Understanding Sedimentary Rocks: Formation, Types & Clasts

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

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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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Sedimentary Rocks 92 Pamela Gore Clay-Dominated Rocks Shale and claystone are fine-grained sedimentary rocks composed of tiny (<1/256 mm) grains of clay minerals, mica, and quartz. The individual grains are too small to see with the naked eye or a hand lens, and the rock feels smooth to the touch, not gritty. Shale and claystone differ in the way that they break (Figure 4.6). Shale is fissile . This means that it splits readily into thin, flat layers. Claystone, on the other hand, is massive, and it breaks irregularly. r The color of shale or claystone can reveal something about its composition. Black shales contain organic matter. They are sometimes called bituminous shales. Red shales contain iron oxide. Kaolin , a white claystone, is composed of the mineral kaolinite. It lacks organic matter and iron oxide. Mud Mud is defined as a mixture of silt and clay. Rocks with both silt and clay are referred to as mudstones or mudshales , depending on whether or not they are fissile. A. Shale is fissile . It splits into ﬂat layers. (Scale in centimeters.) B. Claystone is massive. White claystone is called kaolin. Sandersville, Georgia. (Scale in centimeters.) Figure 4.6 Clay-dominated rocks: shale and claystone. CHEMICAL AND BIOCHEMICAL SEDIMENTARY ROCKS Chemical, biochemical, and organic sedimentary rocks are sometimes referred to as intrabasinal because they form within the basin of deposition, rather than being trans- ported into it. They include chemical precipitates (such as travertine, rock salt, and gypsum), as well as the accumulated remains of organisms that lived within the basin (such as limestones composed of fossil shells). We divide chemical and biochemical sedimentary rocks into four groups: evaporites, siliceous rocks, ironstones, and car- bonate rocks. Evaporites Evaporites are chemical precipitates , which form by precipitation of dissolved miner- als from water during evaporation. There are numerous evaporites, but we concentrate on three: travertine, rock gypsum, and rock salt. Pamela Gore

Sedimentary Rocks 93 Pamela Gore Travertine (Calcite, CaCO 3 ) Travertine forms by evaporation of cave, spring, or river waters. It consists of inter- grown calcite crystals, and it fizzes in hydrochloric acid. Travertine is a dense, crys- talline rock with tan and white bands. It is especially common in limestone caverns, where it forms flowstone and dripstone, including stalactites and stalagmites , recog- nized by their cylindrical shape and internal tree ring-like appearance. Travertine that forms around springs is a more porous, light-colored rock that is sometimes called tufa (Figure 4.7). Because travertine is composed of calcite, it is also mentioned later with the carbonate rocks. A. Laminated travertine in a stalactite. (Scale in centimeters.) Figure 4.7 Travertine. B. Travertine with vague layering and large pore spaces from a hot spring deposit. Travertine is commonly used as tile and decorative stone. (Pencil for scale.) Rock Gypsum (Gypsum, CaSO 4 t2) 2 O) Rock gypsum is softer than your fingernail (you can scratch it). It can be granular, can be glassy with obvious cleavage, or can have a fibrous, satiny luster. It ranges from colorless to white to pink (Figure 4.8). Gypsum forms by precipitation from seawater after about 80% of the water has evaporated. Gypsum is altered to anhydrite (CaSO 4 ) by removal of water, which is generally caused by burial to depths greater than several hundred meters. For purposes of this lab, you do not need to distinguish between gypsum and anhydrite. Figure 4.8 Laminated gypsum, Castile Formation (Permian), Carlsbad, New Mexico. (Scale in centimeters.) Pamela Gore Pamela Gore

Sedimentary Rocks 94 Rock Salt (Halite, NaCl) Rock salt is a glassy, crystalline rock that is easily recognized by its salty taste. It ranges from colorless, to white, gray, pink, or orange, due to impurities (Figure 4.9). Halite forms by precipitation from sea water after about 90% of the water has evaporated. Figure 4.9 Rock salt. (Scale in millimeters.) Siliceous Sedimentary Rocks Siliceous rocks are dominated by silica (SiO 2 ), which precipitates from solution within the basin of deposition. (They do not include quartz sandstones, which are extrabasinal in origin.) The most common siliceous sedimentary rocks are diatomite and chert. Diatomite Diatomite (diatomaceous earth) is a soft, white, powdery rock of low density, composed of the siliceous (silica) skeletons of microscopic algae called diatoms (Figure 4.10). Diatomite can be distinguished from chalk because it does not react with hydrochloric acid. It can be distinguished from kaolinite by its low density. Diatomite floats on water. Chert (Microcrystalline Quartz, SiO 2 ) Chert is a very fine grained silica sediment of chemical or biochemical origin. Some chert contains siliceous skeletons of microorganisms known as radiolarians , which can be seen in thin section (Figure 4.11). Other chert forms through the replacement of limestone, often preserving carbonate textures such as oolites, although the rock has been completely altered to silica. Figure 4.10 Diatomite. (Scale in millimeters.) Figure 4.11 Chert. (Scale in centimeters.) Pamela Gore Pamela Gore Pamela Gore

