Download 47 In nature there is a wide variety of sedimentary rocks and each

47
CHAPTER 3. CLASSIFICATION OF TERRIGENOUS CLASTIC ROCKS
In nature there is a wide variety of sedimentary rocks and each type differs from all other types in terms of physical
properties, composition and/or mode of origin. The classification of sedimentary rocks is a necessary exercise
that provides consistent nomenclature to facilitate communication between sedimentologists (i.e., the classification
sets limits to the attributes of any given class) and most classification schemes are based on characteristics that
have some genetic significance. This chapter briefly describes the classification of sedimentary rocks on various
scales and then focuses on a particular class: terrigenous clastic sedimentary rocks.
A FUNDAMENTAL CLASSIFICATION OF SEDIMENT AND SEDIMENTARY ROCKS
Figure 3-1 shows the the relationship between sedimentary rock classificaiton and the origin of the sediment
that makes up the rocks. All sedimentary rocks are composed of the products of “weathering”, the process that
causes the physical and/or chemical breakdown of a pre-existing rock (termed a source rock). These “products”
include detrital grains (chemically stable grains) and material in solution. Detrital grains are normally dominated
by quartz, with lesser amounts of feldspars, rock fragments, micaceous and clay minerals, insoluble oxides, and a
small proportion (normally less than 1%) of what are termed “heavy minerals” because they have a higher density
than the quartz and feldspars. The heavy minerals may be relatively non-reactive to chemical weathering but form
only a small proportion of a source rock (e.g., tourmaline and zircon) or they may be less stable minerals that comprise
a relatively large proportion of the source rock (e.g., the amphiboles and pyroxenes). Rock fragments (syn. lithic
fragments) may include as wide a range of particles as there are source rocks but only fragments composed of
relatively resistant (physically and/or chemically) minerals withstand transport over great distances. Detrital grains
also include some micaceous and clay minerals and insoluble oxides that are formed by chemical reactions on the
surfaces of some minerals during chemical weathering. The micaceous minerals produced by weathering are
relatively unstable. However, clay minerals, dominated by kaolinite, illite and montmorillonite, and insoluble oxides,
including hematite, bauxite, laterite, and gibbsite, are generally very stable. The exact composition of detrital grains
produced by weathering will depend on the relative importance of chemical and physical weathering and the
composition of the source rock.
Sediment formed from the products of weathering are normally deposited following a period of transport to some
site of deposition. The various types of sedimentary rocks may be most fundamentally classified according to the
type of weathering product from which they form: as chemical sediment, composed of material that was transported
in solution and deposited by precipitation from solution, or clastic sediment, that include all of the particulate
products of weathering (i.e., the detrital grains produced by weathering) that are transported to their site of
deposition by a variety of physical processes: by running water (rivers, currents in lakes, seas and oceans), glaciers,
wind, volcanic eruptions (non-igneous rocks produced by explosions and breakage during lava flow), and gravity
(e.g., landslides).
The chemical sediment may be further subdivided according to the specific mode of formation. Sediment that
precipitates directly from solution is termed orthochemical sediment (e.g., halite, gypsum, some limestone and
dolomite) whereas those that are precipitated by organisms, to form their own shell material, are termed biogenic
sediments. Biogenic sediment is dominated by calcium carbonate (i.e., they form many limestones or have been
diagenetically altered to dolomite) but also include siliceous sediment (e.g., biogenic chert) composed of the
exoskeletons of siliceous-shelled organisms (e.g., diatoms).
Clastic sediment may also be divided into subclasses on the basis of their composition and mode of origin. The
most common is the terrigenous clastic sediment, including all sediment composed of detrital grains (derived from
any source rock) that were transported to their site of deposition. Clastic sediment that is derived from the products
of volcanic eruptions is termed pyroclastic sediment. A third, special type, of clastic sediment that spans between
clastic and biogenic sediment is the bioclastic sediment that is composed of reworked biogenic sediment (i.e., shell
material that is reworked by currents). Each of these subclasses of clastic sediment can be subdivided according
48
l and Chemical weathe
a
c
i
ring
ys
Ph
Source Rock
solid particles
solutions
TRANSPORT
Rivers
Wind
Glaciers
Oceanic currents
Volcanic explosions
DEPOSITION
Cessation of movement
Precipitation
Clastic sediment
olu
ms
fro
dir
l
ria
ate
lm
hel
as s
tion
Chemical Sediment
ect
detrital grains
clay
insoluble oxides
Orthochemical
sediment
Biogenic
sediment
Bioclastic
sediment
Terrigenous
clastic
sediment
Pyroclastic
sediment
re
wo
rk
in
g
Figure 3-1. Illustration showing the relationship between sedimentary rock classification and the origin of the sediment
making up the rocks.
