CLASSIFICATION OF CARBONATE ROCKS ACCORDING TO DEPOSITIONAL TEXTURE1

ROBERT J. DUNHAM8

Houston, Texas

ABSTRACT

Three textural features seem especially useful in classifying those carbonate rocks that retain their depositional texture (1) Presence or absence of carbonate mud, which differentiates muddy carbonate from grainstone; (2) abundance of grains, which allows muddy carbonates to be subdivided into mudstone, •wackestone, and packstone; and (3) presence of signs of binding during deposition, which characterizes boundstone. The distinction between grain-support and mud-support differentiates packstone from wackestone—packstone is full of its particular mixture of grains, wackestone is not. Rocks retaining too little of their depositional texture to be classified are set aside as crystalline carbonates.

INTRODUCTION

The increasing use of thin sections and oiled slabs has shown that most limestone and much dolomite retain their depositional texture, in a more or less ghostly fashion, despite diagenesis. A descriptive classification based on depositional texture is thus a generally helpful adjunct to other classifications, part icularly to those based on genetic kind of particles and to those based on mineralogic composition. I t will not subst i tu te for further description and classification, nor will it produce ready-made interpretat ions, bu t it will focus a t tent ion on whichever few textural proper­ties are chosen as part icularly significant for in-

1 Part of a symposium arranged by the Research Committee, and presented at Denver, Colorado, April 27, 1961, under joint auspices of the Association and the Society of Economic Paleontologists and Mineralogists. Manuscript received, April 23, 1961.

* Shell Development Company, Exploration and Production Division. I am deeply indebted to R. L. Folk, who shared his knowledge of carbonates and classification prior to publication, and repeatedly con­tributed to the evolution of the present concepts; to R. N. Ginsburg, whose studies of Recent carbonates clarified problems in ancient carbonates; and to D. L. Amsbury, K. J. Hsu, S. D. Kerr, O. P. Majewske, P. F. Moore, R. C. Murray, J. M. Parks, B. F. Perkins, G. Rittenhouse, C. I. Smith, B. W. Wilson, and J. L. Wilson, who gave encouragement and stimulating criticism.

terpretat ion of depositional environment, and it will provide the convenience of class names based only on depositional texture.

Ideally, one could confidently divide unaltered carbonate rocks into two main groups, which Grabau (1904) termed clastic and biogenic. Ac­cording to the ideal, one group is controlled mainly by hydraulic conditions. Origin of particles is relatively insignificant, and subdivision is ac­cording to particle size. Calcirudite thus is dis­tinguished from calcarenite. The other group is controlled mainly by the biologic or biochemical processes responsible for producing carbonate . Here , particle size is relatively insignificant, and subdivision is made according to origin of par t i ­cles. Crinoidal limestone thus is distinguished from coral l imestone. As it turns out , most sam­ples are neither clearly in one group nor clearly in the other. Instead, they fall in the middle ground where hydraulic conditions and biologic or bio­chemical processes are in joint control.

Focusing a t tent ion on whether or not the larger particles are transported (fragmented or disarticulated) does not re-establish the sharp boundary between the two groups. Too often one is forced to conclude tha t a carbonate sediment was t ransported a litt le bu t not far, or t h a t some components were transported and others were not,

WAN V

EXPLANATION OF PLATE I

Looser packing would require mud-support. Samples were impregnated on the beach in such a way as to preserve natural packing and were then reimpregnated in the laboratory and sliced. All samples were from Cayo Centra, Cayos Areas, Campeche Banks, Mexico.

a. Coral lime gravel, XI , reflected light. South side of island, AC-64. b. Coral-red algae lime sand, X2S, transmitted cross-polarized light. Northeast side of island, AC-47. c. Red algae lime sand, XS, transmitted cross-polarized light. Southeast side of island, AC-83.

108

PLATE I.—Grain-support in beach deposits.

PLATE II.—Grain-support influenced by grain shape.

C L A S S I F I C A T I O N A C C O R D I N G T O D E P O S I T I O N A L T E X T U R E 111

or tha t degree of t ransportat ion is indeterminate . Indeed, the outs tanding differences between land-derived sediment and carbonate sediment stem from the simple fact tha t , as a rule, land-derived sediment is produced by destruction far from the site of deposition, whereas carbonate sediment is produced by destruction and construction a t or near the site of deposition. The recognized useful­ness of classifications based on origin of particles a t tes ts to the idea tha t carbonate grains are un­like land-derived sediment in distance of t rans­por t ; if grains did no t generally remain near where they were produced, such classifications would lack the environmental significance they are known to have. Possibly, the ease with which carbonate grains are destroyed, or cemented, ac­counts for their relatively short t ranspor t .

