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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
Terminology and Classification of Fine Grained Sedimentary Rocks
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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
Terminology and Classification of Fine Grained Sedimentary Rocks
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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
Terminology and Classification of Fine Grained Sedimentary Rocks
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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)
Terminology and Classification of Fine Grained Sedimentary Rocks
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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-
Terminology and Classification of Fine Grained Sedimentary Rocks
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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).
Terminology and Classification of Fine Grained Sedimentary Rocks
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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
Terminology and Classification of Fine Grained Sedimentary Rocks
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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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