terminology and the classifying fine grained...

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TERMINOLOGY AND THE

CLASSIFICATION OF FINE GRAINED

SEDIMENTARY ROCKS – is there a

difference between a claystone, a mudstone

and a shale?

A.D. WILKINS

Department of Geology and Petroleum Geology,

University for Aberdeen

Summary

Fine grained sedimentary rocks, both clastic

and carbonate, are believed to be the most

abundant rock type on the Earth‟s surface

(Picard, 1971; Blatt, 1982). Fine grained rocks

appear to constitute somewhere in the region

of 70% (Holmes, 1937) and 80% (Clarke,

1924) of all the sediment ever produced.

In sedimentology the size grade scale

most commonly used is that which was

introduced by Udden in 1898, modified by

Wentworth in 1922 and then converted into a

logarithmic form by Krumbien in 1934. For a

single particle, the sand-silt boundary, which is

the distinction between coarser grained

materials and those which are fine grained, is

placed at 1/16 mm (≈63 microns or +4 Φ). For

a given rock the proportion of material

containing particles less than 63 microns in

size is normally greater than 50% to satisfy the

classification criteria of being fine grained.

Despite their abundance, fine-grained

sedimentary rocks, to date, have not been

satisfactorily classified. Perhaps this is because

they are not well exposed in the field compared

to their coarser grained, more weather resistant

counterparts, the sandstones (Pettijohn, 1975;

Spears, 1980); perhaps it is due to their fine

grained nature that makes their study more

testing (Krumbien, 1947; Potter et al., 1980;

MacQuaker & Adams, 2003); or perhaps, as

noted by Rodgers (1950) the problem of

classifying fine grained sedimentary rocks,

simply, but inevitably entails the problem of

terminology.

It is the usage of imprecisely defined

terms that has often led to confusion, and

frustration, when trying to describe and discuss

fine grained sediments. This report will

identify where confusion has arisen, and

determines the type of terminology and

classification that will be used in future works.

TERMINOLOGY – WHAT ARE THE

PROBLEMS?

In the literature there is a plethora of

names for fine grained sedimentary materials –

argillaceous, argillite, clay, claystone, dust,

hydrolysate, loam, lutite, ooze, marl, micrite,

micstone, mud, mudrock, mudstone, pelite,

phyllite, physilite, silt, siltite, siltstone, slate,

wacke, and of course, the ubiquitous term,

shale.

Definition and usage of the word ‘shale’

The term shale has been around for a

few centuries. The first recorded use of the

written word shale appears in Hooson‟s „The

Miners Dictionary‟, published in 1747; within

which the term shale was meant as an

indurated, laminated, clayey rock. Tourtelot

(1960) gives a good summary on the origin and

use of the world shale, and by outlining its

historical use as i) a general term for

describing „laminated clayey rock‟; ii) as a

formation name, e.g. the „Pierre Shale‟; and iii)

it‟s widespread use as a bucket term for the

entire class of fine grained rocks (whether they

are laminated or not) - he has shown quite

clearly just how entrenched the word shale is

in geological literature. Therefore, is it any

wonder that confusion arises given that

Terminology and Classification of Fine Grained Sedimentary Rocks

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„geologists often to prefer to use ambiguous

words‟? (Weaver, 1990, p.6) Regardless of

how the question is presented it is doubtful that

there will ever be unanimous agreement on the

use of the word shale, in fact Selly (1988,

p288) went as far as to say that „The term

shale, however, could perhaps be usefully

abandoned by geologists, except when

communicating to engineers or management‟!

Definition of the word ‘clay’

Various size terms are in common use

and have been adopted by geologists, but due

to the lack of standardisation, the terms often

mean different things to different people. For

example, in the original restricted definition of

the word shale, i.e. a clayey rock, what is the

meaning, in quantitative terms, attached to the

word clay?

Ignoring the compositional connotation

and just purely concentrating on its textural

significance, what is the size limit placed on

the clay grade itself? Even something as

seemingly simple as defining the upper size

limit of clay is fraught with difficulty due to its

duplicitous nature. Hopkins (1899) defined the

clay/silt boundary at 1 micron, a value which is

commonly used today by colloid chemists as

determining clay-sized materials (Burgess,

2006). According to Simonson (1999) it was

Atterberg in 1903 who decided that the upper

size limit for clay should be 2 microns, a value

that is widely accepted today in Europe and

currently being applied by engineering

geologists and civil engineers that have to

adhere to the European Standard EN ISO

14689-1 (ISO, 2003) for size grade scales

when dealing with rock descriptions. The

generally accepted value most commonly used

by sedimentologists and geology students,

most particularly of North America, stems

from the works of Udden (1914) and

Wentworth (1922) which places the division

between silt and clay grade at 1/256mm or 3.9

microns. According to Wentworth (1922),

Diller in 1902 and others such as Grabau

(1913) place the upper size boundary limit of

clay-size particles at 5 microns. Some workers

have even placed the limit at 20 microns

(Correns, 1969).

