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Home , Arenite, Arkose, Clastic rock, Greywacke, Lutite, Matrix (geology)

47

CHAPTER 3. CLASSIFICATION OF T ERRIGENOUS C LASTIC R OCKS

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 C LASSIFICATION O F S EDIMENT A ND S EDIMENTARY R OCKS

Figure 3-1 shows the the relationship between sedimentary classificaiton and the origin of the that makes up the rocks. All sedimentary rocks are composed of the products of “”, 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 , with lesser amounts of , rock fragments, micaceous and , 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., and zircon) or they may be less stable minerals that comprise a relatively large proportion of the source rock (e.g., the and ). 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 , 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 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 and dolomite) whereas those that are precipitated by organisms, to form their own shell material, are termed biogenic . Biogenic sediment is dominated by calcium carbonate (i.e., they form many or have been diagenetically altered to dolomite) but also include siliceous sediment (e.g., biogenic ) 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

nd Chemical w cal a eath ysi eri Ph ng

Source Rock

solutions solid particles detrital grains TRANSPORT clay Rivers insoluble oxides Wind Glaciers Oceanic currents Volcanic explosions

DEPOSITION Precipitation Cessation of movement

Chemical Sediment Clastic sediment as shell material

direct from solution Terrigenous Orthochemical Biogenic Bioclastic Pyroclastic clastic sediment sediment sediment sediment sediment

reworking

Figure 3-1. Illustration showing the relationship between 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 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 Sediment name Rock name Adjectives (mm)

>2 , , well-sorted, etc.

0.0625 - 2 or coarse, medium, fine, well-sorted, etc.

<0.0625 Mud Mudstone or lutite silt or clay

1For 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

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 (dominated clay size sediment), as determined by examination of specimens in . 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 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 “” or, in German, “grauwacke”; commonly abbreviated as “wacke”). All sedimentary rocks with more than 75% matrix are termed in this scheme. The 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 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

MUDSTONES

WACKES

ARENITES

Quartzwacke 100% Quartz arenite Quartz 75% 5 Subarkose 5 Sublitharenite 25 Arkosic wacke 25 Feldspathic Graywacke Lithic Graywacke 50 Percent matrix (<0.03 mm) 15% Arkosic Feldspars 100% Arenite Lithic Arenite

50%

100%

Rock fragments

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).

A. Total rock B. Quartz, feldspars and rock fragments

Component Proportion Component Proportion1 % %

Quartz 26 Quartz 45 20 Feldspar 34 Rock fragments 12 Rock fragments 21 Matrix 42 (∴ a graywacke) Total: 100 Total:100 Total Q, F, and Rf: 58 The proportions above plot in the field classifying this rock as a feldspathic graywacke (see Fig. 2B).

1Calculated 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 Arkosic 10% ROCK FRAGMENTS } Arenite 90

QUARTZ 40% QUARTZ QUARTZ ARENITE 2 Lithic SUBARKOSE 20% FELDSPAR SUBLITHARENITE 80 40% ROCK FRAGMENTS} Arenite

70

ARKOSE 1 60 FELDSPAR ARKOSIC LITHIC ARENITE ARENITE

50

ROCK FRAGMENTS 1010 10

2020 40 20 2

3030

30

4040 30 30 40

5050 50

6060 20 60

7070 70

8080 10 80

FELDSPAR 9090 90

ROCK FRAGMENTS

B. Classification of graywackes QUARTZ see table 2. 3 45% QUARTZ Feldspathic 34% FELDSPAR 90 graywacke 21% ROCK FRAGMENTS } QUARTZ

QUARTZWACKE 80

70

ARKOSIC WACKE 60

FELDSPAR FELDSPATHIC LITHIC GRAYWACKE GRAYWACKE

50

10 10 3 ROCK FRAGMENTS

20 40 20

30

30

40

30 40

50 50

60 20 60

70 70

80 10 80

FELDSPAR 90 90

ROCK FRAGMENTS

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 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 fragments). The rock names given in figure 3-2 may also be modified to refer to the type of cement; e.g., a 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 -transportation-deposition, it tends to become better sorted ( are said to become “cleaner’ as they lose their 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 erosion- transportation-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 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 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 grains). Thus, we can 53

60

50

40

30

Latitude (degrees north of equator) 20 010 20406080 30 50 70 % 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 “” is applied to any rock that was deposited from a (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 , 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 . A tillite is typically composed of poorly sorted clasts, ranging from mud to . 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 R UDITES

Rudites have not been subjected to as much detailed subdivision as the sandstones. However, 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 ()

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 may indicate considerable dis- rounded clasts. tance of transport from source. The signifi- cance will vary with the of the clast (i.e., limestone clasts will become round a short distance from their source whereas will require much greater transport).