SIGNIFICANCE OP MUD

The distinction between sediment deposited in calm water and sediment deposited in agi tated water is fundamental . Evidence bearing on this

problem thus deserves to be incorporated in class names. This can be accomplished in several ways. One is to focus a t ten t ion on average or pre­dominant size, which erroneously assumes tha t all sizes in a sample are equally significant hydraulic-ally. Another is to focus a t tent ion on the size, abundance, and condition of the coarse material brought to the site of deposition. This emphasis on what might be called currents of delivery has long been successful in dealing with land-derived sediments, bu t does not work well in lime sedi­ment because of the local origin of many coarse grains. A third way is to focus a t ten t ion on the fine material t ha t was able to remain a t the site of deposition. This emphasis on what might be called currents of removal seems advisable if we wish to characterize carbonate sediment systematically in terms of hydraulic environment.

Inasmuch as calm water is characterized by mud being able to settle to the bot tom and remain there, it seems tha t the muddy rocks deserve to be contrasted with mud-free rocks, regardless of the

J //f/r

EXPLANATION or PLATE II

Looser packing would require mud-support. Aggregates were made by sedimenting wet grains in quiet water onto a bed of previously deposited grains (Halimeda plates visible at base of aggregate), drying, impregnating with clear plastic, slicing, then staining. Plate W-d was sedimented dry.

Per cent grains is expressed in two ways, both values being rounded to the nearest 5 per cent. Per cent grain-solid, which is measured by water displacement, refers to the volume of solid matter in the grains divided by the volume of the aggregate. Per cent grain-bulk, which is measured by point-counting, differs in that openings and deep indentations in grains are conventionally counted as grain instead of as pore.

a. Well sorted, well rounded, highly spherical porcelain balls, X I , 65 per cent grain-bulk, 60 per cent grain-solid.

b. High-spired snails, X If, 45 per cent grain-bulk, 25 per cent grain-solid. c. Small clams, X l i , 30 per cent grain-bulk, 25 per cent grain-solid. d. Corn flakes, XI , 30 per cent grain-bulk. e. Rose corals, X J, 70 per cent grain-bulk, 20 per cent grain-solid. / . Branching red algae broken to f-inch lengths, XI f, 20 per cent grain-bulk, 15 per cent grain-solid.

igtFd

J p 3 ^ '- . .v **

NffllP

PLATE III.—Indications of grain-support.

C L A S S I F I C A T I O N A C C O R D I N G T O D E P O S I T I O N A L T E X T U R E 113

amoun t and size of included coarse material . Folk (1959) and Bramkamp and Powers (1958) in their new classifications of carbonate rocks have much the same viewpoint. The terms mud and grains are used variously by different au thors . Usage here is based on particle size, grains being larger than 20 microns and mud being smaller than 20 microns. The distinction thus parallels the distinc­tion between matr ix and grains in sandstone (Pet-tijohn, 1957, p . 284). Freedom from mud is taken to mean virtual absence—less than 1 per cent.

GRAIN-SUPPORT AND MUD-SUPPORT

Rocks bearing carbonate mud const i tute the bulk of many carbonate sequences and therefore require subdivision. The most useful textural sub­division of such rocks seems to be on the basis of

abundance of grains. Such subdivision allows mapping of gradients in ra te of product ion of grains relative to ra te of accumulation of mud.

Three degrees of abundance can be recognized. I n the most common case, grains are a b u n d a n t enough to be prominent , say more than 10 per cent, bu t are not so abundan t as to suppor t one another . I n such a texture, the grains are some­times said to be "floating." Here they will be called "mud-suppor ted . " Rocks in which grains are less a b u n d a n t than 10 per cent const i tute a second category. Rocks in which grains are so a b u n d a n t as to suppor t one another, just as they do in mud-free rocks, which are necessarily "gra in-suppor ted ," are a third category.