49
to a variety of characteristics and the remainder of this chapter will focus on the classification of terrigenous clastic
sediment. However, note that many of the criteria for subdividing terrigenous clastic sediment may also be used
to further subdivide pyroclastic and bioclastic sediment.
CLASSIFICATION OF TERRIGENOUS CLASTIC SEDIMENT
Most widely-used classifications of terrigenous clastic sediment or sedimentary rocks are based on the
descriptive properties of a rock (e.g., grain size, grain shape, grain composition). The classifications summarized
here are largely descriptive but they are based on properties that may have important genetic implications (see
below).
A descriptive classification of any rock may be made at various levels and precision. The classification of
terrigenous clastic sediment and rocks given in Table 3-1 represents the simplest subdivision and is based solely
on grain size (note that the boundaries between sediment/rock types are from the Udden-Wentworth grade scale).
This classification should be considered a “first-order” classification and each class may be further subdivided on
the basis of a variety of characteristics.
Table 3-1. Classification of terrigenous clastic sediment/rocks based on grain size.
Grain size1
(mm)
Sediment name Rock name
Adjectives
Gravel
Rudite
cobble, pebble, well-sorted, etc.
0.0625 - 2
Sand
Sandstone or arenite
coarse, medium, fine, well-sorted, etc.
<0.0625
Mud
Mudstone or lutite
silt or clay
>2
1
For the purposes of this general classification we will assign the rock or sediment name shown if more than
50% of the particles are in the size range shown. More detailed classification schemes will limit terms on
the basis of different proportions of sediment within a give size range (see text).
CLASSIFICATION OF SANDSTONES
Basis of Classification
Sandstones may be further classified on the basis of the composition of the grains and the proportion of the
rock that is fine-grained matrix (dominated clay size sediment), as determined by examination of specimens in thin
section. The major components of most sandstones are: quartz (including chert and polycrystalline quartz),
feldspars, rock fragments and matrix; most other minerals are not sufficiently stable to survive significant transport
and comprise only a small proportion of grains in comparison to the major components, and are neglected in most
classifications. Note that sediment with the composition described is commonly termed siliciclastic sediment.
Several schemes for classifying sandstones have been proposed, based on the relative proportions of the major
components listed above. Figures 3-2 and 3-3 show a classification proposed by Dott (1964), defining the
compositional limits of each subclass of sandstone. Note that in this classification Dott defines matrix as all particles
finer than 0.03 mm; within the range of clay-size particles. This classification limits the term arenite to rocks with
less than 15% matrix while a rock with between 15% and 75% matrix is termed a “graywacke” (also spelled
“greywacke” or, in German, “grauwacke”; commonly abbreviated as “wacke”). All sedimentary rocks with more
than 75% matrix are termed mudstones in this scheme. The arenites and graywackes are further subdivided on the
basis of the relative proportions of their major constituents (excluding matrix) by plotting their relative proportions
on a ternary diagram. Figure 3-2 is rather schematic so take a close look at figure 3-3 to see the limits assigned to
each subclass of arenite and graywacke. According to figure 3-3A a quartz arenite contains no less than 90% quartz
grains and a subarkose contains between 5 and 25% feldspars, less than 25% rock fragments (but the proportion
50
ES
TON
DS
MU
S
CKE
WA
ES
NIT
ARE
Quart
75%
ack
e
z aren
ite
Subark
ose
e
Sublith
arenit
25
e
Ark
osic
w
Quart
zwack
100%
Quart
z
5
5
)
25
ose
Felds
pa
Grayw thic
acke
Ark
50
3
0.0
Lithic
Grayw
acke
trix
mm
(<
a
tm
n
rce
Pe
15%
0% rs
10 spa
ld
Fe
Arkos
ic
Arenit
Lithic
Arenit
e
e
Roc 100%
k fr
agm
ent
s
50%
Figure 3-2. Classification of sandstones. After Dott, 1964, as modified by Potter, Pettijohn and Siever, 1972.
