filamentous blue-green algae. The laminae in stromatolites arc usually alternation? of carbonate mud and grainy. often pclletcd, l>Cdimenl. The laminae are. at most. a few millimetres thick and arc often more easily seen in hand specimens than in thin sections. Laminoid fenestrae (p. 68) arc often associated with stromatolites. 121 is a photograph of a polished block showing a stromatolite. Note that the layering is irregular and partly picked out by colour differences. The irregularity of the layering helps to differentiate laminated sediments formed from algal mats from those formed by physical processes. The laminae in stromatolites may form flat or crinkly ?tructures or may build up into columns or domes. Concentric algal laminations about a nucleus give rise to the grains known as oncoids (p. 38). 122 shows a thin section of the same specimen as that illustrated in 121. The laminations consist of alternating thin micritic layers and layers containing a mixture of micrite and sparitc. In some areas th e micrite has a vaguely pcllctcd structure which is eh?tractcristic of stromatolites. The more irregular micrite <trcas may have been coating the algal filaments which then decayed, l eaving a mould which was later Ill led with spa rite cemen t.

121 and 122: Lower Carbonif erous, Carrihe de Ia Vallee
fleureu\'e, Boulonnais, France; 1 2 1 hand specimen, mag? nification x 1.8; lion x 12. PPL.

122 stained thin sec/ion,

magnifica?

53

Carbonate rocks

123, 124, 125

N o n - skeleta l a l gae
Micritization

In the shallow marine environment, some non-skeletal blue-green algae may bore into skeletal material. These are called endolithic algae. The borings, around I 0 J.lm in diameter, arc filled with micrite after the death of the algae. If the process continues, the margin of a shell fragment may become completely replaced by micrite. The process is known as micritization and the replaced shell margin as a micrite envelope. 123 shows micrite envelopes developed on brachiopod shells (the foliated structure) and echinoderm fragments (the speckled plates). Note the irregularity of the contact between the micrite envelope and the unaltered skeleton. This enables micrite envelopes formed by micr itization by itic coat? blue-green algae to be distinguished from micr ings around the exterior of skeletal fragments (77). Repeated micritization may lead to the production of a grain with no remaining recognizable structure. This would then be called a peloid (76). Skeletal algal frag? ments arc often susceptible to this total micritization and it is possible that some of the micritic grains in 123, with their irregular shape and trace of internal structure, were formed by this process. 124 shows the importance of micrite envelopes in preserving molluscan fragments during diagenesis. The original aragonite molluscan shell has been completely dissolved and the mould, outlined by a thin micrite envelope. was then filled by a sparry calcite cement. Although the sparitc is mainly blue-stained fcrroan calcite, there arc thin zones of pink-stained non-ferroan calcite. This is clearly seen in the shell fragment to the left below the centre. Allochems other than bioclasts may become micritized. 125 shows a number of grains with varying degrees of prc?crvation of radial and concentr ic ooid textures (p. 35). I t is possible that the texture was partially lost b) micritiLation, although it might also have been lost during inversion of an original aragonite ooid to calcite (neomor? phism. p. 60).
123: Unstained thin section, Woo Dale Limestone, Lower Carhon(( erous, Peak Forest. Derbyshire, England; magnif? ication x 25. PPL. 124: Stained thin section, Inf erior Oolite, Middle Jurassic. l,ecklwmpton Hill, G/oucestershire, England: magnific? ation x 12. PPL. 125: Stained thin section, Llandy f an Limestone, Lower Carhon({erous, Black Mountains. South Wales; magnific? tlfion x 43. PPL.
54

126, 127

Carbonate rocks

Carbonate cements

The morphology and mineralogy of the pore-filling cement crystals in a limestone can yield information about the environment of cementation. Cements pre? cipitated from marine pore-waters close to the sediment -water interface may be aragonite or high magnesium calcite, but in either case they may form small crystals with a high length-to-width ratio. The crystals are aligned at right angles to the surface on which the cement nucleates. On curved surfaces this means that many marine cements display a radialfihrous fabric. 126 shows a section through a coral skeleton (brownish-stained, structure not clearly visible) in which the first generation of cement is acicular aragonite show? ing a radial-fibrous texture. Note the variation in the length of the crystals which gives a very irregular outer margin to this generation of cement. Such a cement. being aragonite. is not likely to be well-preserved in an ancient limestone. If it undergoes neomorphism (p. 60), the overall radial-fibrous fabric may be retained although detatl will be lost. In the sample shown, there is a second generation of pink-stained fine sparite infilling pores. This is typical of cement deposited from meteoric waters. 127 shows a limestone in which there arc also two cement generations. The first appears as a rim of crystals of equal thickness on all grains (about 2 mm in width in the photograph). Such cements are said to be isopachous. The cement exhibits a radial-fibrous fabric although the length-to-width ratios of the crystals arc not as great as thosein 120. 11 may or iginally have been aragonite, details ofthe texture having been lost during inversion to calcite, or tt may have been a high magnesium calcite mar ine cement in which the crystals were elongate prisms rather than needles. The final pore fill is an equant sparite, blue? ?taincd and thus ferroan calcite. This latter cement is characteristic ofdeposition from meteoric waters or from connate waters fairly deep in the subsurface. In order to incorporate ferrous iron into the calcite lattice to produce a ferroan calcite. reducing conditions must exist. I f the pore-waters arc oxidizing. any ferrous iron present is rapidly oxidized to ferric iron and precipitated as iron h)droxide. Reducing conditions arc more ltkcly to occur at depth than near the surface. Other coarse ferroan calcttc cements are seen in 80, 87. 90 and 124.

tllioll x

116: Swined acetate peel. Quaternary. Momhasa. Kenya; lllllf(!lijicarion x 70. PPL. 127. Stained acerate peel, Ommama11e Formation. Middle Jurassic. Western High Atlas, Morocco: maKnific?
122, PPL.
55

Carbonate rocks

1 28, 129, 130

Ca rbon ate cements
(continued)

/\n early phase of cementation may occur in the vadose lone (above the water table), where pores in the sediment arc not completely water-filled. Water, and hence calcite cement, occur around grain contacts in the form of a meniscus. 128 shows a sediment largely made up of segments of the codiaccan alga Halimeda ( 1 12) . The rock is highly porous, and although impregnated (the brownish-grey background material is the impregnating resin), it has been difficult to take a peel and hence there are numerous air bubbles. The algal segments have been cemented by a small volume of pink-stained fine calcite sparite at grain contacts. This is characteristic of cementation from meteoric waters in the vadose zone. Note the meniscus clfcct leading to the rounding of pore spaces, well seen to the left of centre of the photograph. Another feature which can occur in the vadose :zone is a drip.Holle or microstalactitic cement. In this case water droplets and hence cements are concentrated on the undersurfaces of grains. 129 shows a sediment in which the first generation of cement occurs only on the lower surfaces of some grains. In the photograph it is very pale, brownish-coloured and never more than a millimetre thick. Vadose cements can form from marine pore waters in the intertidal and supratidal zones as well as from meteoric waters. In the former case the cement will have a radial-fibrous fabric. l n the example here, the cement is too fine-grained for its fabric to be resolved at the magnification shown. A later generation of coarse sparitc fills the pores. Cements, especially those deposited in a marine envir? onment, may be micrite. In ancient limestones where pore-spaces are completely filled, it is difficult to distingu? ish micrite cements, which have nucleated on grain surfaces and grown outwards to fill or partially fill pore? spaces, from carbonate mud sediment deposited with the grains. 130 illustrates a sediment comprising fragments of algae and micrite peloids having a matrix which is a mixture of micrite (greenish-brown) and sparite (colour? less). The micrite coats some grains and forms 'bridges' between adjacent grains and it may therefore be a cement. llowcver, it is possible that micrite sediment, deposited along with the grains, became partially lithified and was then subject to erosion by a through flow of water which removed unlithified mater ial.
128: Stained acetate peel, Quaternary, Mombasa, Kenya, magnification x 9, PPL. 129: Unstained thin section, Woo Dale Limestone, Loll'er Carboniferous, Long Dale, Derbyshire, England; magnific? ation x 22, PPL. 130: Unstained thin section, Coal Measures, Upper Carbo? niferous, Meta/lie Tileries, Chesterton, Staff ordshire, ?ngla11d; mag11ijication x 20, PPL.

131, 132, 133

Carbonate rocks

Carbo nate cements
(continued)

The characteristic texture of a cement precipitated i n the meteoric phreatic 7onc (below the water-table) is one in which the crystals increase in size from the margins of pores towards their centres. This is known as a drusy mosaic and results from competitive growth of crystals away from the substrate on which they nucleate. The resulting fabric is one of more or less equidimensional crystals, sometimes known as 'blocky' or ·equant' sparitc. 131 illustrates a drusy mosaic in which cement crystals show compositional zoning, the stain picking out changes in the amount of iron in the calcite brought about by changes in composition of the c irculating ground waters. As explained on page 55, f crroan calcite cements are precipitated under reducing conditions. The zoning mdicatcs the position of crystal faces during growth, and shows that the crystals were cuhcdral at the time, <tlthough growth to completely fill the pore spaces has led to the final crystal shapes being anhedral. Crystal boundaries formed by crystals growing together in this way arc known as compromise boundaries. Where the component grains of a limestone are com? posed of a few large crystals. it is often possible to see that cements 'lave been precipitated in optical cominuity with the grains on which they nucleate. These arc known <lS .syntaxial o1·ergroll'ths or sy11taxial rim cementv and arc most ea?ily seen on echinoderm fragments. 132 and 133 show a sediment in which the cement is composed entirely of syntaxial overgrowths on crinoid plates. The crinoids can be identified by their speckled appearance whereas the cement is clear. The syntaxial nature of the cement is shown in PPL by the cleavage passing through both bioclast and cement. In XPL. the uniform extinction colour of both crinoid and overgrowth can be seen. Fragments of fenestrate bryozoans are abundant in the sample.

