Geology

doi: 10.1130/0091-7613(1978)62.0.CO;2 1978;6;745-748Geology

M. D. Lewan

Laboratory classification of very fine grained sedimentary rocks

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Laboratory classification of very fine grained sedimentary rocks

M. D. Lewan* H. N. Fisk Laboratory of Sedimentology

Department of Geology University of Cincinnati Cincinnati, Ohio 45221

A B S T R A C T

Very f ine grained sedimentary rocks are de f ined as those containing more than 45% by vo lume microscopic material ( < 5 urn). Those containing 65% to 45% microscopic material are designated as muds tones , and those containing m o r e than 65% microscopic material are designated as shales. The increase of detai led studies o n these rock types has revealed the need o f a more descriptive and laboratory-oriented classif ication for them. The proposed classif ication is based o n textural and compos i t iona l attributes o f the rocks. T h e nomenclature consists o f a root name preceded b y a primary adjective, which , in turn, m a y be preceded by a nominal adjective. The root names include c laystone , marlstone, mics tone , and m u d s t o n e and are de f ined on the basis o f vo lume percentage o f microscopic mate -rial and weight percentage o f sil icate minerals in the rock. Primary adjec-tives c o n v e y speci f ic mineralogic informat ion about the rock; nominal adjectives m a y be used to emphas ize other attributes, such as bedding structures, f issi l ity, color, or fossi l content .

INTRODUCTION Shales make up a large proportion of sedimentary rocks,

with some estimates exceeding 80% (Wickman, 1954, p. 108). Studies of fine-grained sedimentary rocks have become more popular as their association with energy resources becomes more apparent. Two examples of this are the degasification studies of Paleozoic shales in the eastern states (Shumaker and Overbey, 1976) and retorting of Tertiary oil shales in the western states (Ash, 1974; Yen, 1976). General classifications for fine-grained sedimentary rocks have been formulated on the basis of grain size and physical character of the rock (for example, Pettijohn, 1975, p. 262; Blatt and others, 1972, p. 375). These types of classifications are quite useful for field descriptions, but with advances in quantitative X-ray diffraction methods, increasing availability of scanning electron microscopes, and development of rapid chemical analysis methods, a more comprehensive and in-formative laboratory classification of these rocks is needed.

Picard (1971) presented a laboratory classification for fine-grained sedimentary rocks that uses conventional sandstone compositional terms (for example, lithic arenite, subarkose, and arkose), which are preceded by the name of the major clay min-eral in the rock. Although this classification is applicable to fine-grained rocks composed of silt-sized material, it cannot be ap-plied effectively to rocks with predominantly microscopic mate-rial ( < 5 fim). This is because it is virtually impossible to iden-tify either rock fragments in very fine grained rocks or the wide variety of additional mineral phases that are normally not in-cluded in the nomenclature of sandstones (for example, carbon-ates, phosphates, clay minerals, zeolites, and iron oxides). For

Present address: Amoco Production Company Research Center, P.O. Box 591, Tulsa, Oklahoma 74102.

this reason I propose the following laboratory classification of very fine grained sedimentary rocks that is based on textural and compositional attributes. I am now using the classification and have found it most helpful in comparing very fine grained rocks from different sedimentary basins and from different deposi-tional environments.

DEFINITION OF VERY FINE GRAINED SEDIMENTARY ROCKS

Very fine grained sedimentary rocks are defined in this classification as those that contain more than 45% by volume microscopic material ( < 5 jon; Fig. 1). This value was chosen from the standpoint that most silt- or sand-sized grains will be

Figure 1. Root names used in the proposed classification of very fine grained sedimentary rocks.

G E O L O G Y , v. 6, p. 7 4 5 - 7 4 8 745

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matrix supported when microscopic material exceeds 40% to 50% by volume of the rock, depending on the sorting, angular-ity, shape, and packing arrangement of its grains. Thus, the in-termediate value of 45% implies that very fine grained sedimen-tary rocks are usually matrix supported.

As shown in Figure 1, very fine grained rocks are subdi-vided into two distinct categories: (1) shale and (2) mudstone. Shale contains more than 65% by volume microscopic material, whereas mudstone contains 65% to 45% by volume microscopic material. Determination of the percentage of microscopic mate-rial may be readily made by point counting the rock in thin sec-tion (for example, Textoris, 1971). Reasonable visual estimates may also be made from thin sections with the aid of visual per-centile comparators (for example, Terry and Chilingar, 1955; Baccelle and Bosellini, 1965). The 5-/im value was chosen from a practical standpoint: I have found that with a petrographic microscope, grain-size analysis below this value becomes quite tedious and yields dubious results. This is especially true for very fine grained carbonates whose constituents are usually less than 5 tan but commonly straddle the clay size boundary at 4 /m.