Even the clay minerals themselves vary

significantly in size, see Figure 1.

Figure 1. Relative sizes and thicknesses of common clay

minerals (After Cerato, 2001)

CLASSIFICATION SCHEMES

Any undertaking of a study on fine

grained sedimentary rocks will ultimately

involve the classification and naming of the

materials in question.

A fine grained sedimentary rock

classification will be a useful tool when using

accepted, standard terminology that will

provide a clear and consistent means of

communication between those with different

backgrounds, i.e. between managers and

scientists, either academic or industrial.

Defining classification schemes

Given the confusion surrounding even

the simplest of sediment scalar properties


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which could be used for classification, i.e.

particle size boundary placement, it is not

surprising that there have been multiple

attempts to create an ultimate/universal

classification of fine grained sedimentary

material. Despite numerous attempts to classify

these materials, none appear to have gained

general widespread acceptance within the

scientific geological community: as Twenhofel

(1939, p297) stated „Classification of clays and

silts and their indurated equivalents on almost

any basis gives little satisfaction to other than

the classifier and often not even to him‟.

Is the difficulty in classifying fine

grained sedimentary materials in part due to

the confusion surrounding the used, or more

simply due to the fact that they are geological

materials formed from a complex mixture of

many different properties that could be used

for classification purposes?

Usually in classifications the aim is to

group objects of concern into classes of

similarly defining properties, properties which

are meaningful and significant, and that

facilitate giving those divided classes a name.

More simply classification is the naming and

„grouping together of phenomena that have

something in common‟ (Blatt et al., 1980, p

13).

Currently only one, two or three fine

grained sedimentary rock properties are ever

chosen for classification. The measurable

properties that any one sedimentary fine

grained rock may possess runs into the tens of

numbers, historically classifications of fine-

grained sediments and sedimentary rocks have

been based on the limited combinations of

several main, perhaps considered by some to

be fundamental (Griffiths, 1967, p31),

properties which include:

1) Texture (Particle size)

2) Degree of Induration

3) Stratification

4) Fissility

5) Chemical Composition

6) Clay Mineralogy

7) Mineral Composition

8) Tectonic Association

9) Environment of Deposition

The first seven criteria have commonly

been used in descriptive classifications,

whereas the final two criteria are of use in

genetic classification systems which group

geological materials of a similar origin

together.

It must be noted that all properties have

some relevance but there will be those that

have a particular relevance to some workers

depending upon their field of research or

industrial application, but the same said

properties may be notably less relevant to other

workers and will be of less use to them for

classificatory purposes.

The following brief summaries of the

various classification schemes are used to

illustrate the highly vacillating, and sometimes

recondite, nature of fine grained rock

classification.

Descriptive Field Classification

For the field description and

classification of rocks perhaps texture, in terms

of particle size, is the most significant

property.

When quantitatively describing a single

particle, it is very clear where that particle

should be placed on the size grade scale. e.g. a

sand particle lies between 63microns and

2mm. However, when it comes to describing


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the particle size distribution of rocks it

becomes more difficult as they are effectively

a mixture, an aggregate, of various sized

particles of differing proportions. In effect

there are no sharp boundaries that exist to

divide between different kinds of sedimentary

rocks but a gradational variation: nevertheless,

it should be noted that in general terms of

particle size, clay is finer grained than mud,

see Figure 2.

Figure 2 a) silt b) mud c) clay. Relative change in particle

size going from the coarsest (silt) to the finest grained

(clay).

Twenhofel (1937) created a

classification, see Table 1, based upon

composition, degree of induration and level of

metamorphism. Mudstone was used by

Twenhofel as a general term to include all fine

grained detrital rocks; whereas shale was

specifically an indurated, fissile, non-

metamorphosed, mud.