131: Stai11ed acewte peel. Woo Dale Umesttme. Loll'er Carboniferous, Wol/.lcote /)a/e. Staffordshire. England; magnificatioll x 22. PPL. 132 and 133: Stained thin section. ?yam Limestone. Loll'er Carbonif emus, Once-a lt'l'ek Quarry. Derhphire, England; magnification x 17. 132 PPI ? . 133 XPL. 57

Carbonate rocks

134, 135, 136

C o m pact i o n

Apart from cementation, the major process leading to porosity reduction in sediments is compaction. Early stages of compaction in uncemented sediments involve the readjustment of loose grain fabrics to fit more tightly together, the fracture of delicate shells, the squashing of soft grains, and the dewatering of carbonate mud. 134 shows a peloidal limestone in which either the outer layers of the peloids, or a very thin early generation of cement, has flaked off during compaction. The micritic grains must have been rigid or compaction would have resulted in their deformation. Compaction was followed by the precipitation of a coarse sparitc cement which 'healed' the fractures caused by the flaking off of the rinds of the grains. 135 shows a cross-section of a gastropod preserved as a cast. The inner wall of the organism is marked by a micrite envelope and a thin generation of early cement (sec for example the chambers in the upper part of the photo? graph). The wall of the shell has been fractured and some fragments disoriented during compaction. Both micrite envelope and early cement arc fractured and the fractures then healed by a coarse sparite cement. Thus after deposition, the mollusc was micritized and then cemented by a thin early generation of fine carbonate. Then the aragonite wall was dissolved and fracturing occurred, before the rock was finally cemented. The sample also shows a vein running from top left to bottom right of photograph and brown-coloured replacement dolomite crystals are scattered throughout the sediment. 136 illustrates a highly compacted bioclastic sediment, consisting of complete two-valved ostracods as well as single ostracod valves and long, thin bivalve fragments. Most fragments are aligned parallel to the bedding but some still show folding and fractur ing (e.g. upper left). The complete ostracods have withstood considerable pressure but most eventually fractured.

134: Stained acetate peel, Red Hill Oolite. Lower Carb? onif erous. Cumbria, England; magnification x 31, PPL. 135: Unstained thin section. Woo Dale Limestone, Lower Carboni f erous. Derbyshire, England; magnification x 14, PPL. 136: Unstained thin section, Coal Measures. Upper Carb? oniferous, Cobridge Brickworks, Hanley, Staffordshire, Englcmd; magnification x 19, PPL. 58

137, 138, 139

Carbonate rocks

Pressu r e - s o l u t i o n and d eformat i o n

Pressure-solution is the process whereby a sediment. because it is under load. is subject to selective dissolution. In limestones it is normally calcium carbonate that is dissolved and any less soluble material such as clay and quartz is concentrated along seams. 137 illustrates a case of grain-to-grain pressure? solution. Before the pores of a rock arc filled by cement, ?tress is concentrated at the points where the grains meet and part of one or both the grains dissolves. In the example, ooids have undergone solution. The later ce? ment is a mauve-stained, slightly fcrroan sparite. Note the small rhombic areas of fine calcite spar (e.g. midway up, half-way between centre and left-hand edge). These are calcite pseudomorphs after dolomite (dedolomite, p. 74). 138 shows a limestone which has undergone pressure? solution to such an extent that most grain boundaries have been modified and the rock is pervaded by thin dark seams. Many of these have the fine saw-tooth appearance character istic of stylolites. This type of pervasive pressure-solution is known as sutured-seam solution. 139 shows a limestone which has been subjected to some stress. Speckled echinoderm plates arc recognizable, together with syntaxial overgrowths. Most of the calcite crystals are twinned, a feature which may develop as a rt)Uil of pressure. and the twin planes can be seen to be slightly bent.

/J7· Stained thin section, Upper Jurassic. Cap Rhir, \Iarocco: magnification x 52. PPL. 118 Stained acetate peel, Woo Dale Limestone, Loll'er (urboni f erolls, Long Dale, Derbyshire, Engl and: magniftc? urum x3/, PPL. 139. Stained thin section, Torquay Limestone, Devonian. llt1pe's Nose. Devon. Enf(land; magnification x 31, PPL.

59

Carbonate rocks

140, 1 4 1 , 142

N e o m o r p h ism
Microspar, Pseudospar

The term neomorphism refers to all transformations between a mineral and the same mineral, or another of the same general composition. During diagenesis, aragonite components of limestone may be transformed to calcite without the development of significant porosi ty. There is usually an ac companying increase in grain size (aggrading neomorphism). In pa rticular the micritic material of limestones may be altered to coarser calcite. The terms microspar and pseudospar are used for crystal mosaics of neomorphic origin having a mean size of between 4 and IOttm and > IOttm respectively. It is not always possible to ditfcrcntiatc between neomorphic fabrics and fine spar cements. or sediments composed of primary sih-Siled part1cle?. As a guide. neomorphic spar generally has irregular crystal boundar ies and patchy grain size distri? bution. often with relicts of micrite and floating skeletal gra ins 140 shows a limestone in which the matrix is fine pseudospar. It is cloudy and contrasts with the coarse clea r mosaic replacing the wall and in filling the chamber of the mollusc on the right of the photograph. Its grain si1c varies pa tchily and it is thus li kely to be neomorphic, ha v ing originally been a micritic sedimen t . 141 shows a very fine grained limestone ( note magn ific ation) composed almost entirely of calcium carbonate grains of m icrospar size. There appear to be no micritic relicts and this fabric may be a p rimary one as a result of deposition of carbonate mud of fine silt-sized particles, rather than a product of neomorphism of micrite. 142 illustrates a limestone with a few dolomite rhombs (dark-coloured) in a 'matrix' of pseudospar with patches of microspar and micrite. ote how crystal size and shape \ary irregularly throughout the mosaic. This is character? Istic of a neomorphic fabric.
' ' . ?

140: Swined thin section. Carboniferous Limestone. Llan? gollen, North Wales; magnification x 43. PPL. 141: Stained acetate peel. Blue Lias. Loll'er Jurassic. l.m·emo< 'k Point, South Wales: magnificmion x 72. PPL. 142: S/(/ined acetate peel, Woo Dale Limestone, Lower Carhoni f erous. Woo Dale. Derbyshire. England; magnific? otion x 43, PPL. A neomorphic f abric is also shown m 1 6 1 .

143, 144, 145

Carbonate rocks

Neomorphism
Bioclasts

Most aragonite bioclasts arc preserved as calcite casts with no trace of the ori gina l microstructure of the wall (sec p. 40). Occasionally however. aragonite bioclasts in1crt to calcite in situ. Th is is a form of neomorp h i sm . 143 illustrates parts of t h e shells of bivalves which have been subject to neomorphism. The shells consist of a blue? >tained fcrroan calcite -;parry mosaic. but there are lines or mclusions cutting across cry:.tal boundaries and in? ?\1:.?\\?\\\c ? original foliated structure of the shell. Many cn'\.1'' arc a\so brown-coloured because of their inclu? ''" content The sediment between the shells is muddy 1, , '17 /. JJn> itbund;wr qua rt7 (unstained). /J.I;md 145 show sections through a colonial scleract? mtan coral, origmally com posed of aragonite and now calctte. 144 shows the coral at low magnification. The coral walls and septa arc inclusion-rich. non-fcrroan calcite of which the detailed fabric is unclear. Pore space tS filled with an inclusion-free spa ri te cement, very p ale mauve-stained and hcn?e slightly ferroan. 145 shows the colony at higher mag ni ficati on in a secti o n which has been ground slightly thinner than is usual. The coral walls comprise an irregular mosaic of crystals of varying gra i n silCS and shapes which is neither the original micro? structure nor ad rusy mosaic, but a product of ncomorp h 1\m, Note that some crystal boundaries cut across from pore filling spa ri tc to coral septa and t hus some crystals arc partly neomo rphic and partly cement.
. -

/4.l Stamer/ 1hin .1ec1ion. Weald en, Lower Cretaceous, "'llfllt'm I:11Kimul: magnification x 2 1 . PPL. /U am/ 145: S/(/ined thin sec/ions, lgKui e/ Behar Form?
t/1/1111, (ppa Jura,\Sil'. lmou::er de.v ld a-ou- T anane. Wes?

ltlll 1/igh Alias, Morocco: magn!ficmion 144 x 12: 145 x4.1 PPL.

61


Carbonate rocks

--

Limestone Classification
Two of the most popular limcslonc classifications are those of Folk ( 1 959, 1962) and Dunham ( 1 962). These are summarized in Tables 3 and 4 and Fig. E.

Origin al components not organically bound together during deposition

Components organically bound d uring

Table 3.

Classification o f limestones according to Dunham (1962) Rock names are in capita/ letters

deposition
no carbonate

contains carbonate mud

mud

mud-supported
<

10% allochems > 10% allochems
MUDSTONE WACKESTONE

grain-supported

PACKSTONE

GRAINSTONE

DOUNDSTONE

allochem

volumetric
composition

> 10% allochems Sparry calcite > Micrite > Micrite Sparry calcite

<

10% allochems
<I% allochems

Table 4 "' ""

1-10% allochems

Classification o f limestones based on the scheme o fFolk ( 1959, 1962) Rock names are in capita/ letters

?
6 ...