CLASSIFICATION OF SHALE The nomenclature used for shale consists of a root name

and a preceding primary adjective. As shown in Figure 1, the root names are dependent on the percentage by weight of the silicate fraction in the rock. The silicate fraction is

Percent of (tektosilicates + phyllosilicates + chalcedony + glass)

- ' X 100. Percent of (tektosilicates + phyllosilicates + chalcedony + glass + carbonates + phosphates + chlorides + sulfates

+ Fe oxides + Mn oxides + A1 oxides)

The tektosilicates include such minerals as quartz, cristobalite, tridymite, opal, feldspars, and zeolites, and the phyllosilicates

include the clay minerals and micas. It should be noted that the calculation does not include sulfides or organic matter. The re-sidual of the silicate fraction is called the nonsilicate fraction, and its equation is footnoted at the bottom of Table 2.

Once the silicate fraction has been determined, the shale may be assigned the proper root name: (1) claystone (silicate fraction = 100% to 75% by weight), (2) marlstone (silicate frac-tion = 75% to 25% by weight), or (3) micstone (silicate fraction = 25% to 0% by weight; see rationale for name in discussion section). These subdivisions of shale are broad enough so that a reasonable estimate of the silicate fraction may be made by the integration of petrographic examination of a thin section and a qualitative X-ray diffraction evaluation. Depending on the type of minerals present in the nonsilicate fraction, the percentage of insoluble residue (Ireland, 1971) may also be useful in determin-ing the silicate fraction.

After the root name of a rock has been determined, it is then modified by a preceding primary adjective. The purpose of the primary adjective is to reveal more detailed compositional in-formation about the rock. The primary adjectives for shale with silicate fractions that exceed 50% are given in Table 1, and those with silicate fractions less than 50% are given in Table 2. Gen-erally, chemical names are used for primary adjectives that de-note a group of minerals from a particular chemical group, whereas mineralogic names are used for primary adjectives that denote one mineral or a mineral group. When the summation of several minerals is designated for a primary adjective (for ex-ample, siliceous, argillaceous, calcareous, and calcophosphatic), only two of the designated minerals are required to be present in the rock for assigning the primary adjective. More than one primary adjective may be applicable to a rock by this definition, but the primary adjective revealing the most detailed mineralogic information about the rock is the one that should be used. A micstone whose nonsilicate fraction consists entirely of calcite, aragonite, and dolomite may by definition be considered calcar-eous, calcophosphatic, or calcosaline; however, the absence of phosphate, sulfate, and chloride minerals in this rock indicates

Primary ad jec t i ve

PRIMARY ADJECTIVES FOR SHALES AND MUDST0NES WITH SILICATE FRACTIONS THAT EXCEED 50% BY WEIGHT

Designated mineral or summation of minerals must comprise 503! or more by weight o f the s i l i c a t e f r a c t i o n

TABLE 2. PRIMARY ADJECTIVES FOR SHALES AND MUDST0NES WITH NONSILICATE FRACTIONS THAT EXCEED 50% BY WEIGHT

Primary ad jec t i ve

Designated mineral or summation of minerals must comprise 50% or more by weight of the n o n s i l i c a t e f r a c t i o n

Carbonates

~ S iE Si 1iceous Quartzose Chalcedonic C r i s t o b a l i t i c V i t r i c Opaline Arg i l laceous S i a l l i t i c Ch lo r i t i c I l l i t i c K a o l i n i t i c Smec t i t i c * Micaceous S i a l f e l d s p a t h i c Feldspathic A l b i t i c K - fe ldspa th ic Z e o l i t i c Z e o s i a l l i t i c Tektosi1iceous

Calcareous C a l c i t i c Aragon i t i c Dolomi t ic S i d e r i t i c A n k e r i t i c Sodic Calcophosphatic Phosphatic Sal ine H a l i t i c Gypsic Calcosal ine Ferruginous Ferromangano Mangano Ferroaluminous Aluminous

*Rocks conta in ing smect i te in q u a n t i t i e s exceeding 75% by weight are usua l l y con-s idered bentoni tes and are termed such i n t h i s c l a s s i f i c a t i o n .