Table 3-2. Example of the treatment of data collected by determining the proportions of quartz (Q), feldspars (F),
rock fragments (Rf) and matrix, as seen in thin section. A. Total composition, including matrix, indicates that the
rock is defined as a graywacke. B. Proportions of quartz, feldspars, and rock fragments "normalized" to 100% so
that the data may be plotted on a ternary diagram (see Fig. 3-2B).
B.
A. Total rock
Component
Proportion
%
Quartz
Feldspar
Rock fragments
Matrix
26
20
12
42
Quartz, feldspars and rock fragments
Component
Quartz
Feldspar
Rock fragments
100
58
45
34
21
(∴ a graywacke)
Total:
Total:
Total Q, F, and Rf:
Proportion 1
%
100
The proportions above plot in the field classifying this rock
as a feldspathic graywacke (see Fig. 2B).
1
Calculated as the proportion of each component in the
total rock divided by the total proportion of quartz,
feldspars and rock fragments (in this example this total is
58).
51
A. Classification of arenites
QUARTZ
1 60% QUARTZ
30% FELDSPAR
10% ROCK FRAGMENTS
90
90
QUARTZ
SUBARKOSE
2 40% QUARTZ
QUARTZ ARENITE
SUBLITHARENITE
20% FELDSPAR
40% ROCK FRAGMENTS
80
80
} Arkosic
Arenite
} Lithic
Arenite
AR
KO
SE
70
70
EN
TS
1
60
60
RA
GM
LITHIC
ARENITE
RO
CK
F
50
50
100
110
2
200
40
40
400
440
300
330
220
30
30
550
500
NT
880
900
FE
990
800
10
10
S
700
770
660
600
20
20
ME
AR
SP
LD
FE
ARKOSIC
ARENITE
RO
CK
AR
FR
SP
AG
LD
B. Classification of graywackes
QUARTZ
see table 2.
3 45% QUARTZ
34% FELDSPAR
21% ROCK FRAGMENTS
90
} Feldspathic
graywacke
QUARTZ
QUARTZWACKE
WA
CK
E
80
AR
KO
SIC
70
NT
S
60
ME
LITHIC
GRAYWACKE
50
20
40
50
50
TS
ME
N
90
FE
90
80
10
80
70
70
60
20
60
40
30
40
30
30
20
10
3
10
RO
CK
AR
FR
SP
AG
LD
FE
FELDSPATHIC
GRAYWACKE
CK
RO
AR
FR
SP
AG
LD
Figure 3-3. Details of the classification of arenites and graywackes as depicted in figure 3-2. Note that the corners of the
triangles represent 100% of the constituent indicated and solid and dashed lines (at 5% intervals) within the ternary
diagrams delineate lines of equal proportion of each component, decreasing to 0% for a given component on the side of the
triangle opposite each corner labelled for that component.
52
of feldspars always exceeds the proportion of rock fragments) and between 50 and 95% quartz. Figure 3-3A also
shows the compositions of two rocks and points, based on the relative proportions of their constituents, plotted
on the ternary diagram. Note that the proportions plotted on a ternary diagram must be recalculated from the original
data describing the total composition of the rock so that quartz, feldspars and rock fragments total 100% (i.e., the
proportions of quartz, feldspars and rock fragments must be “normalized” to 100%; see table 3-2). This procedure
must be applied to all such data that includes any proportion of matrix (i.e., arenites and graywackes).
Note that clastic sediment may contain detrital grains made up of chemical sedimentary rocks (i.e., they have
been eroded from a source rock that was a chemical sediment and subsequently transported to the site of deposition
of the clastic rock in which they occur). Particles derived from chemical sediment are generally relatively unstable
(with obvious exceptions like chert) and do not survive transport to a distant site of deposition and are not
considered here. However, the classification of terrigenous clastic rocks may be more specific than that shown here.