> 25% Intraclasts

INTRASI1ARITE

INTRAMICRITE

Intraclasts INTRACLASTBEARING MICRITE

v ..<:1 0

:.0

>25% Ooids

OOSPARITE

OOMICRITE

!!! "' (j "' ...
"'

"' -o ·o " "'
·- "' o rl)

., ..c
3:1 BIOSPARITE
OIOMJCRJTE

"' E

· - v

Ooi ds

]
N

]. ?

"' v

?

0 8 ? :.0 ? .2 '
v ? ...

to

3: I
BIOPELSPARITE OIOPELMICRITE

-o c ::s .D
?

(;j c "' "'

_g

0010-

..c (.) ? ? o. _
? el !-;: :
"'
u ... c o .

"'

"' .....

-o Q

BEARING MICRITE

fBioclasts

? i Q. CI'I - "' - Q.

-e .:: .? "0

? -o <.>
c

1:3

?

?

FOSSILII'EROUS MICRITE Peloids PELOIDBEARING MICRITE

? ? ... 1w

;:l
!-

i

;;: :J ;;

0 >

§

v

!0

1:3 PELSPARITE

I'ELMICRITE

Over J mic rite matrix

()-1%
Allochems

Allochems

1-10%

Allochems

1()-50%

Over 50% Allochems

Subequal spar& micrite

Over i spar cement Sorting poor Sorting good Rounded & abraded

Micrite & dismicrite

Fossiliferous micrite

Sparse biomicrite

Packed

Poo rlywashed ' b1ospante

Unsorted b1ospante

Sorted b1ospante

Rounded b1ospante

-

Micrite matrix

B Sparry calcite cement

Fig. E

The range in textures shown by carbonate rocks, illustrated using the rock names o fthe Folk classification (after Folk, 1959)

62

146, 147

Carbonate rocks

Li mestone classi f i cation
(continued)

146 illustrates a grainstone. The rock is grain-supported with a spar cement. The sediment is loosely-packed, suggesting that cementation occurred before significant compaction. The allochems arc a mixture of ooids (some arc superficial ooids, see p. 35) and bioclasts. It is therefore an oosparite according to Folk. Since the allochems are rounded it would be a rounded oosparite, using Folk's textural spectrum. 147 shows a packstone. The rock shows two sizes of grains, having large and small peloids. The former have a trace of oolitic structure in places and may be micritized ooids (p. 54). The latter arc probably faecal pellets. The sediment contains some fcrroan calcite cement but also much carbonate mud sediment in the matrix. It is nevertheless grain-supported and thus a packstone. According to Folk's classification it is a poorly-washed oosparite.

146: Unstained thin section . .!uras.tic. unknown locality. Enl!,land; maf!.nification x 23. PPL. /117: Stained acetate peel, I n f erior Oolite, Midd/e Jurassic, Cooper's Hill. Gloucestersllire. Enl!,land; ma?:n[fic? ation x 13, PPL. Other grainstones are shown (F olk classification in

brackets) in 73 (oos parite). 14 (oosparite). 15 (sorted pels parite). 77 (unsorted intrasparite) . 87 (unsorted bio? sptlrite) and 124 (unsorted biosparite). Other packstones are shown in 72 (poorl y-washed oos? parite) . 79 (packed intramicrite) 96 (poorly-washed biosparite) anc/ 115 (packed biomicrite). 63

Carbonate rocks

148, 149, 150

L i m estone c l ass i f i cat i o n
(continued)

148 :.hows a wackestone. The grains are bioclasts. mainly echinoderm plates with some bryozoans (e.g. lower left part). These grains arc supported by a matrix of car? bonate mud in which many small particles are visible at this magnification. 149 shows a mudstone, being a matrix-supported limestone with less than 10% allochems. In this case the allochems arc microfossils foraminifera and calcite casts of radiolaria. The sediment is cut by thin vei ns of pale blue-stained ferroan calcite. This sample is a .fossilif erous micrite according to Folk's classification. A boundstone is a limestone in which sediment is bound together by organisms, such as occurs in many reef's. Textures are often more clearly visible at hand? specimen scale. ISO shows a thin section of a reef limestone comprising growths of a number of problema? tic organi sms (probably algae or foraminifera ) which have encrusted one another while incorporating fine? grained sediment into the rock framework.

148: Stained t1cetate peel, Wenlock Limestone, Silurian Shropshire, England; magn(fication x I I , PPL. 149: Stained thin section. Upper Crewceous, Pindos Zone, Greece; magnification x 43. PPL. 150: Stained thin section, Red Hill Oolite, Lower Carbon? if erous, Elliscales Quarry, Dalton-in-Furne.s·s, Cumbria, England; magnification x 12, PPL. Other wackestones are shown (Folk classification in brackets), in 105 (biomicrite) and 156 (biomicrite). 64

Carbonate rocks

Limestone Porosity Any description of a limestone should include an evaluation of the amount and type of porosity in the sediment. Porosity may be primary, having been present in the rock since deposi tion or secondary, having developed as a result of diagenesis. A classification of porosity types is shown in Fig. F. The terminol ogy of porosity types illustrated here with limestones, i s also a ppl icabl e to sandstones.
.

Fabric selective

Interparticle

lntraparticle

lntercrystal

Mouldic

Fen estral

S h elter

Growth framework

Not fabric selective

Fracture

Channel*

Vug?

Cavern*

?

Cavern applies to man-sized or larger pores of channel or vug shapes

Fabric selective or n ot

Breccia

Boring

Burrow

Shrinkage

Fig. F

fter Choquelte and Pray, 1970) Basic porosiry rypes in sediments. Pores shaded hlack (a

65

Carbonate rocks

151, 152, 153

Li mestone porosity
(continued)

151 and 152 show an oolitic/peloidal sediment in which much of the depositional space between grains is unfilled by sediment or cement. The rock is said to show primary intergranular porosity. When deposited. such a sediment may have had as much as 50% pore-space. This has been red uccd by compaction and by the introduction of some cement. Two types of cement are present - a fine spar. forming coatings on most grains (about l mm thick at this magnification and best seen in XPL) and syntaxial overgrowths on echinoderms (lower left). Although loca? liLed. the latter are volumetrically more significant. A common type of secondary porosity is mouldic porosity, usually formed by the dissolutiOn or aragomtc bioclasts. 153 shows a sediment having primary inter? granular and secondary mouldic porosity. Thin micrite envelopes have supported the shell moulds, although that supporting the fragment seen in the lower part of the photograph has partly collapsed. The bluish-grey interference colours seen in the inter? granular pores and the shell moulds of 152 and 153 are c.-used by strain in the mounting medium.

Jurassic. Dorser. England: magn{ficarion x 27, !51 PPL:
152 XPL. 153: Srained r!tin seuion, Porrland Stone. Upper Jurassic.

151 and 152: Stained r!tin si>crion. Porrland Sume. Upper

Dorser. England: magnificcuion x I I . XPL.
66

lS4, ISS, IS6

Carbonate rocks

Limestone porosity
(continued)

154 shows a limestone composed mainly of echinoderm fragments in a pink-stained, non-ferroan, calcite spar ite cement. However, a number of grains comprising a small e chinoderm fragment nucleus, surrounded by a zone of blue-stained f erroan calcite cement, are also present. This cement is interpreted as a late infilling of pore-space formed by the dissolution of an aragonite coating to the echinoderm fr agments. Such a coating is likely to have been oolitic and after solution the sediment would have exhibited oomouldic porosity. Porosity may develop as a result of the burrowing and bor ing activities of organisms. ISS Shows a section through a boring made by an organism in an oolitic sediment. Note that grains arc truncated around the margins of the boring, indicating that the sediment was lithified when the organism was at work and hence the structure is a boring rather than a burrow. The boring is infillcd with a f erroan calcite cement, some of which has been lost during the making of the section. Shelter porosity occurs below curved shell fragments which arc preserved in a convex-up position. 156 shows bivalve fragments in a carbonate mud sediment. Those preserved in a convex-up position. including the large f ragment extending right across the field of view, have areas of sparite cement below them which was pre? cipitated during the infilling of shelter cavities. Sediment was unable to fill the cavities because of the 'umbrella' effect of the shell.

154: Stained acetate peel, Oolite Group, Loll'er Carbo? f ? niferous, Daren Cilau. Lkmgaffock , South Wales; magn( ication x 15, PPL. 155: Stained thin seerion, Inf erior Oolite, Middle Jurassic. Cooper's Hill, Gloucestershire, England; magnific? lllion x 16, PPL. 156: Stained thin section, Lower Carboni f erous, Arbigland, Dumfries. Scotland; IIWf{nijicarion x 16, PPL. 67

Carbonate rocks

157, 158

L i m estone porosity
(continued)

Fenestrae is the name used for pores in a carbonate sediment which are larger than grain-supported spaces. They usually become infilled with internal sediment or cement, or a combination of the two. Fenestrae can be different shapes and sizes depending on their mode of origin. 157 shows spar-filled fenestrae in a micrite. Most are irregular in shape and probably formed as a result of the entrapment of fluid in a sediment during desiccation, although the elongated f enestra in the centre may have been a burrow. Fenestrae of this type are sometimes called birds-eye slmctures. The sediment contains a few micrite? walled foraminifera. Fenestral micrites were called dis? micrile by Folk (see Table 4). 158 shows fenestrae in a fine pellet grainstone. They show a tendency to be elongate parallel to the bedding. Fencstrae ofthis type arc known as laminoidf enestrae and may form from the decay of organic matter associated with algal stromatolites (p. 53).

157: Swined thin section , Lower Jurassic . Alias. Moroao: magn(/icalion x 14. PPL.
CarboniF erous.

