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 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 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 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 Sediment name Rock name Adjectives (mm) >2 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 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 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 MUDSTONES WACKES ARENITES Quartzwacke 100% Quartz arenite Quartz 75% 5 Subarkose 5 Sublitharenite 25 Arkosic wacke 25 Feldspathic Graywacke Lithic Graywacke 50 Arkose 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