For example, the lithic arenites may be further classified on the basis of the relative proportion of the types of rock
fragments (e.g., proportions of sedimentary, metamorphic or igneous rock fragments). The rock names given in
figure 3-2 may also be modified to refer to the type of cement; e.g., a calcareous quartz arenite would have a calcium
carbonate cement. Howe in these notes we will limit the level of classification to that shown in figure 3-2.
Genetic implications
Rock names based on the relative proportions of their constituents not only provide us with a basis for
systematic classification but also tell us something about the history of the rock.
Textural maturity refers to the maturity of a rock in terms of its grains size distribution and shape. As a population
of sediment undergoes more and more transport, and/or cycles of erosion-transportation-deposition, it tends to
become better sorted (sands are said to become “cleaner’ as they lose their silt and clay fractions) and its’ particles
become rounder and more spherical in shape (see the section on Grain Shape and consider the generalizations made
here in light of all of the constraints on grain shape). A sedimentary rock is said to be mature if it well-sorted and
consists of rounded clasts. Thus, a quartz arenite, with less than 15% matrix, is texturally more mature than a lithic
graywacke (in terms of sorting and also in terms of grain shape; graywackes commonly have more angular grains
than arenites). Clearly, the name applied to a terrigenous sedimentary rock reflects is textural maturity and, therefore,
has implications related to the distance from the source that the sediment was transported prior to deposition and/
or the nature of the source-rock that produced the sediment.
Compositional maturity refers to the relative proportions of stable and unstable grains comprising a sediment
(quartz is the most stable component whereas feldspars and rock fragments are less stable). Like textural maturity
the degree of compositional maturity of a rock increases with transport and number of cycles of erosiontransportation-deposition (i.e., as a sediment matures it loses its less stable components and becomes better sorted).
The unstable grains are destroyed by a variety of processes during weathering and transport: these processes
include physical processes (e.g., removal of unstable minerals by breakage) and chemical processes (e.g., solution
or transformation of unstable minerals to produce clay minerals). For example, the average proportion of feldspars
in igneous and metamorphic rocks is approximately 60% whereas the average proportion of feldspars in sandstones
is 12%. The difference is due to the relative ease with which feldspars may be destroyed by abrasion and/or chemical
weathering, in comparison to quartz that dominates most sandstones, and the fact that source rocks commonly
include older sandstones that have already been through the geologic cycle (maybe several times). Rock fragments
are also generally less stable than quartz grains and so that their proportions are smaller in mature sandstones than
in immature sandstones. As such, a quartz arenite is the most compositionally mature clastic sedimentary rock. The
ultimate formation of a quartz arenite commonly requires several passes through the geologic cycle. Clearly, textural
and compositional maturity go hand in hand, both depending on many of the same factors.
The composition, and therefore the rock name derived from the above classification, will also reflect something
of the nature of the source rock and the tectonic setting of the source area (referred to as the provenance of a
sediment). Taking a very simplistic view, we can think of the feldspars in a sediment as reflecting the contribution
from a granitic source and the rock fragments as reflecting a volcanic or low-rank metamorphic source (these typically
fine-grained rocks tend to produce abundant rock fragments rather than individual mineral grains). Thus, we can
Latitude (degrees north of equator)
53
60
50
40
30
20
0
10
20
30
40
50
60
70
80
% Feldspar
Figure 3-4. Proportion of feldspars in sands plotted against the latitude at which the sands were collected. Data are from
eastern and southern North America as summarized in Pettijohn, Potter and Siever (1973).
make some broad inferences regarding the nature of the source area of a sediment comprising a sedimentary rock,
given its formal name and an understanding of the basis for the name: e.g., an arkose represents a sedimentary rock
with sediment derived from a source area with abundant granitic rocks, a shield area for example. Of course,
knowledge of the specific type of feldspar or the specific composition of the rock fragments will tell much more about
the source rock and the tectonic setting of the source area.