T h e distinction between mud-suppor t and grain-support seems to be more meaningful than

^ '3JJS

EXPLANATION OP PLATE I I I

Floored interstices, shelter effects, embayed contacts, and overly close packing indicate grain-support. a-d—Floored interstices are produced by fine sediment filtering into coarser sediment, and by concurrent

deposition of fine and coarse sediment. Marine infiltering reaches deeply only if the size difference is large, and even then does not fill interstices to the roof.

a. Oyster shells infiltered by lime mud in the laboratory, XI , reflected light. Mud floors (Fi) are layered, and drape over irregularities. Shrinkage during drying reduced the amount of mud in the interstices, but voids (V) were beneath the shells before drying.

b. Gravel, Halimeda, and ooliths infiltered by very fine sand in the laboratory, XI , reflected light. Sand largely fills interstices in the gravel layer, reaches part way through the Halimeda layer, and does not enter the oolith layer. The voids (V) beneath the pebbles are original shelter effects—the sediment did not shrink during drying.

c. Beach sand infiltered by lime mud in the laboratory, X5, reflected light. Infiltering (M) is negligible except in the top layer of grains. Note the shelter effects at the large grain (S).

d. Floored interstices (Fd) produced by concurrent deposition of ooliths and Halimeda in the laboratory, in volume ratio of 3:1, X3, reflected light. Ooliths making floors (Fi) in lower quarter of picture filtered into inter­stices in previously deposited bed of pure Halimeda.

e. Embayed contacts, X20, transmitted unpolarized light. Lake Valley limestone, Mississippian, Sacramento Mountains, New Mexico, AGN.

/ . Overly close packing, X50, transmitted unpolarized light. Mission Canyon formation, Mississippian, Shell Richey NP-1, Richey field, Dawson County, Montana, NP 1-7312.

114 ROBERT J. DUNHAM

any alternative percentage boundary. A grain-supported rock is full of its particular mixture of grains, whereas a mud-supported rock is not. In land-derived sediments, the figures 65 per cent grains, 35 per cent porosity or interstitial debris, have been used to mark the boundary between grain-support (framework) and mud-support (dis­rupted framework). If all carbonate grains were as equidimensional as ooliths, a percentage boundary might equally well be substituted for visual distinction between mud-support and grain-support in carbonate rocks; but carbonate grains commonly are shaped like potato chips and twigs instead of like marbles. Because of this, a grain-supported rock whose grains are Ivanovia, a platy alga shaped rather like a cornflake, or Halimeda, would contain a far smaller percentage of grains than would a grain-supported rock whose grains are crinoid ossicles. A 65 per cent boundary, or a 50 per cent boundary, would put the Ivanovia rock in one group and the crinoid rock in another group; yet both rocks would contain as much of their particular mixture of grains as their volume allows them to hold.

Objection may arise that differentiating mud-supported from grain-supported rocks is impos­sibly subjective because of the need to envision a three-dimensional arrangement of irregular shapes

by looking at a two-dimensional view. The diffi­culty is real but not so bad as it at first seems. (The errors are not much greater than those en­countered in attempting to measure or estimate per cent grains in rocks having hollow or indented grains.) Experience gained in examining mud-free carbonates, which are necessarily grain-supported is an aid in determining what the kind of support is in muddy carbonates. Other aids are the floored interstices, embayed grains, overly close packing, and sheltering effects seen in grain-sup­ported rocks and not in mud-supported rocks. Illustrations of these and of grain-supported natural and artificial aggregates are shown in Plates I - I I I .

The phenomenon of grain-support has the added importance of bearing on the postdeposi-tional history of the sediment. Soluble grains that are grain-supported are in contact with each other, and thus have a chance of forming a con­nected network of molds (or incomplete molds) unlike the relatively disconnected molds in mud-supported rocks. Furthermore, compaction affects grain-supported sediment differently than it does mud-supported sediment. The grains in grain-supported sediment carry the weight of the over­burden. This tends to cause weak grains to brec-ciate. It also tends to protect from compaction

777777*

EXPLANATION OF PLATE IV

(X5, transmitted unpolarized light) o. Coral lime boundstone. Toronto limestone, Pennsylvanian Virgil, Greenwood County, Kansas, L-38-8b. b. Laminated lime boundstone (stromatolite). Cabbage-head structure in West Spring Creek limestone,

Ordovician, Arbuckle Mountains, ADN-1. c. Floored lime boundstone. Floored openings (F) constructed by intergrown complex (C) of encrusting Fo-

raminifera, algae, and hydrozoans? Permian Wolfcamp. Shell State ETA-5, Townsend field, Lea County, New Mexico, ETA5-10368.2.

'iXM?&

m ynA»n I, 1̂1

PLATE IV.—Binding in boundstone.

is* L ilpB

•

5' B|K^pj

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US

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PLATE V.—Mudstone and wackestone.