Central High

158: Stained ace/ate peel, Woo Dale Limestone. Loll'er Derbyshire. England: magnification x 7. PPL
68

159, 160

Carbonate rocks

Li mestone porosity
(continued)

Pore-space in limestones may be filled with sediment as well as cement. Sediment partially infilling cavities, particularly in fossils or fenestrae, will indicate the horizontal plane at the time of its deposition. Such . sed imenl infills are known as geopetal infills. 159 shows geopctal sediment within a gastropod. On deposition the gastropod would have had a primary porosity within its chambers (intragranular porosity). This was partially infilled by micritic sediment and the cavity finally filled by f e rroan calcite cement. lnclusions within the shell wall of the gastropod and surrounding bioclasts suggest that they inverted to calcite during neomorphism (p. 61 ), rather than being cement-filled casts. Some pore-spaces have hydrocarbons within them or have evidence that hydrocarbons have passed through. 160 shows a limestone in which a few pores are filled with black hydrocarbon and others arc lined by a thin coaling ofit. Examination of its relationship to the cement shows that the hydrocarbon cntere,d the rock after an early generation ofisopachous cement (marine?) and before the final (illing of coarse blocky cement (meteoric).

160: Unsrained thin seuion, Bee Low Limestone. Lower Carbonif erous. Windy Knoll, Derbyshire. Enxland; mag? nification X 16, PPL.

159: Stained rltin section. Purbeck Marble. Upper Jurassic. Dorser, Enxland; magni ficarion x 12, PPL.

69

Carbonate rocks

Dolomitization
Introduction Dolomite. CaMg(C03)2, is a major component of limestones. It is usually secondary. replacing pre-existing carbonate minerals. Unlike calcite, it often occurs as euhedral rhomb-shapcd crystals. However. since its optical properties arc similar to those of calcite, it can be difficult to distinguish between the two. For this reason etching and staining of sections with Alinrin Red S is carried out (see p. 34). Dolomitic rocks arc classified according to their dolomite content as follows: 0 to 1 0 % dolomite
10 to 50% dolomite

50 to 90% dolomite 90 to 100% dolomite

limestone dolomitic limestone calcitic dolomite dolomite

Since the term dolomite is used for both the mineral and the rock, some workers prefer the term dolos/One for the rock. although the term has not been universally accepted and is not employed here.

70

161, 162

Carbonate rocks

D o l o m itization
{continued)

161 shows a dolomitic limestone containing 20 to 30% dolomite. The dolomite is unstained and occurs as cuhcdral rhomb-shapcd crystals which contain inclu? s ions. probably of calcite. and arc thus cloudy. The unaltered limestone surrounding the dolomite is pink? stained, non-ferroan calcite and shows a patchy texture of micrite and sparite with few recognizable grains. This is a neomorphic fabric (p. 60). 162 shows a calcitic dolomite in which the original calcite matrix has been wholly replaced by dolomite (unstained) but the micritic allochems (peloids) have resisted dolomitization and are only partly replaced (dolomite unstained, calcite red). Where replacement is incomplete, euhcdral rhomb-shaped crystals are visible. Where replacement is complete, crystals have grown together and the euhedral shape is lost.

161: Stained thin section, Woo Dale Limesrone, Lower Carboni f erous. Derbyshire, England; magnification x 20. PPL. 162: Stained thin section. Middle Jurassic. Jebel Amsiuen. Morocco; magnification x 14. PPL.
71

Carbonate rocks

163, 164, 165

Dolom itization
(continued)

163 shows a sediment in which the original limestone has been totally replaced by dolomite. The result is a mosaic of anhedral crystals. Although the section was immersed in the staining solution, no stain at all is apparent, indicating the completeness of the replacement. 164 shows a dolomite in which the crystals arc distinctly loned. Although the crystal f abric is tightly interlocking, the rhombic shape of the dolomite crystals is clearly outlined by the zoning. The zoning may be partly caused by chemical dirlcrcnccs in the dolomite but it is probably due mainly to varying amounts of foreign matter in? corporated in the growing crystals. The mineral dolomite may contain iron substituting for magnesium. When the iron content reaches 10 mole %. the term ankerite is used. 165 shows a ferroan dolomite approaching ankerite.in composition. The ferroan nature of the mineral is ?hown by the wrquoise stain colour (sec p. 34), although the section was stained for a longer time than usual to enhance the colour, which is why the calcite present is red rather than pink-stained. The iron content of the dolomite is also shown by the dark brown margins to some crystals, where iron has been oxidized, producing limonite.

163: Swined thin section, Pemnaen Burrows Limes/one, Lower Carhon(f erous. Caswell Bay. South Wales: magni f ? imtion x 43, PPL. 164: l.'nstained thin section, Woo Dale Dolomite, L01rer Carboni f erous. Woo Dale. Derbyshire. England: nwgnific? (l(ion x 56. PPL. 165: Stained thin section. Woo Dale Dolomite. Lower Carhon{/ erous. Woo Dale. Derbyshire. England; magnific? ation x J I . PPL.

72

166, 167, 168

Carbonate rocks

Dolomit izat i o n
(continued)

166 shows a loosely-interlocking network of cuhcdral dolomite crystals (unstained) with the intcrcrystal spaces infillcd by a coarse sparry calcite cement (pink-stained). Sometimes depositional textures arc preserved in a rock despite complete replacement of the original sedi? ment by dolomite. 167 shows a dolomite rock in which the matrix has been replaced by more finely crystalline dolomite than the allochems (perhaps originally ooids). The result is a 'ghost' texture. 168 shows a highly porous dolomite rock, some of the pores having been filled with a slightly f erroan calcite cement which is stained very pale mauve in thin section. but is too faint to reproduce well in the photograph. The dolomite is very fine-grained and the outlines of original allochems have been preserved as a ghost texture. Poros? Ity in sediment replaced by dolomite is known as inter? crptal porOSity (Fig. F. sec p. 65).

166: Stained thin section. Woo Dale Dolomite. Loll'er Carlumi/eroll.\, C11nning Dale. Derbrshire. En?land: mag? m fi<'lltton x 15. P PL. 167: Stained thin section. 011anamane Formation. Middle Jurassic. 011adim, Western High A tla? . Morocco: magn( ( ? ication x JJ. PPL. 168: Stained thin sec!ion. MuJ?nesian Limes/one, Permian. Smllh )'orkshire, England: magni fic(l/ioll x 38. PPL. 73

Carbonate rocks

169, 170

D e d o l o m itization

Dolomite may be replaced by calcite, usually by the action of oxidizing meteoric waters. This process of dedolomitiz? ation yields rhomb-shaped crystals of calcite or rhomb? shaped areas which comprise a mosaic of replacement calcite (dedolomite). 169 shows large rhomb-shaped areas which are now pink-stained calcite crystals. The morphology of these areas suggests that they were originally single dolomite crystals. Note that the 'dedolomite' is full of brown inclusions of iron oxides. This is a common feature since dedolomitization often occurs in oxidizing conditions, where any ferrous iron in the precursor dolomite is oxidiled to produce iron oxides rather than be in? corporated in the replacement calcite. A sparry calcite cement can be seen together with numerous hexagonal sections through an unstained mineral with low relief, which is authigenic quartz. Before dedolomitization this sediment would have been similar to the calcitic dolomite shown in 166. 170 illustrates a dedolomite in which the former dolomite crystals have been replaced by a mosaic of small calcite crystals. Micritic calcite sediment occurs between the rhomb-shaped areas.

169: Stained thin section. Woo Dale Limestone. Lower Carboni/'erous, Cunninl{ Dale, Derbyshire. En[{land: IIU/1( n(/ication x 27. PPL. 170: Swined thin section. Upper Jurassic. Jebel Amsitten. Morocco: lllllf{llijication x 42. PPL. Dedolomite can also be seen in 137.
74

Other sedimentary rocks

I ntroduction

In this section we include phoLOgraphs of thin sections of i ronstones evaporites cherts, phosphorites and carbonaceous rocks. Even taken together, these rock types form a very small proportion of the total sedimentary record. However these groups of rocks have always attracted petrographic Study out of proportion to their abundance, partly because they include rocks of great economic importance and partly because they show unusual f eatures which have attracted considerable interest.
,

,

77

Other sedimentary rocks : Ironstones

171, 172

I r o nstones

Scdimc111ary rocks with more than I S % iron, which have usually been worked as orcs, arc known as irons/ones. Phanerozoic ironstones arc usually local accumulations of fossiliferous oolitic deposits and are called ooli1ic irons/ones, whereas Precambrian ironstones are much more extensive in area and comprise bedded <liternations of iron minerals and silica. The latter arc known as banded

iron f ormalions. 171 and 172 show a Jurassic oolitic ironstone. The olive?

green mineral making up most of the ooids and the initial cementing material is clwmosile. Chamosite is an iron slltcatc with a structure similar to that of chlorite. It occur? as fine-grained aggregates and, as the lower photograph shows, it has a birefringence of nearly zero. The brown-stained areas within the grains having high btrcfringcnce arc siderile. iron carbonate, whereas the mtcrgranular cement is calcite.

171 and 172· Nonlwmplon Sand Ironstone. Middh? Jurassic. i\'onhampton.111ire. Englallll: magni f ication x 37: 171 PPL. 172 XPL.

78

173, 174

Other sedimentary rocks : Ironstones

I r o n stones
(continued)

Chamosite readily oxidizes to limonite, sometimes on the sea-floor soon after deposition, but more commonly, much later after uplift and weathering. 173 shows an ironstone containing both partially-altered chamosite ooids, which arc yellow to golden-brown in colour with darker limonite zones, and totally-replaced grains of pure limonite which are almost opaque. The cement is calcite. The sediment is fairly soft and some grains have been plucked out during the making of the section, leaving holes which appear colourless in the photograph. 174 illustrates an ironstone in which ellipsoidal grains have been completely replaced by opaque limonite. The sample contains a few rounded shell fragments (e.g. left? hand side above centre) and scattered quartz grains, which although colourless and showing low relief, arc sometimes surrounded by thin rims of dark limonite (examples can be seen near the centre). The cement is calcite.