To summarize the above discussion, the class of terrigenous clastic rock, by virtue of its basis on texture and
composition, reflects something of: (1) the intensity of weathering that the material experienced (related to the climate
ad relief of the sources area); (2) the extent of transport that the material has undergone; and (3) the nature of the
source rock (original mineralogy and/or rock type: e.g., igneous, sedimentary or metamorphic) and the tectonic
setting of the source area. To illustrate, consider the data plotted in figure 3-4 which shows a general decrease in
the feldspar content of sands in the southward direction, through eastern and southern North America (these sands
would form arenites, specifically arkosic and quartz arenites, if they were cemented). This southward decrease
reflects several factors. First, in the north the source rocks are dominated by rocks of the Canadian Shield that include
a variety of feldspathic igneous and metamorphic rocks. Such source rocks provide a local supply of feldspars so
that the sands are relatively rich in that mineral. In contrast, to the south there are fewer igneous and metamorphic
source rocks and sediment is derived, to a greater extent, from weathering of pre-existing sedimentary rocks that
have gone through a least one cycle of weathering and lost a proportion of their feldspars. The second factor is
the difference in the style of weathering in the north and south. In the south, a warmer, moist climate facilitates
chemical weathering that readily alters feldspars, producing soluble products and clay minerals. In the north,
physical weathering is more important (especially during the Pleistocene glaciation of the region that originally
produced much of the sand-size sediment in modern rivers of glaciated areas). Thus, the chances of feldspars
surviving weathering are greater in the north. Finally, for the data set described, from north to south, the average
transport distance from the original source tends to increase. The sands in the north are closer to their richest source
of feldspars than the sands in the south that include particles that originated on or near the Canadian Shield but
which have lost much of their feldspar content due to abrasion and further chemical weathering over the great
distance of transport. These are broad generalizations and the extensive scatter of points in figure 3-4 reflects the
complex interaction of these and other factors.
As noted earlier, other minerals only rarely make up more than a few percent of terrigenous clastic sediment but
these may be of great interpretive importance. For example, a sandstone may consist of a relatively large portion
of detrital carbonate, such as limestone or dolomite particles, derived from a carbonate source rock. However, these
grains will be destroyed within a short distance of transport from their site of origin. Thus, the presence of detrital
54
carbonate grains in a sediment reflects close proximity to exposed carbonate rocks at the time that the sediment was
deposited.
Level of classification
How specifically a rock is classified depends on the purpose of the study for which the classification is made.
In many cases classification based only on grain size will be adequate (especially if the origin of the sediment particles
is not of interest). However, there are many different types of study that require a more detailed classification. In
studies that aim to delineate the geological history of a region the identification of the various classes shown in
figure 3-2 will help with the interpretation of aspects of the nature of the source rock and source area and the extent
of transport from the site of weathering. In another situation a sedimentologist may be required to provide
information to engineers who are planning to excavate or drill through sedimentary rocks. In this case the
classification based on composition will be necessary to determine the cost of the work in terms of time required
and the type of excavating or drilling apparatus that must be used. Both time and equipment influence the cost of
such a project so that a sedimentologist must conduct the necessary petrographic analyses to describe the rock
and give it a name (that reflects its’ composition). For example, an arkosic graywacke will contain a smaller proportion
of quartz than a quartz arenite. Because the quartz grains, that make up more than 95% of a quartz arenite, are harder
than the matrix and feldspars that make up a relatively large proportion of a feldspathic graywacke, the cost of
excavating or drilling an arkosic graywacke may be less than for the quartz arenite.
Note on genetic classification of sedimentary rocks
It is worth commenting here that some rock and sediment names that are commonly used are based on the mode
of origin of the rock (i.e., based on a genetic classification). The broad classification into clastic and chemical
sediment described at the beginning of this chapter is such a genetic classification. The classification of sandstones
is descriptive but those rocks may also be classified according to their origin at very specific levels. For example,
the term “turbidite” is applied to any rock that was deposited from a turbidity current (a type of sediment gravity
flow). A turbidite may be composed of carbonate or siliciclastic sediment that may range in grain size from gravel
to mud, but will contain a certain arrangement of internal structures and will occur in a particular stratigraphic context.