C L A S S I F I C A T I O N A C C O R D I N G T O D E P O S I T I O N A L T E X T U R E

TABLE I. CLASSIFICATION OF CARBONATE ROCKS ACCORDING TO DEPOSITIONAL TEXTURE

117

D E P O S I T I O N A L T E X T U R E R E C O G N I Z A B L E

Original Components Not Bound Together Ouring Deposition

Contains mud ( particles of clay and fine silt size )

Mud-supported

Less than 10 percent grains

Mud s to ne

More than 10 percent grains

Wack e s tone

Grain-supported

Pack s tone

Lacks mud and is

grain-supported

Gra i ns tone

Original components were bound together during deposition...

as shown by intergrown skeletal matter,

lamination contrary to gravity, or sediment-floored cavities that

are roofed over by organic or questionably organic matter and are too large to be interstices.

B o u n d s t o n e

DEPOSITIONAL TEXTURE NOT RECOGNIZABLE

C r y s t a l l i n e C a r b o n a t e

{ Subdivide according to

classifications designed to bear

on physical texture or diagenesis.)

any sheltered mud beneath strong grains, perhaps making the mud more susceptible to leaching. Collapse brecciation improved permeabil i ty in muddy grain-supported limestone composed mostly of Ivanovia in the Paradox formation of the Desert Creek field, Utah (Murray , 1960, p . 66). Leaching of mud from interstices in muddy grain-supported limestone composed mostly of crinoids caused the porosity in some of the Devonian reservoirs in Andrews County, Texas (F. J . Lucia, personal communication).

BINDING

Most carbonate rocks retaining their deposi­tional texture are lithified sediment made of clearly discrete and originally loose particles. The rocks that differ by showing signs of being bound

during deposition are scarce, but are worth special a t tent ion . Three signs of binding during deposi­tion are recognized (PL IV). One is interconnected skeletal mat te r , such as occurs where colonial corals or encrusting Foraminifera grow one on the other. Another is lamination contrary to gravity, such as occurs in the crinkly laminat ion of s tromatol i tes . The third is sediment-floored cavi­ties t h a t are too large to be interstices and are roofed over by organic or questionably organic mat ter , such as the large and small tunnels and grottoes in coral reefs.

CLASSES AND NAMES

The concepts outlined above allow five textural classes to be recognized (Table I ) . The names tentat ively a t tached to these classes are fairly

~*. TOW

EXPLANATION OF PLATE V

(X25, transmitted unpolarized light)

a. Ostracod lime mudstone, less than 1 per cent grain-bulk. Duperow formation, Late Devonian, Brown's Gulch section, Blaine County, Montana, BG-17.

b. Mixed-fossil dolomite wackeslone, 20 per cent grain-bulk. The rock contains no calcite. San Andreas Permian, Shell D. Roberts No. 15, 5,124 feet, Wasson field, Yoakum County, Texas, 1-300.

c. Ostracod-lithiclast lime wackeslone, slightly dolomitized, 40 per cent grain-bulk. Duperow formation, Late Devonian, Shell Richey NP-1, 8,900 feet, Richey field, McCone County, Montana, 1.562.

118 ROBERT J. DUNHAM

short, meaningful English nouns that apply only to texture. They can be combined with names for grain-kind classes and mineralogic classes, as shown in the figure captions, and they also can stand alone where mineralogy and grain-kind are not at issue.

Mudstone.—Muddy carbonate rocks containing less than 10 per cent grains (10 per cent grain-bulk as defined in PL II) are termed mudstone PI. V). The name mudstone is synonymous with calcilu-tite, except that it does not specify mineralogic composition and thus avoids such ambiguities as dolomite calcilutite, and it does not specify that the mud is of clastic origin. The significance of mudstone, aside from the implication of calm water, is the apparent inhibition of grain-produc­ing organisms.

Wackestone.—Mud-supported carbonate rocks containing more than 10 per cent grains (10 per cent grain-bulk) are termed wackestone (PI. V). The name has much against it, but it does have the advantage of calling to mind a mixture of mud and grains similar to that seen in some sandstones, and it is less awkward than expressions such as calcarenitic calcilutite or calcarenitic limestone.

Packstone.—Grain-supported muddy carbon­

ate rocks are termed packstone (PI. VI). Grain-support is generally a property of rocks deposited in agitated water, and muddiness is generally a property of rocks deposited in quiet water. A rock exhibiting both properties is peculiar, and it is well to have it isolated for further study. It may record simple compaction of wackestone, as is suggested where interstices are completely filled with mud. It may record early or late infiltering of previously deposited mud-free sediment, or prolific production of grains in calm water, as is suggested where interstices are floored with mud. It may record mixing by burrowers or incomplete winnowing or partial leaching of mud, as is sug­gested by patchily distributed mud.