173: Northampton Sand Ironstone. Middle Jurassic. Northwnpton1·hire. England: lllagni fication x 37. PPI.. 174: Frodinglw111 lronswne. Lower Jurassic. Scunthorpe. England; nwgn!frcation x 35, PPL.

79

Other sedimentary rocks : Ironstones

175, 176, 177

I ronstones
(continued)

175 ill ustrates the abundant marine fauna found in many oolitic ironstones. The speckled fragments arc echinod? erm plates (a large example can be seen just below the centre). The concentric structure of the ooids is picked out by alternations of green chamosite and opaque iron oxides. The matrix contams iron oxides and small quartz grains (colourless, low relief). Ooids in Phanerozoic ironstones are often squashed during compaction, suggesting that they were soft for some time after deposition. This contrasts with calca? reous ooids which are rigid grains and thus retain their shape. 176 and 177 show chamosite ooids which have been squashed and hooked as a result of compaction. Such grains arc known as spastoliths. The very low birefringence of the chamosite, with dark grey inter? ference colours, is well seen in XPL. The matrix between the ooids contains dark, greenish-brown coloured chamosite mud and small roughly equidimensional crystals with high birefringence. In some places these can be seen to be rhombic in cross-section and are siderite (iron carbonate). Larger brown-stained siderite crystals can be seen replacing the margins of some of the ooids.

175: Loll'er Jurassic. Skye. Scotland; magnification x 16, PPI.. 176 and 177: Raasay Ironstone, Lower Jurassic, Raasay. Scotlmul; magn{{ication x 43; 176 PPL, 177 XPL.
80

178, 179, 180

Other sedimentary rocks : Ironstones

I ronstones
(continued)

178 shows a limestone in which bioclasts have become impregnated by opaque iron oxide. The iron oxide has infilled the pores in crinoid fragments and partly replaced their skeletons, leading to the development of a distinctive reticulate structure. Bryozoans also have been impreg? nated by iron oxide and two examples can be seen just above and left of the centre of the photograph. 179 shows a thin section of a Precambrian banded ironstone comprising alternations of dark, iron oxide-rich layers and colourless chert layers. 180 is a higher magnif? ication view ofpart oft he same section taken with crossed polars and showing the fine-grained quartz which makes up the chert layers.

178: Rhiu·bina Iron Ore. Lou'l!l' Carbon(f'erous. South Wales: magnification x 20. PPL. 179 and 180: Precambrian. Trai/SI'aal: 179. magn(fic? arion x 9. PPL: IBO. ma!tni fication x 32. XPL. 81

Other sedimentary rocks : Cherts

181, 182

C h e rts

Cherts arc rocks composed of authigenic silica usually in the form of fine-grained quartz. Cherts may be primary, in wh ich case most ofthe silica i s in the form of hard parts of siliceous organisms such as radiolaria, diatoms and some sponges. Much chert, however, is secondary, usu? ally replacing limestone. Scattered grains of authigenic replacement quartt, often hexagonal in cross-section, arc not uncommon in limestones and are illustrated in 73 and
169.

Radiolaria arc siliceous microfossils which accumulate in sediments of the deep ocean floor. 181 and 182 show a radiolarian chert with spherical radiolaria skeletons and their long thin spines. The matrix contains fine-grained iron oxide. hence the red-brown colour. Radiolaria arc origina lly opaline silica. an isotropic form containing. water. This has been converted to fine-quartz (micro? quart/) showing low first order interference colours i n
-

XPL.

181 o/1(/ 182: 1-rllt'er Cretaceous. Greece: ation x 32: 181 PPL. 182 XPL. Calcite w.1ts l?/' radiolaria are shown in 89. 82

nwgnific·

183, 184

Other sedimentary rocks : Cherts

C h e rts
(continued)

183 and 184 illustrate features found in limestones, ironstones and terrigenous rocks as well as in cherts. The sediment contains terrigenous quartz grains, both mono? crystalline and polycrystallinc (p. 5), which arc clear in PPL. It contains bioclasts, including an cndopunctatc brachiopod shell (upper left) and echinoderm plates impregnated with iron oxide (e.g. upper right). These are composed of calcite and have been stained pale pink by the use of Alizarin Red S (sec p. 34). The sediment also contains silicified grains which arc unstained and brown? ish in colour. These include both structurelcss examples, which arc seen to be made up of microquartz in varying orientations (e.g. below the centre), and those with a nucleus of terrigenous quartz and an outer zone of silica with an oolitic structure (e.g. left of centre, ncar the top). These arc interpreted as silicified ooids. Evidence from the XPL photograph suggests that in some of these at least. the silica replacing the ooid cortex has grown syntaxially with the terrigcnO\JS quartz nucleus (shown by uniform mtcrference colour) e.g. the grain just below half-way up, right of centre.

183 and 184: Stained tltin section. YPL.

Carbonijc'/'(1/1.1, Middle Atlas, Central Morocco: magn{frcation x 17: 183 PPL. 184

83

Other sedimentary rocks : Cherts

185, 186

C h e rts
(continued)

185 and 186 show a silicified limestone in which silicific? ation is not quite complete. The brownish grains seen in PPL are unaltered calcite, as indicated by their high birefringence when viewed with crossed polars. Those grains which are clear in PPL show first-order inter? ference colours when the polars arc crossed, and have been entirely replaced by quartz. Although the limestone has undergone considerable alteration, the ghost texture visible shows that the original sediment comprised small rounded grains (peloids) and a few shell fragments.

/85 and 186: AJ(e and locality unknown; maf(nific? ation x 22: 185 PPL. 186 XPL. 84

187, 188

Other sedimentary rocks : Cherts

Cherts
(continued)

187 and 188 show a variety of quartz types. The circular to elliptical areas of fine quartz (microquartl) may be the original sediment grains replaced by silica. The surround? ing areas consist of clear and cloudy Lones of radial? fibrous quartz. known as cha/cedonic quartz. The final generation of brown silica illustrates most clearly the radial-fibrous structure. Chalcedonic quartz is often a pore-fill rather than a replacement. This is supported from the evidence of the sample illustrated. in that there are straight boundaries between adjacent growth of chalcedonic quart.? and triple points where three growths meet. These polygonal boundaries arc characteristic of radial-fibrous, pore-filling cements. The upper right ofthe photographs shows coarse equant quartz (macroquartz) which contains inclusions of highly birefringent car? bonate. indicating that the silica has probably replaced limestone.

187 and 188:

Upper Jurassic, Dorset. En?:land; magn({ic? mion x 43; 187 PPL, 188 XPL.
85

Other sedimentary rocks : Evaporites

189, 190, 191

Eva p o r i tes

Evaporites are rocks composed of minerals which pre? cipitate from natural waters concentrated by evapor? auon. Although only a few minerals are abundant in marine evaporite deposits, complex textures may develop as a result of the replacement of one mineral by another during diagenesis. On evaporation of seawater, the first minerals to precipitate after carbonate arc the calcium sulphates. The hydrous form, gypsum, (CaS04.2H20) occur? only ncar the earth's surface, whereas anhydri1e (CaS04) is formed at the surface and also replaces gypsum at depth. 189 and 190 shO\\ laths of gypsum partially filling a ca\ it) in a dolomite rock. The dolomite shows the very high relief and strong birefringence of a carbonate, whereas the gypsum shows low relief and \\cak birefrin? gence. The photograph taken with crossed polars shows typical gypsum interference colours, up to first-order pale grey. 191 and 192 show a thin section through a sediment composed almost entirely of anhydrite. It can be distingu? ished from gypsum by its higher relief and stronger birefringence. In the example shown, the anhydrite is Ill the form of laths with a radiating habit. The view taken with crossed polars shows the bright second-order inter? ference colours characteristic of anhydrite. 193 and 194 show a sediment composed ofgypsum and dolomite. The dolomite is very fine-grained and almost opaque in the photograph. The gypsum is in t\\0 fonns. At the ba?e and top ofthe photograph it is in the form of a network of irregular crystals, whereas in the centre It is 111 ight angles to the bedding. the form of fibres aligned at r The former t)pC is characteristic of gypsum replacing anh)dritc. \\hcrcas the fibrous gypsum is filling a vein n1nning parallel to bcddmg.

86

192, 193, 194

Other sedimentary rocks : Evaporites

Eva p o r i tes
(continued)

.

g n

189 (IIU/ 190: Carhoni/erou.\ J.illll'\lmre. T a/ Ts Well. South Wale.1: magn(frmtion x lO: 189 XPL. /90 XPL. /9/ and 192: Permian. Bi//inglwnr, Teevside. f:"ngland: magni fication x 16. 191 PPI . . /92 XPL.

magnification x 9: 193 PPL. 194 XPL.

I 93 and 194: Permian, Bi/linj?lwm, Tee.uide. /;'nJ?Iand:

87

Other sedimentary rocks : Evaporites

195, 196

Eva p o r i tes
(continued)

Gypsum may replace anhydrite on uplift of evaporite sequences and when removal of the overburden brings them ncar the surface. Textures arc often of the type hown in 193 and 194, with small irregular gypsum crystals. but sometimes large cuhcdral crystals form. 195 and 196 ?how gypsum porphyroblasts replacing fine? grained (aphanitic) anhydrite. Note the six-sided gypsum crystals with low relief and first-order interference colours. contrasting with the anhydrite showing moderate relief and bright second-order interference colours. Note that the distribution of relict anhydrite inclusions within the gypsum porphyroblasts has some? times led to the dcvclonment of a texture similar to "hour? glass· LOning, a f eature found in some minerals in igneous and metamorphic rocks.