Note: Nons i l i ca te f r a c t i o n i s Weight percent of (carbonates + phosphates + ch lo r ides + su l fa tes + Fe

oxides + Mn o);1des + A1 oxides) Weight percent o f ( t e k t o s i l i c a t e s + p h y l l o s i l i c a t e s + chalcedony + glass

+ carbonates + phosphates + ch lo r ides + su l f a tes + Fe oxides + Mn oxides + A1 oxides)

100.

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A t t r i b u t e

TABLE 3. EXAMPLES OF NOMINAL ADJECTIVES THAT MAY BE USED OR REDEFINED AT THE DISCRETION OF THE INVESTIGATOR

Examples of nominal adject ives

(A) NEW ALBANY SHALE SAMPLE

Color Red, brown, black, green, gray, variegated S p l i t t i n g Blocky = equant masses. Slabby = slabs that exceed 5 mm in thickness.

Platy = plates that range from 2 to 5 mm in thickness. F i s s i l e = th in chips that do not exceed 2 mm in thickness. Earthy = loosely aggregated clods

Mineral Analcimic, c l i n o p t i l o l i t i c , p y r i t i c , i l l i t i c , c a l c i t i c , hemat i t ic , content col lophanic, gypsic

Bedding Bedded = layers th icker than 5 mm. Laminated = layers not exceeding 5 mm in thickness. Streaked = discontinuous layers with thicknesses 0.3 x t he i r lengths. Massive = st ructure less

Structures Load s t ructures, convolute bedding, slump s t ructures, b io tu rba t ion , ba l l -and-p i l l ow structures

Organic Nonorganic =

TABLE 4. CRITERIA USED TO DEFINE FINE-GRAINED SEDIMENTARY ROCKS

Reference Grain s ize Composition S p l i t t i n g Bedding

Shale P e t t i j o h n (1975, p. 261)

Clay and s i l t N.S. F i s s i l e Laminated

B l a t t and others (1972, p. 374)

Clay and/or s i l t

Substant ia l amounts o f c lay minerals

F i s s i l e N.S.

Jackson (1970, p. 377)

Clay and s i l t N.S. F i s s i l e Laminated

Moorhouse (1959, p. 361)

Very f i n e grained

Arg i l laceous

Mudstone

F i s s i l e N.S.

P e t t i j o h n (1975, p. 261)

Clay and/or s i l t

N.S. Non f i ss i l e Nonlamina ted

B l a t t and others (1972, p. 375)

Clay = 67%-33%, s i l t = 33%-67%

N.S. Non f i ss i l e N.S.

Jackson (1970, p. 377)

Fine-grained N.S. Nonf iss i I t ; N.S.

Dunham (1962, p. 118)

Mud w i th less than 10% grains

Carbonate

Claystone

N.S. N.S.

P e t t i j o h n (1975, p. 261)

Clay N.S. Nonf iss i l o N.S.

B l a t t and others (1972, p. 375)

Clay = 100%-67%, s i l t = 33%-0%

N.S. Nonf iss i I s N.S.

Moorhouse (1959, p. 366)

F ine-gra ined Arg i l l aceous N o n f i s s i l Nonlaminated

Picard (1971, p. 185)

Clay = 100%-50% N.S. N.S. N.S.

Marlstone (mar l ) P e t t i j o h n (1975, p. 285)

Clay = 65%-35% Carbonate = 35%-65% N.S. N.S.

Jackson (1970, p. 384)

N.S. Subequal amounts o f c lay and carbonate

N.S. N.S.

Moorhouse (1959, p. 378)

N.S. Equal amounts o f c lay and carbonate

N.S. N.S.

Note: N.S. 3 Not spec i f i ed .

this classification are given in Table 4. In comparison to these definitions, the proposed classification places more emphasis on composition and places less emphasis on fissility. As, shown in Table 4, it is common practice to differentiate shale from mud-stone on the basis of the former having fissility. Field observa-tions indicate that fissility of a rock is usually a function of weathering, with fissility increasing as the degree of weathering increases. As one digs into a weathered outcrop composed of very fine grained rocks, the earthy rock at the surface grades in-ward into a fissile rock, which grades into a platy or slabby rock, which, in turn, grades into a blocky nonfissile rock. The depth into the outcrop and the thickness of each of these weath-ering stages depends on the texture of the rock, its composition, and degree of weathering. This weathering series has been ob-served in humid and arid climates, and has been observed on many rock types (for example, claystone, marlstone, and oil shale) with massive and laminated bedding. It is likely that most laboratory studies of very fine grained sedimentary rocks would concentrate on unweathered field samples or subsurface samples, and the nomenclature distinguishing fissile from nonfissile rocks would be inappropriate. For this reason the proposed classifica-tion does not place primary significance on fissility, but instead gives it secondary importance as a nominal adjective to be used at the discretion of the investigator.