Breccia A rudite composed predominantly of Generally indicates that the clasts have not angular clasts. traveled far from their source or were trans- ported by a non-fluid medium (e.g., gravity or glacial ice).

Diamictite A rudite composed of poorly sorted, mud Commonly refers to sediment deposited from to gravel-size sediment, commonly with glaciers or sediment gravity flows, particularly angular clasts. debris flows.

Note: in the following the rock names are given for rudites consisting of rounded clasts (conglomerates) but the term may be replaced with the term "" if the clasts comprising the rock are angular.

Orthoconglomerate A conglomerate in which all clasts are in Clast-supported framework is typical of grav- (clast-supported conglomerate) contact with other clasts (i.e., the clasts els deposited from water flows in which gravel- support each other). Such conglomer- size sediment predominates. Open framework ates may have no matrix between clasts suggests an efficient sorting mechanism that (open framework) or spaces between caused selective removal of finer grained sedi- clasts may be filled by a matrix of finer ment. Closed framework suggests that the sediment (closed framework). See figure transporting agent was less able to selectively 3-5. remove the finer fractions or was varying in competence, depositing the framework-filling sediment well after the gravel-size sediment had been deposited.

Paraconglomerate A conglomerate in which most clasts are Typical of the deposits of debris flows or water (matrix-supported not in contact; i.e., the matrix supports flows in which gravel size clasts were not conglomerate) the clasts. See figure 3-5. abundant in comparison to the finer grain sizes.

Polymictic conglomerate A conglomerate in which clasts include Conglomerates that include clasts from a wide- several different rock types. variety 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 Suggests that the source area was nearby or made up of only one rock type. source rock extended over wide geographic area.

Intraformational A conglomerate in which clasts are de- conglomerate rived locally from within the deposi- Deposition in an environment where tional basin (e.g., clasts composed of accumulated. Muds were in very close proxim- local muds torn up by currents; such clasts ity to the site of deposition as the clasts would are commonly termed "rip-up clasts" or not withstand considerable transport. "mud clasts").

Extraformational A conglomerate in which clasts are ex- Clasts derived from a distant source. conglomerate otic (i.e., derived from outside the depo- sitional basin). 56

Orthoconglomerates Paraconglomerate { {

Clast-supported Clast-supported Matrix-supported (open framework) (closed framework)

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 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 .

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 All sediment finer than 0.0039 mm. Fissility 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). Argillaceous sediment A sediment containing largely clay-size particles (i.e., >50%). A dense, compact rock (poor fissility) composed of mud-size sediment (low grade , cleavage not developed) Psammite 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

Percentage clay-size 0 - 32 33 - 65 66 - 100 constituents

Field Loamy Fat or slick adjective Gritty

BEDDED BEDDED BEDDED thick

Beds SILT MUD CLAYMUD > 10 mm

LAMINATED LAMINATED LAMINATED SILT MUD CLAYMUD NONINDURATED thick Laminae < 10 mm

BEDDED MUDSTONE CLAYSTONE

thick SILTSTONE Beds > 10 mm

LAMINATED INDURATED MUDSHALE CLAYSHALE SILTSTONE thick Laminae < 10 mm LOW QUARTZ ARGILLITE ARGILLITE

QUARTZ SLATE

METAMORPHOSED PHYLLITE AND/OR Degree of HIGH

Table 3-5B

Thickness Stratification Parting Composition 30 cm Thin 3 cm Slabby Very Bedding thin 10 mm

Thick Flaggy 5 mm

Medium Platy 1 mm

Thin Lamination Fissile

0.5 mm Increasing clay and organic content

Very Increasing sand, silt, and carbonate content thin Papery