195 ond 196: Pl'rmitm. South Durham. Enr,/wu/: llttlf!11( fic? mion x 8: 195 PPI? . /96 XPL. 88

-

197, 198, 199

Other sedimentary rocks : Evaporites

Evaporites
(continued)

The two most common chloride minerals in evaporite sequences arc halite (NaCI) and sylvite (KCl). 197 shows these minerals together. The refractive index of halite is close to that of the mounting medium, so that it shows very low relief, whereas the sylvite has a moderate negative relief. Some of the sylvite is reddish-brown in colour owing to the presence of a small amount of hematite. although the crystal in the lower right-hand corner is hematite-free. The perfect { I 00} cleavage of both minerals is visible in some crystals and the halite shows some evidence of zoning. Both halite and sylvite arc cubic and thus isotropic. 198 and 199 show a layered anhydrite-halite rock. The thin layers of fine-grained anhydrite show moderate relief in PPL and bright second-order interference colours with polars crossed. The halite has low relief and is isotropic. The halite layers also contain scattered rectangular anhydrite crystals.

197: Permian. Fison 's Borehole. Robin llood's Bay. North Yorkshire. England: magnif ication x 20. PPL. 198 and /99: Permian. Fordon No. I Borelwle. Scar? borough, Nonh Yorkshire. England: magnif ication x 9: 198 PPL. 199 XPL 89

Other sedimentary rocks : Evaporites

200, 201

Evaporites
(continued)

200 and 201 show an evaporite in which the minerals now present arc halite (low relief. isotropic) and anhydrite (moderate to high relief, second-order interference colours). There is also some carbonate between the small anhydrite crystals which is too fine-grained to be resolved at the magnification shO\\ n here. The irregular six-sided shapes which arc now composed principally of halite and scattered anhydrite laths. have the form of gypsum porphyroblasts (sec 195 and 196). They are thus interpre? ted as gypsum crystals which have been replaced. The gypsum itself was probably replacing anhydrite; this Illustrates the complexity of diagenetic reactions which may occur in evaporites.

20() and 20 / · Permian. IIm,·,/.,er Borehole. near ll'ltith r. \ ortlt rork.\1111'(', England; magni{lcation X 8: 200 pPl.. 201 XPL. 90

202, 203, 204

Other sedimentary rocks : Evaporites

Eva porites
(continued)

Polyhalite, K2MgCaz(S04)4.2H20, is a common mineral

in some marine evaporite sequences. 202 shows a rock composed essentially of polyhalite and halite. Both minerals have a similar relief and crystals arc not easily distinguishable in PPL. Thus only a view taken with crossed polars is shown. The halite is isotropic and so appears black. The polyhalite is partly fine-grained and partly coarse-grained. The larger crystals show simple twinning. Polyhalitc has a fairly low birefringence, and interference colours up to low second-order can be seen. 203 and 204 show a rock which is predominantly a fine? grained polyhalite. Two large porphyroblasts of anhy? drite with a typical lath shape can also be seen. The distinct difference in relief of the two anhydrite crystals results from their different orientations with respect to the polarizer. The anhydrite crystal ncar the lower edge shows the rectangular cleavage. The anhydrite is being replaced by the polyhalite. The dark spots seen in 203 are granules of bituminous carbonate.

202: Permian, Fort/on No. 1 Borehole, Scarborough. North Yorkshire, England: magnification x 16, XPL. 203 and 204: Permian, Aislaby Borehole. near Whitby . North Yorksh ire . England: magnification x 20; 203 PPL, 204 XPL.

91

Other sedimentary rocks : Evaporites

205,206, 207

Evapo rites
(continued)

205, 206 and 207 show a rock composed of anhydrite and long narrow crystals of a carbonate mineral. The anhy? drite occurs as separate rectangular crystals and as sheaves of sub-parallel laths. One of these sheaves, having first-order grey i nterference colours, can be seen towards the lower, right-hand corner of the photograph. Most of the anhydrite crystals show two cleavages at 90?, and bright second- and third-order interference colours. The two photographs taken in PPL show the marked change in relief of the carbonate mineral, achieved by rotating the polarizer through 90?. 207, taken under crossed polars, shows that this mineral has very high order interference colours. I t has been ident ified as magnesite, MgC03.

205, 206 and 207: Permian, Aislaby Borehole, near Whitby, North Yorkshire, England,· magnification x 20,· 205 and 206 PPL, 207 XPL.

92

208,209

Other sedimentary rocks

:

Evaporites

Eva porites
(continued)

Carnallite (KCI.MgCI2.61120) is one of the most soluble
of evaporite minerals and thus preparation of thin

sections containing it is difficult. 208 and 209 how a thin section of an evaporite rock which is slightly thicker than the usual 30 Jlm. The carnallite is showing bright inter? f erence colours. Crystals in the centre and in the lower part oft he field of view show the multiple twinning which is a characteristic of this mineral. The isotropic mineral is halite and the sediment also contains small rectangular cr)stals of anhydrite showing high birefringence.

208

and 209: Permian, Fison '.1 Borehole, Robin 1/ood's Bay. near Whitby. North Yorkshire: ma?Jni /lcllfion x 19:
.

208 PPL. 209 XPL.

93

210, 211, 212

P h osphatic sed i m ents

Some marine sediments contain authigenic phosphate, usually in the form of a cryptocrystalline carbonate hydroxyl fluorapatite, known as collophane. It commonly occurs as ooids and pellets, or as biogenic material such as fish teeth and scales or bone fragments. Sedimentary rocks rich in phosphate are called phosphorites. 210 and 2 1 1 show a phosphorite containing small greyish-brown pellets of collophane set in a coarse calcite cement. The view taken with polars crossed shows the isotropic nature of collophane and the high-order inter? ference colours of the calcite. Visible in the lower left? hand quadrant is a grain of secondary quartz showing first-order grey interference colours. 212 and 213 illustrate a phosphorite in which the grains are principally brown-coloured pellets of isotropic collo? phane. The colourless fragments, some ofwhich show very weak birefringence, are also phosphate. Some show a trace of internal structure (e.g. the grain to the right and below the centre). These are probably fish teeth and bone fragments. In contrast to the phosphorite shown in 210 and 2 1 1 where the cement is calcite, the cement here is fine-grained quartz. 214 and 2 1 5 show a limestone which has been stained with Alizarin Red S and potassium ferricyanide (see p. 34). The fragments are mainly oysters (pink-stained) and sections of hollow calcareous worm tubes (mauve) set in a ferroan calcite cement (blue). The sediment contains rounded pebbles of brown-coloured collophane (iso? tropic), containing small quartz grains and scattered pellets of green-coloured glauconite (see p. 17). The sediment also contains a few large, rounded quartz grains (e.g. the grain in the upper right corner).

94

213, 214, 215

Other sedimentary rocks : Phosphatic sediments

Phosphatic sed i m ents
(continued)

210 and 211: Carboniferous. Road f ord. Co. Clare, Repub? lic o f Ireland; ma?Jnification x 23; 210 PPL. 2 1 1 XPL. 212 and 213: Duwi Phosphare Formarion. Eocene. Red Sea Coasr. E gypt: magnification x 10: 212 PPL. 213 XPL. 214 and 2 15: Tour de Croi Nodule Bed, Upper Jurassic. Wim('reux. France; magn( fication x 1 1 : 114 PPL. 115 XPL. 95

Other sedimentary rocks : Carbonaceous rocks

216,217

C o a l s a n d coal b a l l s

Coals arc usually examined microscopically under high power 111 reflected light using oil-immersion objectives and therefore their detailed petrology is beyond the scope of this hook. 216 is a thin section of a coal viewed in transmitted light. The photograph shows durain. the dull material in coal, composed of the more resistant plant matter. The br ight yellow fragments are spore-cases distorted during compaction of the coal. Coal balls are carbonate concretions formed before compaction of the plant material in coal. They may be up to a few tens of centimetres across and 217 shows a thin section of part of one. The photograph shows that they contain well-preserved cellular tissue of plants.

216: Coal Measures. Upper Carbonif erous. England; magni/lwtion x 14. PPL. 217: Coal Measures. Upper Carbonif erous. Lancashire. Enxland: IIWKII{/iwtion 96
x