The major advantages of this classification include: (1) the nomenclature reveals a significant amount of textural and com-positional information about the rock; (2) the classification is

quantitative in part, but it also allows an investigator to quali-tatively emphasize other attributes of a rock (3) the nomenclature is detailed enough to allow lithologie subdivisons of what were once considered monotonous stratigraphie sections of shale and marlstone; (4) the classification is in a format that may be easily coded for computer processing. The intention of this classifica-tion is to improve communication among investigators and to initiate a more descriptive and laboratory-oriented nomenclature for very fine grained sedimentary rocks.

REFERENCES CITED Ash, H. O., 1974, Current status of oil shale development in United

States: American Association of Petroleum Geologists Bulletin, v. 58, p. 905-906.

Baccelle, L., and Bosellini, A., 1965, Diagrammi per la Stima Visiva Della Composizione Percentuale Nelle Rocce Sedimentarie: Annuali Dell' Universit Di Ferrara, v. 4, p. 59-62.

Blatt, H., Middleton, G., and Murray, R., 1972, Origin of sedimentary rocks: Englewood Cliffs, N. J., Prentice-Hall, Inc., 634 p.

Cook, H. E., and others, 1975, Methods of sample preparation and X-ray diffraction data analysis, X-ray mineralogy laboratory, Deep Sea Drilling Project, University of California, Riverside, in Kaneps, A. G., ed., Initial reports of the Deep Sea Drilling Project, Vol. 28: Washington, D.C., U.S. Government Printing Office, p. 999-1007.

Devine, S. B., Ferrell, R. E., Jr., and Billings, G. K., 1972, A quantita-tive X-ray diffraction technique applied to fine-grained sediments of the deep Gulf of Mexico: Journal of Sedimentary Petrology, v. 42, p. 458-475.

Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture, in Ham, W. E., ed., Classification of carbon-ate rocks, a symposium: American Association of Petroleum Geologists Memoir 1, p. 108-121.

Ireland, H. A., 1971, Insoluble residues, in Carver, R. E., ed., Procedures in sedimentary petrology: New York, Wiley-Interscience, p. 479-498.

Jackson, K. C., 1970, Textbook of lithology: New York, McGraw-Hill Book Co., 552 p.

Moore, C. A., 1968, Quantitative analysis of naturally occurring multi-component mineral systems by X-ray diffraction: Clays and Clay Minerals, v. 16, p. 325-336.

Moorhouse, W. W., 1959, The study of rocks in thin section: New York, Harper and Row, 514 p.

Pettijohn, F. J., 1975, Sedimentary rocks: New York, Harper and Row, 628 p.

Picard, M. D., 1971, Classification of fine-grained sedimentary rocks: Journal of Sedimentary Petrology, v. 41, p. 179-195.

Shumaker, R. C., and Overbuy, W. K., Jr., 1976, Devonian shale produc-tion and potential: Proceedings of the Seventh Annual Appala-chian Petroleum Geology Symposium, Morgantown, West Virginia, MERC/SP-76/2, 271 p.

Terry, R. D., and Chilingar, G. V., 1955, Summary of 'Concerning some additional aids in studying sedimentary formations' by M. S. Shvetson: Journal of Sedimentary Petrology, v. 25, p. 229-234.

Textoris, D. A., 1971, Grain-size measurement in thin section, in Carver, R. E., ed., Procedures in sedimentary petrology: New York, Wiley-Interscience, p. 95-108.

Wickman, F. E., 1954, The "'total" amount of sediment and the composi-tion of the "average igneous rocks": Geochimica et Cosmochimica Acta, v. 5, p. 97-110.

Yen, T. F., 1976, Science and technology of oil shale: Ann Arbor, Michigan, Ann Arbor Science Publishers, Inc., 226 p.

ACKNOWLEDGMENTS Reviewed by Roy C. Kepferle and Paul E. Potter. Acknowledgment

is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research.

MANUSCRIPT RECEIVED MAY 30, 1978 MANUSCRIPT ACCEPTED SEPTEMBER 25, 1978

748 PRINTED IN U.S.A. DECEMBER 1978

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