Grainstone.—Mud-free carbonate rocks, which are necessarily grain-supported, are termed grainstone (PI. VII). Grainstones are not all of the same hydraulic significance. Some are current laid; some are the product of mud being bypassed while locally produced grains accumulate, or of mud being winnowed from previously deposited muddy sediment; and some, conceivably, are the product of locally produced grains accumulating too rapidly to be contaminated by mud. Com­monly, origin cannot be definitely known from

EXPLANATION OF PLATE VI

(X5, transmitted unpolarized light) a. Crinoid lime packstone overlying lens of lime mudstone. Overly close packing (75 per cent grain-bulk)

suggests that grain-support was acquired during compaction. Mission Canyon formation, Mississippian, Shell Richey NP-1, Richey field, Dawson County, Montana, NP 1-7312.

b. Hydrozoan? lime packstone. Floored interstices (F) indicate that grain-support is original and suggests infiltering. Permian Wolfcamp, Shell Hilburn No. 1, Townsend field, Lea County, New Mexico, SH-1-10,556.

c. Coated-grain lime packstone. Abraded nuclei of grains indicates turbulent water; presence of mud indicates quiet water; patchy distribution of mud suggests that burrowers mixed originally interbedded sand and mud together. Toronto limestone, Pennsylvanian Virgil, Kansas, Bed A.

PLATE VI.—Packstone.

PLATE VII.—Grainstone.

C L A S S I F I C A T I O N A C C O R D I N G T O D E P O S I T I O N A L T E X T U R E 121

the single sample tha t is being classified. The class name thus denotes merely the absence of mud, and, of course, the corollary tha t the grains are supported by each other. If the grains were not self-supporting, the former presence of now-recrystallized mud would be indicated. Subdivid­ing grainstone, so as to include more evidence in class names, is a temptat ion. M a n y possible sub­divisions suggest themselves. Grain size is, of course, one possibility. Sorting is another , for the well-sorted grainstones are scarce enough to be noted. Wear is another . Names such as calcirudite, sortedslone, and wornstone are useful in s tudying some suites of rocks, but , a t present , none of these properties seems of wide enough application to warrant being incorporated in a general classifica­tion.

Boundstone.—Carbonate rocks showing signs of being bound during deposition are termed boundstone (PL IV). The signs of binding are specific, and they occur within the sample being classified. Except for that , the concept of bound­stone plays the same role as the reefoid carbon te , biohermal carbonate, constructed biogenic car­

bonate , klinti te, and biolithite of other classifica­tions.

Crystalline carbonates.—Inasmuch as this clas­sification is concerned with depositional environ­ments , the rocks retaining too litt le of their deposi­tional texture to be classified must be set aside. Such rocks are here termed crystalline carbonates (specifically, crystalline dolomite, crystalline lime­stone). Although depositional texture is lacking, relics or ghosts of grains commonly allow classifi­cation according to origin of grains; for example, crinoid-bearing crystalline dolomite.

REFERENCES

Bramkamp, R. A., and Powers, R. W., 1958, Classifica­tion of Arabian carbonate rocks: Geol. Soc. America Bull., v. 69, no. 10, p. 1305-1318.

Folk, R. L., 1959, Practical petrographic classification of limestones: Am. Assoc. Petroleum Geologists Bull., v. 43, no. 1, p. 1-38.

Grabau, A. W., 1904, On the classification of sedimen­tary rocks: Am. Geologist, v. 33, p. 228-247.

Murray, R. C , 1960, Origin of porosity in carbonate rocks: Jour. Sed. Petrology, v. 30, no. 1, p. 59-84.

Pettijohn, F. P., 1957, Sedimentary rocks, 2d ed.: New York, Harper and Bros.

"^ TO5?

EXPLANATION or PLATE VII

(X5, transmitted unpolarized light)

a. Oolith lime grainstone (oolite). Note the sheltering done by the coated brachiopod shell (5) near the center —grains are smaller and more numerous above the shell than below the shell; compare Plate III-c. Wapanucka limestone, Pennsylvanian Morrow, Bromide, Oklahoma, W-23.

b. Fusulinid lime grainstone, druse-cemented. Seemingly loose packing is due to fusulinids being unusually elongate. Uniform orientation (long axis perpendicular to page) and breakage indicate that the grains are current-laid. Back-reef limestone of Capitan reef, Permian Guadalupe, Pinery Canyon, Guadalupe Mountains, Texas, PN-6-A.

c. Lithiclast lime grainstone. Note the sand-floored interstices (F); compare Plate Ill-d. El Paso limestone, Ordovician, Franklin Mountains, Texas, AGE.