Hence, the term turbidite is largely independent of the fundamental properties of the rock and is defined in terms
of the mode of origin of the rock. The term “tillite” is another rock name based on mode of origin: a rock deposited
as glacial till. A tillite is typically composed of poorly sorted clasts, ranging from mud to boulders. Therefore, the
classification of a rock as a tillite requires a knowledge of the overall depositional environment that can only come
from a regional study of the tillite and associated rocks. In contrast, descriptive classifications of rocks may be made
equally well in the field, in the original stratigraphic context, or in hand specimens where the stratigraphic context
may not be known. In any study of a suite of sedimentary rocks it is usually advisable to classify rocks according
to their descriptive properties, at least in the beginning, possibly later classifying them on a genetic basis when
the depositional setting is better understood.
CLASSIFICATION OF RUDITES
Rudites have not been subjected to as much detailed subdivision as the sandstones. However, rudites may
be further classified on the basis of shape, packing and the composition of the lithic fragments that dominate this
class of terrigenous clastic sediment. Table 3-3 reviews the classification of rudites by summarizing the common
terminology, including a brief description of the distinctive characteristics and possible genetic significance of each
type of rudite. This classification is largely descriptive but in some cases the basis includes an understanding of
the genesis of the clasts (e.g., the intraformational and extraformational rudites). While this discussion of the
classification of rudites is limited to the broad generalizations contained in Table 3-3, it is important to realize that
the concepts of textural and compositional (lithological ) maturity apply to rudites in a manner similar to sandstones.
CLASSIFICATION OF LUTITE (SHALE)
A detailed treatment of the classification of lutite, that is dominated by the fine-grained clay minerals produced
by weathering, is beyond the scope and purpose of these notes. The definitions given in Table 3-4 should be learned
55
Table 3-3. Definition of terms used to classify rudites.
Term
Distinguishing Characteristics
Genetic Significance
Conglomerate
A rudite composed predominantly of
rounded clasts.
Rounded clasts may indicate considerable distance of transport from source. The significance will vary with the lithology of the clast
(i.e., limestone clasts will become round a short
distance from their source whereas quartzite
will require much greater transport).
Breccia
A rudite composed predominantly of
angular clasts.
Generally indicates that the clasts have not
traveled far from their source or were transported by a non-fluid medium (e.g., gravity or
glacial ice).
Diamictite
A rudite composed of poorly sorted, mud
to gravel-size sediment, commonly with
angular clasts.
Commonly refers to sediment deposited from
glaciers or sediment gravity flows, particularly
debris flows.
Note: in the following the rock names are given for rudites consisting of rounded clasts (conglomerates) but the term
conglomerate may be replaced with the term "breccia" if the clasts comprising the rock are angular.
Orthoconglomerate
(clast-supported conglomerate)
A conglomerate in which all clasts are in
contact with other clasts (i.e., the clasts
support each other). Such conglomerates may have no matrix between clasts
(open framework) or spaces between
clasts may be filled by a matrix of finer
sediment (closed framework). See figure
3-5.
Clast-supported framework is typical of gravels deposited from water flows in which gravelsize sediment predominates. Open framework
suggests an efficient sorting mechanism that
caused selective removal of finer grained sediment. Closed framework suggests that the
transporting agent was less able to selectively
remove the finer fractions or was varying in
competence, depositing the framework-filling
sediment well after the gravel-size sediment
had been deposited.
Paraconglomerate
(matrix-supported
conglomerate)
A conglomerate in which most clasts are
not in contact; i.e., the matrix supports
the clasts. See figure 3-5.
Typical of the deposits of debris flows or water
flows in which gravel size clasts were not
abundant in comparison to the finer grain
sizes.
Polymictic conglomerate
A conglomerate in which clasts include
several different rock types.
Conglomerates that include clasts from a widevariety of source rocks, possibly derived over
a wide geographical area or a smaller but
geologically complex area.
Oligomictic conglomerate
A conglomerate in which the clasts are
made up of only one rock type.
Suggests that the source area was nearby or
source rock extended over wide geographic
area.
Intraformational
conglomerate
A conglomerate in which clasts are derived locally from within the depositional basin (e.g., clasts composed of
local muds torn up by currents; such clasts
are commonly termed "rip-up clasts" or
"mud clasts").