9. PPL.

Appendix 1 Preparation of a th in section of a rock

It is sometimes believed that complex and expensive equipment is required for making thin sections of rock of standard thickness of 0.03 mm. but as the following instructions indicate. this is not the case. Thin sections can be made by the amateur with a little patience and perseverance. If a diamond saw is available to cut a slab of rock I 2 mm in thickness. the process is considerably speeded up. However, a chip of rock not more than 8 10 mm in thickness can usually be broken from a hand specimen with a small hammer and then a thin section can be made. The operations requ ired to prepare a thin section after obtain ing the fragment of rock arc set out below. Using I 00 micron particle size ( 120 grade) carborundum abrasive. one surface of the rock fragment is ground flat on a piece of glass measuring about 30cm x 30cm and up to I em in thickness; ordinary window glass is satisfactory if thicker glass is not available. Only a small amount of carborundum (half a teaspoonful). just moistened with water. is used for grinding. If too much water is present the carborundum tends to extrude from underneath the rock. and in con?cquence is much less etr ective for grinding. After gri ndi n g with a rotary movement for about half a minute, the noise of the grinding changes because the carborundum grains lose their sharp cutting edges. The glass plate is washed clean and a fresh slu rry of carborundum made on the plate. The time spent on grinding a f lat surface will of course depend on how irregular the surface of the rock chip was to begin with. When the surface of the rock is flat. the sample should be thoroughly cleaned w1th a jet of water before gnnding with a finer grade of carborundum. The second stage of grinding should be carried out with 60 m1cron site (220 grade) carborundum and two periods of grinding. for about a minute each. with a fresh quantity of carborundum is all that i? required at this stage. After washi ng. a final grinding of one surface is made for about a minute with 1 2 micron size carborundum ( 3 F grade) . Again, after cleaning. the rock sample may be polished using cerium oxide (O.H mic ron size) but this is not essential. The next stage is to glue the smooth surface of the rock to a m icroscope slide in one of two ways. It can be achieved by using a cold? setting epoxy resin. which usually consists of two fluids which must be thoroughly mixed. The maker's instructions for using these should be f ollowed carefully because these matcriab should not be allowed to come in contact \\ith the skin and the vapour ?hould not be inhaled. The refractive mdiccs of epoxy resin\ vary but most arc somewhat higher than the value of 1 .54. For any work involving comparison of the refractive index of mincrab with the mou nt ing material, the refractive index of the cold resin should be ascertained. The chief disadvantage of usi ng an epoxy resin is that it is very difllcult to remove. if, for exa mple. it became necessary to transfer the rock chip to

another glass slide. The alternative method is to use a material known a? Lake ide 70C cement. 1 which is supplied in short rods and must be melted on a hotplate. This material begins to soften about 85 ·c. so a hotplate which reaches 10o·c is quite suitable. A flat piece of aluminium or steel placed on a gas stove or on the clement of an electric cooker at very low heat can be used for this stage. if !)O electric hotplate is avai la ble. A glass microscope slide and the rock specimen should both be heated on the hotplate until they are just too hot to touch, then some La kcside cement is melted on the flat surfaces of the rock and the slide by touching them with the rod of Lakeside cement. Whether the cold-setting epoxy resin or the Lakeside cement is used, the procedure is the same at this stage in that thc flat surface oft he rock chip must be attached to the glass slide with no air bubbles between the two surfaces. The rock chip is placed on the glass slide and, with a ?light pressure and circular movement. the excess mounting material and air bubbles are squceLcd out. The slide is then turned over to observe whether any air bubbles have been trapped between the rock and the slide: any bubbles must be gently extruded by pressure and. in the case of t he Lakeside cement, this has to be done before the cement cools and becomes too viscous for the bubbles to escape easily. However. it can be reheated to render it fluid again. With the epoxy resin. since the hardening ta kes place over a period which depends on the variety, more t ime is av<Ji la ble for extru di n g the air bubbles. but in this case the sample should not be heated because this only speeds up the hardening process. If a diamond saw is available the rock fragment can now be cut from II!> original thickness of 5 1 0 mm to about I mm. otherwise it must be ground by hand. Its thickness :.hould be reduced to about 0.2 mm (200 micron:.) using 100 micron site carborundum: at this thickness it IS pos!>iblc to see through the transparent minerals. Carborundum of 60 micro n size should be used to reduce the thickness from 0.2 mm to 0. I mm and at this stage qua rtt and f eldspars should show bright second-order interference colours when examined under crossed The final stage of grin din g from 0.1 m m to 0.03 mm is accomplished usi n g 1 2 micron size carborundum. This is the stage in the whole process of section making which requires the most skill. The grinding ha? to be done very carefully to ensure that the section is o f uniform thickness over its "hole area. otherwise the edges tend tO be ground
polars.

preferentially and become too thin. The slide must be examined between each stage of grinding to ched.. on the uniform reduction of the interference colours.
a/111111//rl ,ed !11
1 l.akeside cellll'lil i.1 llw pmprit?1ary 11111/lt'.for a malerialmanufaclured in /he USA

f<olf(l. Flee/. Hampshire.

llw Uni1ed l\i11Kd11111 h.r f'roduC'IitJII Tech11iques Lui. . I I

'lm?i.llock

97

In the making of thin sections. it is generally assumed that the rock will contain some quartz or feldspar. These show first-order grey and white interference colours in a thin section of standard thickness and neither should show a first-order yellow or red colour. Thus a thin section in which quartz or feldspar shows colours in Newton's scale higher than first-order white is too thick. In making thin sections of limestones or evaporites where quartz and feldspar are absent, it is very difficult to estimate thickness: only an experienced thin section maker can do so accurately. With limestones where the minerals show high? order interference colours, the section should be ground until sparitc crystals and the internal structures of shell fragments arc clear. Micrite will remain difficult to resolve even at high power. It is usual to cover the section. either by painting the surface with a transparent cellulose lacquer or better still with a glass cover slip as lacquer tends to scratch easily. This is done traditionally using Canada balsam diluted in xylene. but t he process of heating the mixture at the correct temrcraturc for the correct time requires some experience. We have found that it is quite satisfactory to fix the cover glass by either using the same epoxy resin which was used to attach the rock to the microscope slide. or by using a clear lacquer painted or sprayed onto the surface of the rock. As in the process of fixing the rock to the microscope slide. care must be taken to ensure that no air or gas bubbles arc trapped between the cover glass and the rock. This is particularly impo rtant if the material has been applied by a spray. because some of the propellant may be dissolved in the clear lacquer Any bubbles which arc visible in the liquid after spraying should be allowed to burst before applying the cover slip. Only sufficient lacquer or Canada balsam to cover the slide with a thin layer of liquid should be applied. The cover slip should touch the liquid on the slide at one end and be allowed to f all slowly onto the liquid. If any air bubbles are visible they can be extruded by gentle pressure on the cover glass. The excess lacquer or epoxy resm must be extruded to render it as thin as possible othcn\ isc the minerals cannot be brought into focus with a high-power lens because of the short working distance of lenses of magnification more than x 40. Finally when the mounting material has set hard, the excess can be scraped from round the edges of the cover glass with a razor blade or sharp knife.
. , . .

98

Appendix 2 Staining a thin section of a limestone

The procedure detailed below, adapted from Dickson ( 1965), has been found generally satisfactory and has been used in preparation of most of the stained sections shown in this book. Two stains are required AliLarin Red S and potassium ferricyanide.

I. Prepare a thin section of the rocks as described i n Appendix I but
omitting the coverslip. Ensure that no dirt or grease adheres to the surface. Prepare two staining solutions: Solution A: Alizarin Red S concentration of 0.2 gj I00 ml of 1 . 5 % hydrochloric acid ( 1 5 ml pure acid made up to I litre with water). Solution 8: Potassium ferricyanide concentration 2 g/ I 00 rnl of 1.5% hydrochloric acid. Mix solutions A and B i n the proportion 3 parts by volume of A 10 2 parts of B. Immerse the thin section in the mixture of solutions for 30-45 seconds, agitating gently for at least part of the time to remove gas bubbles from the surface. Wash the stained section in running water for a few seconds. Allow to dry. Cover with polyurethane varnish or a coverslip in the normal way.

2.

3.
4.

5. 6.

7.

Note: The solution of Alizarin Red S in acid may be made up beforehand and will keep. but the potassi um lerricyanide must be made fresh each time. A large number of sections can be stained with 250 rnl of stain solution.

99

Appendix 3 Preparation of a stained acetate peel of a limestone

The following procedure ha? been found to work well with most lithified limestones of low porosity and has been used to make most of the peels shown i n this book. Porous li mestones should first be impregnated with resin, otherwise evaporation of the acetone will draw up W<ttcr onto the stained surface after step 6 (below).
1 . Prepare a slab of rock, grinding f1at the surface to be peeled . The final gri nd ing should be made using 3F grade carborund um powder . 2. Prepare sta111 solutions A and B in the concentrations described in Appendix 2. 3. Mix solutions in the proportion A: B. 3:2. and pour into a shallow contai ne r large enough to allow the whole o f the ground surface to be i n contact with the solution. 4. A fter ensuring that the surface to be peeled is free from dirt or grease, immerse the rock slab in the stain solution so that the surface to be peeled is completely covered by solution. This is done best by holding the specimen with the ground surface downwards, either by hand or in a clamp and retort stand . otherwise the solution will be wasted on the unprepared surfaces. The specimen should he immersed in the solution for 90 seconds. Agitate the solution occasionally to remove gas bubbles from the undersur face of the slab. 5. Rinse the stained surface with water and leave for a few minutes for excess water to evaporate . 6. Flood the surface with acetone allowing it to run oiT, taking the excess stain with it. 7. Cut out a piece of thin acetate sheet (0.003 inch thickness is suitable) slightly larger than the sample. 8. Arrange the rock sample with its stained surface uppermost and hori70ntal. taking care not to tOuch the surface. 9. Flood the surface wi th acetone. 10. Lower the acetate sheet onto the ?urfacc gently. taking care to expel any air bubbles which may have f ormed. No pressure is req ui red . 1 1 . Leave the slab and peel for half an hour at least. to allow the peel to harden. 1 2 . Gcntl) peel the ac etate sheet from the sam ple. 1 :1. Trim and mount immediately between two pieces of glass to keep the peel flat. ormal glass slides for thin sections arc suitable for small samples.
?

using 500 ml or solution will depend on their surface areas.
Using samples averaging about 5 em square, I 0-15 samples can be accommodated. although it will be necessary to increase the time in the solution as the acid becomes weaker. After

I0-15 samples, the solution must either be discarded or strengthened with a 2 3 ml of concentrated hyd rochloric acid.

(c) All chemicals should he handled with great care. It s i
recommended that protective gloves are worn throughout tire making o fpeels. Take care nor to inhale the acetone f umes.

The peel is now ready for examination u nde r the microscope.

ole:

(a) To makl: another peel of the same sample it is necessary 10 re-grind the surface \\ ith only the finest grit before repeating
steps 4 1 3 above.