Unindurated Indurated

After

Incipient

Metamorphism

Mudstone

Silt Siltstone

Argillite Mud Shale (fissile)

Clay Claystone

Table1. Twenhofel‟s 1937 classification of fine grained

sediments

On the basis of texture and composition

alone, Shrock (1948) proposed a field

classification where a fine grained rock of

unknown composition and indeterminable fine

grained size is designated shale; and a

mudstone, which is a subset of shale along

with claystone and siltstone, is a partly

indurated argillaceous rock which slakes

readily during repeated wetting and drying

cycles. This classification yet again sees the

use of shale as a bucket or reconnaissance term

before detailed identification work is

undertaken.

Alling (1945), Ingram (1953) and

McKee and Weir (1953) all created

classifications based primarily upon the

layering properties of fine grained layered

rocks, be it fissility or stratification, see Tables

2 and 3.

Table 2. Nomenclature of mudrocks containing more than

50% silt and or clay (After Ingram, 1953)

Thickness Stratification

Terms

Splitting

Terms

>120cm Very thickly bedded Massive

60-120cm Thickly Bedded Blocky

5-60cm Thin bedded Slaby

1-5cm Very thinly bedded Flaggy

2mm-1cm Laminated Shaly

<2mm Thinly Laminated Papery

Table 3. Comparison of quantitative terms used in describing layered rocks. (After McKee and Weir 1953)

Stratification is a syndepositional

sedimentary structure, i.e. layering that forms

or is constructed during sedimentation within a

sedimentary bed; whereas fissility is the

tendency of a rock to split along relatively

smooth surfaces which are believed to parallel

to the bedding (Pettijohn, 1975; Tucker, 1991,

p91). Stow (1980) also uses particle size and

fissility as a means of mudrock classification,

see Table 4.

Amount of

silt and clay

No connotation

to breaking

characteristics

Fissile Massive

Silt predominates

Siltrock Siltshale Siltstone

No connotation

as to relative

amounts

Mudrock Mudshale Mudstone

Clay

predominates Clayrock Clayshale Claystone


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Particle size &

proportions Fissile Non-fissile

4-63m, >66% Silt-shale Siltstone

<63m, no proportions Mud-shale Mudstone

<4m, >66% Clay-shale Claystone

Table 4. Classification of mudrocks (After Stow, 1980)

Tucker (1991, p91) only states that a

mudstone is a blocky non fissile rock and that

a shale is usually laminated and fissile.

Krynine (1948) determined that fine

grained rocks were either siltstones or shales,

with the rocks being either gritty or unctuous

to the touch respectively.

There have been many textural

classifications for recent, unconsolidated fine

grained sediments which are based on ternary

diagrams, for example see Figures 3 and 4,

whose end-points are clay, silt and sand

(Trefethen, 1950; Shepard, 1954; Folk, 1954;

Krumbien and Sloss, 1963; Folk et al, 1970

and Flemming, 2000). Although Shephard has

no name for a sand-silt-clay mixture of roughly

equal proportions, others such as Flemming

call this mixture of particle sizes „mud‟ and

unlike the previous multi parameter

classifications given in Tables 2 to 4, the

ternary diagram is a single parameter

classification based upon particle size.

Figure 3. Sediment nomenclature based on sand-silt-clay

ratios. (After Shepard, 1954)

Flemming (2000) believes that his

textural classification also incorporates a

genetic element by distinguishing between

different hydrodynamic regimes, i.e. lines can

be used to delineate energy gradients between

those of a lower energy (i.e. clay dominated

mud) to higher energy levels where silt

dominated mud occurs .

Figure 4. Hydrodynamic nomenclature based on sand-silt-

clay ratios (After Flemming, 2000)

Very few of the sediment ternary

diagrams to extend the terminology to the

sediments‟ respective indurated equivalents:

however, Folk (1974) and MacQuaker and

Adams (2003) both successfully achieve this,

see Figures 5 and 6.

Figure 5. Nomenclature of fine grained rocks (After Folk,

1974)


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It is worth noting that whist Folk sees a

distinction between a claystone, a siltstone and

mudstone, each still contain up to 10% sand.

On the other hand MacQuaker and Adams note

that all rocks (those with greater than 50% of

materials being 63microns or less in size)

which contain a mixture of clay, silt and sand

are mudstones.

Figure 6. Nomenclature of mudstones (After MacQuaker

and Adams, 2003)

Lundegard and Samuels (1980) devised

a simple classification for fine grained rocks

based upon particle size and the presence, or

otherwise, of stratification. Mudstones are

described as non-laminated rocks containing

between 33% and 66% silt; whereas shale, is

reserved as a suffix for laminated rock

materials containing 66% to 33% silt,

(mudshale) and 33% or less of silt (clayshale).

