LABORATORY 4.docx

LABORATORY 4 Sedimentary Rocks Sedimentary rocks are important because they contain the historical record of ancient environments and life on Earth. Throughout this course we will be studying sedimen- tary rocks, the fossils they contain, and the history that they record. Sediment is loose particulate material, which can form in several ways. Sediment may be derived from the weathering and erosion of preexisting rock (called terrig- enous , or detrital , or clastic sediment). Sediment also may be formed from chemical, biochemical, or biological materials, such as minerals formed by the evaporation of seawater, seashells, or plant remains. Sedimentary rocks are formed when sediment is compacted and cemented together. Approximately 75% of the rocks exposed at Earth’s surface are sedimentary rocks. Sediment accumulates in subaqueous environments, such as lakes, rivers, bays, deltas, beaches, and ocean basins. Sediment also may be deposited in other types of environments, such as deserts or glaciated areas. The characteristics of the sediment (grain size, shape, sorting, and composition), and the sedimentary structures are clues to the environment in which the sediment was deposited. In general, it takes more energy (greater water velocity) to transport larger grains. CLASSIFICATION OF SEDIMENTARY ROCKS Sedimentary rocks are grouped according to their origin: r Terrigenous sedimentary rocks (also called clastic or detrital sedimentary rocks) form from fragments of preexisting rocks. r Chemical and biochemical sedimentary rocks form as chemical precipitates or from the shells of organisms. r Organic sedimentary rocks are composed of organic matter or carbon. TERRIGENOUS (CLASTIC OR DETRITAL) SEDIMENTARY ROCKS Terrigenous sedimentary rocks are derived from preexisting rocks. They are com- posed of rock fragments and mineral grains that have been weathered, eroded, transported, deposited, and cemented together to form a sedimentary rock. They are sometimes referred to as extrabasinal because they are derived from rocks out- side of the basin of deposition. The individual grains (or clasts) in these rocks are mechanically durable (able to withstand abrasion during transport) and chemically stable. Typical clasts are made of quartz, feldspar, muscovite, clay minerals, or rock fragments.

Sedimentary Rocks 87

Sedimentary Rocks 88 Texture Most clastic sedimentary rocks have three textural components (Figure 4.1). 1. Clasts are larger pieces of sedimentary debris (gravel, sand, silt). 2. Matrix is fine-grained material surrounding clasts. 3. Cement is the chemical glue that holds the rocks together. The most common cements are calcite, iron oxide, and silica cement. a. Calcite cement makes any sedimentary rock fizz in hydrochloric acid. b. Iron oxide cement gives the rock a reddish brown, pink, red, or orange color. c. Silica (quartz) cements do not fizz in hydrochloric acid and are not red- dish brown. They are best recognized by process of elimination. Clast Size Most terrigenous sedimentary rocks are classified by the size of the clasts of sediment they contain. The size ranges of sedimentary grains are shown in Tables 4.1 and 4.2. Table 4.1 gives a simplified version to memorize, and Table 4.2 gives a more complex version for reference. Figure 4.1 Clasts and matrix (labeled), and iron oxide cement (reddish brown color surrounding clasts). (Scale in millimeters.) TABLE 4.1 SIMPLIFIED GRAIN SIZE CLASSIFICATION Particle Name Particle Diameter Gravel > 2 mm Sand 1/16 - 2 mm Mud Silt 1/256 - 1/16 mm Clay <1/256 mm Pamela Gore

Sedimentary Rocks 89 TABLE 4.2 CLASSIFICATION OF GRAIN SIZE Particle Name Particle Diameter Textural Term Gravel Boulders > 256 mm Rudite Cobbles 64–256 mm Pebbles 4–64 mm Granules 2–4 mm Sand Very coarse sand 1–2 mm Coarse sand 0.5–1 mm Medium sand Fine sand 0.25–0.5 mm 0.125–0.25 mm Arenite Very fine sand 0.0625–0.125 mm Mud Silt 0.004–0.0625 mm (1/256 to 1/16 mm) Clay < 0.004 mm (<1/256 mm) Argillite Textural Terms Rudites: Sedimentary rocks with gravel-sized clasts are sometimes referred to as rudites. Rudite means “gravel.” Arenites: Arenaceous sedimentary rocks or arenites are those with sand-sized grains. Arenite means “sand.” The word is derived from the sand that covered the floor of the Roman arenas where the gladiators fought. Argillites: Argillaceous sedimentary rocks or argillites are those with mud. (Mud is defined as a mixture of silt and clay.) Argillite means “mud.” Clast Shape Shape of clasts is important in naming sedimentary rocks with gravel-sized clasts. Gravel may be rounded or angular (based on the sharpness of the corners of the clasts). Gravel rapidly becomes rounded in the first few miles of transport. Sorting Sorting refers to the distribution of grain sizes in a rock. If all of the grains are the same size, the rock is well-sorted . If there is a mixture of grain sizes, such as sand and clay, or gravel and sand, the rock is poorly sorted . Rocks with Gravel-Sized Clasts Conglomerate (Figure 4.2) and breccia (Figure 4.3) contain gravel-sized clasts sur- rounded by finer-grained matrix. Conglomerate has rounded clasts. If the particles are angular , the rock is a breccia. In a conglomerate, the larger clasts are generally more rounded than the smaller clasts.

Sedimentary Rocks 90 A. Conglomerate. Figure 4.2 Conglomerate. B. Conglomerate with rounded quartz clasts in a sandy matrix. (Scale in centimeters.) A. Breccia. Figure 4.3 Breccia. B. Breccia with angular clasts of dolostone in a sandy matrix. (Scale in centimeters.) Rocks with Sand-Sized Clasts Sandstones contain sand-sized clasts. Sand grains may be either rounded or angular, and they are generally more or less the same size ( well-sorted ). The sand grains are held together by cement, which may be silica (quartz), calcite , or iron oxide . Sand- stones are classified according to the composition of the sand grains into three main groups (Figure 4.4): Quartz sandstone or quartz arenite is composed mainly of quartz sand grains. Arkose is composed mainly of pink or white feldspar grains, with quartz and generally some muscovite mica or sand-sized rock fragments. Litharenite (meaning “rock-sand”) or lithic sandstone or graywacke is pre- dominantly composed of dark sand-sized rock fragments , with some mica, quartz, and feldspar grains in a clay-rich matrix. A wacke is defined as a “dirty” (or muddy) sand. The term graywacke is best used loosely; there is no strict defi- nition of the term with which all geologists agree. A litharenite is more strictly defined as a rock primarily composed of sand-sized rock fragments. Pamela Gore Pamela Gore

Sedimentary Rocks 91 Pamela Gore Feldspar Quart z ARKOSE QUARTZ SANDSTONE LITHARENITE OR LITHIC SANDSTONE (GRAYWACKE) Rock fragments A. Simplified sandstone classification diagram. B. Quartz sandstone or quartz arenite. (Scale in centimeters.) C. Arkose with pink feldspar grains. (Scale in centimeters.) Figure 4.4 Types of sandstone. D. Litharenite or lithic sandstone, sometimes called graywacke. (Scale in centimeters.) Rocks with Silt-Sized Grains Siltstone is intermediate in texture between sandstone and shale (Figure 4.5). The grains are difficult to see with the naked eye because they are so small, but the rock has a distinct gritty feel to the fingernails. Figure 4.5 Siltstone. (Scale in millimeters.) Pamela Gore Pamela Gore Pamela Gore

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