(b) The number of samples which can he peeled successfully
100

-

References

Carver, R. E., 1 97 1 , Procedures in Sedimentary Petrology. Wil ey?

Bathurst, R. G. C., 1975, Carbonate Sediments and their Diagenesis. Elsevier. Amsterdam. 2nd Edition. Interscience, New York.

classifica tion of porosi ty in sedimentary carbonates. Bull. Am. Assoc. Pe1rol. Ceo!. . 54, 207-50.
Dickson, J. A. D., 1965, A modified stai ni ng tech niq ue for carbonates in thin section. Nawre. 205. 587. Dunham. R. L 1962, Classification o f carbonate rocks according to depositi onal texture. In W. E. Ham (Ed.). Classification o f carbonate rocks. Am. Assoc. Petrol. Ceo/. Mem. / , 1 08-2 1 . Fl iigel, E., 1982, Micro{acies analysis o(/imeslones. Springer, Berlin. Folk, R. L., 1 9 5 1 . Stages of textural maturity in sedimentary rocks. J. Sedim. Petrol., 2 L 127-30. Folk , R. L.. 1 959. Practical petrographic classification of limestones. Bull. Am. Assoc. Petrol. Ceo/. . 43, 1-38. Folk, R. L., 1962. Spectral subdivision of limestone types. In W. E. Ham (Ed.), Class(fication o f carbonate rocks. Am. Assoc. Pe1rol. Ceo/.
Mem. I, 62-84.

Choq uette. P. W. and Pray, L. C., 1 970, Geol ogic nomenclature and

Folk. R. L., 1974, Petrology r ?{Sedimen1my Rocks. Hemphills. Austin,

'? Horowitz, H. S. and Potter, P. E. .

Texas.

1 9 7 1 , Introductory Petrography r?f Fossils. Springer. Berlin. Johnson. J. H., 1961, Limes/one-building algae and algal limes/ones.

Colorado School of Mines. Golden, Colorado. MacKenzie, W. S .. Donaldson, C. H. and Gu il ford , C.. 1 982, Alias of
igneous rocks and their textures. Longman, Harlow. M ajewsk e , 0. P., 1969, Recognition o f invertebratejossilf ragments in rocks and lhin-sections. Int. Sed. Pel. Ser. 13, 101 pp. Brill, Leiden.

Pettijohn, F. J. 1975. Sedimenlary Rocks. Harper and Row. New York, 3rd Edition. Pett ijo h n, F. J . , Potter, P. E. and Siever, R. 1973, Sand and Sandrtone. Springer. Berlin. Scholle, P. A., 1978. A color illustrated guide to carbonate rock
consti/Uems. textures. cemems and porosites. Am. Assor. Petrol. Ceo/. Mem. 27. Sorby, H. C., 1 8 5 1 , On the m ic ro scopical structure of the Calcareous

Grit of the Yorkshire coast. Q. J. geol. Soc. London . 7, I 6. So rby, H. C .. 1879, On the st ructu re and origin of limestones. Q. J.
geol. Soc. London. 35, 56-95. Wray. J. L. 1977. Calcareous algae. Elsevier. Amsterdam.

1 01

Index

Plate numbers in bold type. Page numbers, in lighter type, are given where there are no photographs, or where the discussion of the feature is on a different page to the plate. References to plates arc given where illustrated material is described in text. Other photographs showing the same feature are mentioned at the end of each section of text. acetate peels Appendix 3
126 78, 79

bryozoans

101-3 1 1 2-17 40-2,

crinoids

96 , 97 113

calcareous algae
51,52

dasycladacean algae dedolomite discocyclinids
169, 170 108

calcite cement (sandstones) calcispheres
120

carbonaceous rocks carbonate cements carbonate mud carnallite

216, 217 55

dismicrite dolomite dolostone

62, 157
161-8

34, 36, 37

70
129 131

acicular aragonite aggregate grains algae
ll2-17

208, 209

dripstone cement drusy mosaic durain
216

cavern porosity cements

65

1 8-21 , 55-7 187, 188

algal stromatolite Alizarin Red S allochems anhydrite ankerite aragonite arkose 34

121, 122 34,

chalcedonic quartz chamosite

echinoderms echinoid spines embayed quartz endolithic algae

96-8 98 50 54 90

Appendix 2

171-3, 175-7

channel porosity charophyte algae chert rock fragments cherts chlorite
181-8 31, 32 62 24

65
114 27,28

191, 192, 195, 196,

198-201, 203-7

72
34 54, 55

endopunctate brachiopods endothyracids etching evaporites extraclasts
34 189-209 80 110

authigenic quartz cement 38, 39, 42 replacement 169, 183-8 banded iron formations
180 78, 179,

classification limestones sandstones clay minerals coal
216

15, 45, 46

fabric selective porosity faecal pellets
75 9-16

65

bioclasts biomicrite biosparite biotite

39, 84-120
62 62 68, 69 157

coal balls coated grains

21.7 77 112

f eldspars

codiacean algae collophane compaction limestones sandstones composite quartz

feldspathic greywacke feldspathic litharenite f enestrae
157, 158

64,65 24

210-15 58 22 3-5 131 47

birds-eye structures bitumen bivalves
160

fenestrate bryozoans ferroan calcite cements ferroan minerals flexible sandstone fluid inclusions foraminifera
8 107-11 34

101 127, 131

84, 85, 87-9 117 62, 150 90-5 95

blue-green algae boundstone brachiopods

compromise boundaries concavo-convex contacts coralline algae corals
99, 100 116

48, 49

brachiopod spines
102

fossiliferous micrite

62, 149

fracture porosity gastropods

65

meteoric water cements 131 micas 29, 30, 68, 69 34

127, 128,

poikilitic texture

40, 41 3-5

polycrystalline quartz polyhalite porosity limestones sandstones 202-4 65 23

84, 86

geopctal sediment
Girvanel/a

159

micrite

117 33-5 Ill 11 I 62, 146

micrite envelopes micritization 54

123-24

glauconite globcriginids globorotalids grainstone

microcline feldspar microcrystalline calcite microquartz microspar 181-4 141

II, 12 34

potassium ferricyanide Appendix 2 pressure-solution limestones 59 22 sandstones pr imary porosity 65

34,

grain-to-grain pressure 137 solution grapcstones greywackes
37

microstalactitic cements miliolids 65 109 153

129

pseudopunctate brachiopods pseudospar punctae I, 2 140, 142 90, 91 53

91, 95

24, 62-7

mouldic porosity molluscs 84-9

growth framework porosity gypsum
Halimeda

43, 44, 189, 190, 193-6 112 197-201 160 23-6, 58, 92

monocryslalline quartz mudstone (carbonate) muscovite neomorphism non-skeletal algae nummulites oncoids 107 81, 82 81, 82 114
72-4, 171, 172

62, 149

quartz arenite

halite

29, 30, 68, 69 60, 6 1
53, 54

hydrocarbons

quartz cement 38, 39, 42 1-8 detrital grains replacive 183-8 radial-fibrous fabric radiolaria red algac 181, 182 116 17-28 55

igneous rock fragments 59 impunctate brachiopods
Inoceramus

88 65, 168 65, 47, 151,

intercrystal porosity intergranular porosity 152 intraclasts 77, 79

oncoliths oogonia ooids

rock fragments roundness rugose corals 3

99

oolitic ironstones 65, 159 ooliths oomicrite

171-173, 175-177 sandstone classification sandstone rock fragments 154 schistose quartz scleractinian corals 15, 16 secondary porosity sedarenite 19, 20 45, 144, 145 24 21, 22

intragranular porosity intramicrite intrasparite ironstones 62 62 171-80

72-4, 171, 172 62

oomouldic porosity oosparite 127, 160 62, 146

isopachous cements itacolumite
Koninckopora

orthoclase f eldspar ostracods oysters 87 62, 147 104, 105

65

48,49 113 158 62

24,56, 57 21,

sedimentary rock fragments 22, 27,28,56,57 sericite 111 9, 10 17, 18 5 65, 156 65

laminoid f enestrae limestone classification limonite litharcniles lithic arkose 173, 174 56-0l 24

packstone

pelagic foram inifera pellets pelmicrite peloids 75 62 75, 76 62

shale fragments sheared quartz shelter porosity shrinkage porosity siderite

lithic grcywackes lithoclasts
Lithostrotion

62, 63, 66, 67

pelsparitc

171, 172, 176, 177 183-8 68-71 50

80
99

perthitic feldspar phosphatic sediments phosphorites phreatic cements phyllarenite pisoids 83 24

13, 14 210-15

silicification siltstones

210-13 131

skeletal calcareous algae skeletal particles slate fragments sorting 24
34

macroquartz magnesite maturity

187, 188 205-7 24

84-120 17, 18

metamorphic rock fragments 17-20

plagioclase feldspar planktonic bivalves

9, 10 89

sparite

sparry calcite

34
103

spastoliths sphericity
Spirorbis

176, 177 3 119 216 34, Appendices 2 and
5

sutured seam solution sylvite 197

1 38

vacuoles

8 128, 129 23-6

vadose cements

spore-cases stainin'g 3

syntaxial overgrowths quartz 38,39 132, 133 calcite syntaxial rim cements 132, 133

volcanic rock fragments volcanic arenite vug porosity 65

24, 58, 59

stretched metamorphic quartz stromatolites stylolites subarkose sublitharenite superficial ooids 138 24 24, 60, 61 72 121, 122

wackestone tabulate corals trilobites 106 100 worm tubes

148 1 18, 1 1 9

toned cements 4, 5, 48, Udden-Wentworth scale undulose extinction 6, 7 3 zoned dolomite zooecia 101

131 164

sutured grain boundaries 49. 138

104

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