Picard (1971) presented both a field

classification and a laboratory classification of

fine grained sedimentary rocks. In the field

Picard proposes to use textural criteria, namely

particle size. In the field a mudstone is

specifically a rock composed of a mixture of

clay, silt and sand. Under no circumstances is

the term mudstone to be used as a general term

for materials for which the size distribution is

unknown. Picard‟s laboratory classification

builds upon the field-based textural terms

supplementing them using clay mineralogy and

modal analysis of mineral composition.

However, the final resultant name becomes all

rather confusing when the textural root is then

followed by conventional sandstone

compositional terms which in turn is preceded

by clay mineral terms e.g. a silty mudstone,

illite-sublitharenite.

Descriptive Laboratory Classifications

Lewan (1979) attempts a laboratory

classification of very fine grained (less than 5

microns) material only which is based purely

upon textural and compositional criteria.

Mudstones contain 65% to 45% by volume of

microscopic material; whereas shales contain

more than 65% by volume. Shales are further

subdivided into named subclasses of

claystones, marlstones and micstones

depending upon the silicate content of the rock.

The root names, i.e. mudstone or shale

derivative, are given a preceding primary

adjective based upon mineral composition e.g.

calcitic marlstone.

Spear‟s (1980) classification is based

upon the percentage of quartz and fissility, see

Table 5.

%

quartz Fissile Non-fissile

>40 Flaggy siltstone Massive siltstone

30-40 Very coarse shale Very coarse mudstone

20-30 Coarse shale Coarse mudstone

10-20 Fine shale Fine mudstone

<10 Very fine shale Very fine mudstone

Table 5 Classification of fine grained rocks (After Spear,

1980)

In this classification particle size data is

an uncertain quantity and it is far better to use

the quartz content directly as determined by X-


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ray diffraction (XRD). He summarises simply

that „shale is a fissile or laminated rock,

whereas a mudstone is neither fissile nor

laminated‟.

Weaver (1980) was dissatisfied with the

poor definition of rock names and the

inconsistency of terms such as shale and

mudstone, and so did away with both terms

and proposed a simple classification based

upon particle size and the percentage of

phyllosilicates (physils), i.e. clay minerals,

determined by XRD, see Table 6.

Particle size &

proportions >50% Physils <50% Physils

4-63m., >50% Physil Siltstone Physilic Siltstone

<4m, >50% Physil Claystone Physilic Claystone

Table 6. Classification of fine grained rocks (After

Weaver, 1980)

Although very straight forward

Weaver‟s simple and logical classification is

problematic and does not correspond easily to

usage: for example what would happen in the

situation where a fine grained rock containing

more than 50% siliclastic material, less than 63

microns in size, is actually a three component

aggregate of clay, silt and sand, where neither

clay nor silt were in excess of 50% - how

would it be classified?

Figure7. Nomenclature based upon composition (After

Selley, 1988)

Selley (1988), like Weaver, is also unhappy

that the terms mudstone and shale are so

poorly defined in literature and alternatively

uses a composition ternary diagram where the

apices represent pure carbonate, pure clay

minerals and pure organic matter to determine

the nomenclature to be used for fine grained

sedimentary rocks, see Figure 7.

Moore (2005) using a combination of

particle size data, pore size data and

petrographic observations devised a

classification which suggests that fine grained

rocks are either a) floc-dominated whose

structure is supported by the clay matrix or b)

silt or sand-rich mudrocks whose structure is

supported by a silt/fine sand framework.

Genetic Classifications

Some classification systems have

utilised both descriptive and genetic criteria

such as Grabau (1913) where he devised an

elaborate classification system using particle

size and composition but also went further to

further subdivide the classes based on the

agent of formation or environment of

deposition.

Krumbien (1947) who divided fine

grained rocks based upon their aerial extent,

uniformity and lithological associations

produced a tectonic classification of fine

grained materials into „platform‟ and „basin‟

types.

Other classifications which are genetic,

but not specific to fine grained rocks, include

the works of Trowbridge (1914); Weeks

(1952) and Doeglas (1968).


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Discussion

The original question asked – „is there a

difference between a mudstone and a shale?‟

Well it‟s clear that it depends upon who you

ask, as there is no consensus.