Extraformational
conglomerate
A conglomerate in which clasts are exotic (i.e., derived from outside the depositional basin).
Deposition in an environment where muds
accumulated. Muds were in very close proximity to the site of deposition as the clasts would
not withstand considerable transport.
Clasts derived from a distant source.
56
Orthoconglomerates
Paraconglomerate
{
{
Clast-supported
Clast-supported
(open framework)
(closed framework)
Matrix-supported
Figure 3-5. Schematic illustrations of orthoconglomerates and paraconglomerate. Refer to Table 3-3.
in order to begin to understand nomenclature that has developed around this class of terrigenous clastic sediment
and sedimentary rocks. Table 3-5 outlines several descriptive properties of lutite and offers a descriptive
terminology. Table 3-5A summarizes a detailed classification of lutites that is promoted by Potter, Maynard and
Pryor (1980), based on the composition and bedding characteristics of lutite. Note that the term indurated (Table
3-5A) refers to any rock that is hardened by pressure and/or cementation; an indurated sediment is a rock and a
non-indurated sediment is an unconsolidated sediment. Table 3-5B summarizes the terms used to describe the
layering (stratification) of lutite and the manner in which a lutite “parts” or breaks along planes that are parallel to
primary bedding. Lutites, in particular, are characterized by their parting which is well-developed due to the parallel
alignment of platy minerals along the bedding planes (rendering the bedding planes particularly weak and termed
“parting planes”). In Table 3-5B “thickness” refers to the thickness of slabs of lutite that break along parting planes.
For those with additional interest the book by Potter, Maynard and Pryor (1980) is an invaluable text on the topic
of lutites.
Table 3-4. Definition of terms used to desribe mudrocks.
Term
Definition
Shale
The general term applied to this class of rocks (> 50% of particles are finer than
0.0625 mm).
Lutite
A synonym for "shale".
Mud
All sediment finer than 0.0625 mm. More specifically used for sediment in which
33-65% of particles are within the clay size range (<0.0039 mm).
Silt
A sediment in which >68% of particles fall within the silt size range (0.0625 - 0.0039
mm).
Clay
Fissility
Mudstone
Argillaceous sediment
Argillite
Psammite
Siltstone
All sediment finer than 0.0039 mm.
Refers to the tendency of lutite to break evenly along parting planes. The greater
the fissility the finer the rock splits; such a rock is said to be "fissile".
A blocky shale, i.e., has only poor fissility and does not split finely (see table 5).
A sediment containing largely clay-size particles (i.e., >50%).
A dense, compact rock (poor fissility) composed of mud-size sediment (low grade
metamorphic rock, cleavage not developed)
Normally a fine-grained sandstone but sometimes applied to rocks of predominantly
silt-size sediment.
A rock composed largely of silt size particles (68-100% silt-size)
57
Table 3-5. A. Classification of lutite. B. Terminology for stratification and parting in lutites. From Potter, Mayard
and Pryor (1980).
Table 3-5A
BEDDED
CLAYMUD
LAMINATED
MUD
LAMINATED
CLAYMUD
MUDSTONE
CLAYSTONE
MUDSHALE
CLAYSHALE
INDURATED
METAMORPHOSED
LAMINATED
SILT
BEDDED
SILTSTONE
LAMINATED
SILTSTONE
Degree of metamorphism
> 10 mm
thick
BEDDED
MUD
BEDDED
SILT
< 10 mm
thick
Fat or slick
> 10 mm
thick
Loamy
< 10 mm
thick
Gritty
Beds
Field
adjective
NONINDURATED
66 - 100
Laminae
33 - 65
Beds
0 - 32
Laminae
Percentage
clay-size
constituents
LOW
QUARTZ
ARGILLITE
ARGILLITE
QUARTZ
SLATE
SLATE
PHYLLITE AND/OR MICA SCHIST
HIGH
Table 3-5B
Very
thin
Slabby
10 mm
Thick
Flaggy
Medium
1 mm
Thin
Lamination
5 mm
Platy
Fissile
0.5 mm
Very
thin
Papery
Composition
Increasing sand, silt, and carbonate content
Thin
3 cm
Parting
Increasing clay and organic content
Stratification
Bedding
Thickness
30 cm