Rodgers (1950) felt that the term

mudstone „has been used in so many senses

that it should be abandoned.‟ Whereas it is felt

by Selley (1988) that the term shale „could

perhaps be usefully abandoned by geologists‟.

Since geologists cannot agree on which terms

to preserve and retain, it could be a reason

why Carr and Hibard (1991) did not include

any fine grained rocks in their „open-ended‟

rock classification. The classification is based

upon a rocks textural characteristics and

mineralogical composition. It allows a neural

network computer algorithm to accesses a very

limited database and compare it to any given

unknown sample: but by their own admission

if someone was to examine the database „rocks

such as shales/clays, and siltstones are absent‟

altogether. Not a very practical classification

for fine grained rocks.

Despite the lack of agreement upon

rock names, it would appear that particle size

is being used for classification to the greatest

extent, with indurated rocks having the same

particle size as the unconsolidated materials

from which their name has been derived from,

e.g. silt and siltstone. Although this will work

adequately for material such as sand and

sandstone, it would appear that not much

thought has gone into the significant textural

changes that clay-mineral rich fine grained

materials undergo during burial diagenesis, i.e.

the particle size becomes coarser. Clay-sized

clay minerals notably increase in size and

become silt-sized with an increase in

temperature and pressure during burial

(Weaver, 1990, p10). Put simply, rocks can

be clay-mineral rich but not clay sized.

Also, there seems to be a general

consensus by most that shale is a fissile fine

grained sedimentary rock and that mudstones

are non-fissile. If fissility was taken as a

defining characteristic of fine grained rocks

there is still contention regarding its

applicability. As Aplin and MacQuaker (2010)

state “The word shale...should be used with

caution as it implies fissility”. It is believed by

some that that fissility is a derived property

determined by the degree of weathering and

water content (Ingram, 1953). As all rocks are

massive when subjected to overburden

pressure, it stands to reason that rocks

commonly do not develop fissility until they

have been brought to the surface and

weathered to some degree. Therefore shales, in

the common sense, do not exist in the

subsurface (Weaver, 1990) but laminated fine

grained rocks such as mudstones may be

potential shales.

Wadell (1938) noted that

„sedimentological nomenclature…lacks the

foundation of a rational and logically

developed classification, resulting in an

incomplete, truncated, and inconsistent

nomenclature‟. This is well demonstrated in

the aforementioned non-exhaustive list of

classifications for fine grained sedimentary

rocks.

Pettijohn (1957, p239) stated „The

argillaceous group of sediments… is a hybrid

class… A satisfactory classification of the

materials (has) yet to be worked out.‟ Over 50

years later and a universal classification has

still not been worked out.

Perhaps there will never be a uniform

classification of fine grained sedimentary


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rocks. Various classification schemes have

been and will continue to be created to meet

the specific individual needs of an

investigation, be it for a given region, basin or

stratigraphic unit (Schieber and Zimmerle,

1998) rather than a single universal solution.

Conclusions

For the purposes of future study all fine

grained sedimentary rocks that contain more

than 50% of material finer grained than 63

microns will have the generic group term

argillaceous rocks applied: using the definition

of argillaceous to mean „clay-mineral rich‟

(Millot, 1970, p35).

Ideally any classification should be

comprehensive, scientifically sound,

unambiguous, practically oriented, easy to use

and avoids unclear, undefined terminology.

The advantages of such a classification are the

standardisation in the reporting of results and

their effective communication between users.

We believe that particle size is the most

important parameter to be used for describing

and classifying argillaceous rocks. Therefore

the classification scheme proposed here

subdivides argillaceous rocks into subclasses

of siltstone, mudstone or claystone, based upon

the following particle size definitions and

proportions, see Figure 8.

Figure 8. Classification of argillaceous rocks.

Stratification is also considered to be

an important parameter and in agreement with

Weaver (1980) the presence of fissility will not

be used as criteria for classification, as the

materials being dealt with are from the

subsurface and have not been subjected to any

weathering per se. The terms for sedimentary

stratification which will be applied, are given

below in Table 7, and are based upon the

works of Ingram (1954) and Hallsworth and

Knox (1999).

Thickness of

Layer (mm) Term

No apparent

internal structure Massive

>1000 Very thick bedded

300-1000 Thick bedded

100-300 Medium bedded

30-100 Thin bedded

10-30 Very thin bedded

5-10 Thick laminated

1-5 Medium laminated

0.5-1 Thin laminated

<0.5 Very thin laminated

Table 7. Terminology for sedimentary stratification in

argillaceous rocks.

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