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Sedimentary Rocks 95 Pamela Gore Chert can be recognized by its extremely fine grain size, smooth feel, and hard- ness (it scratches glass). Chert may be black, white, tan, gray, or greenish gray. A red variety is called jasper. Flint is sometimes used as a synonym for chert, but the term is used loosely and is best reserved for artifacts, such as arrowheads. Sedimentary Ironstones Some sedimentary rocks are dominated by iron-bearing minerals such as hematite. Common examples of sedimentary ironstones include banded iron formations , oolitic hematite or oolitic ironstone , and iron oxide concretions , typically in sand (Figure 4.12). Banded iron formations consist of layers of red chert (jasper) containing oxidized iron, alternating with gray, unoxidized iron (hematite or magnetite). Banded iron formations are Precambrian (Paleoproterozoic and Archean) in age, 2.6 to 1.8 billion years old. A. Banded iron formation. This specimen also has a few layers of yellow tiger’s eye or silicified asbestos. (Scale in centimeters.) B. Oolitic ironstone, Silurian Red Mountain Formation, Birmingham, Alabama. (Scale in centimeters.) C. Iron oxide concretion, Lumpkin, Georgia. (Scale in centimeters.) Figure 4.12 Sedimentary ironstones (Continued) . D. Road cut through oolitic ironstone of the Silurian Red Mountain Formation, Birmingham, Alabama. (Geology students for scale.) E. Pamela Gore Pamela Gore Pamela Gore

Sedimentary Rocks 96 Pamela Gore 3 F. Dark, irregular layer of iron oxide cemented sand and concretions overlying white Providence Sand (Cretaceous). Iron layer is 10 to 15 cm thick. (Lumpkin, Georgia.) G. Iron oxide concretions are commonly hollow and irregularly shaped. (Scale in millimeters.) Figure 4.12 Sedimentary ironstones. H. Iron oxide concretion, Hyattsville, Maryland. (Scale in centimeters.) Carbonate Rocks Limestone and dolostone are called carbonate rocks because they are composed of carbonate minerals, which have a CO 2- group in their chemical formula. The carbon- ate minerals include calcite and aragonite (both CaCO 3 , calcium carbonate) and dolo- mite (CaMg(CO 3 ) 2 , calcium-magnesium carbonate. Note that calcite and aragonite are polymorphs of calcium carbonate. Calcite is the stable form, and aragonite is metastable . Aragonite alters to calcite over a long time. (It is not necessary to distinguish calcite from aragonite in the lab.) Rocks that contain abundant calcium carbonate are often referred to as calcareous rocks (derived from the word calcite ). Limestone is primarily composed of the minerals calcite and aragonite. Lime- stones are generally gray, but may be tan, pink, white, black, or other colors. Dolostone is primarily composed of the mineral dolomite . Weathered surfaces of dolostones are commonly yellowish or brownish gray because of the presence of Pamela Gore Pamela Gore

Sedimentary Rocks 97 small amounts of iron associated with the magnesium in dolomite. Dolomite results from the addition of magnesium to calcium carbonate sediments after deposition, in arid climates. Magnesium chemically replaces some of the calcium in calcite to turn a limestone into a dolostone. Carbonate rocks and minerals are easy to identify because they react with hydro- chloric acid. Limestone effervesces (fizzes) readily in hydrochloric acid. Dolostone fizzes weakly in hydrochloric acid, but only after it has been powdered. Carbonate rocks most commonly form in warm shallow seas in places such as the Florida Keys and the Bahamas (Figure 4.13). Figure 4.13 Carbonate tidal ﬂats in the Florida Keys. Calcareous algae are plants with calcium carbonate skeletons. They live in warm shallow seas in areas such as Florida Bay (Figure 4.14). When calcareous algae die, the organic tissues decompose, releasing fine particles of calcium carbonate (aragonite) that accumulate to form lime mud. Lime mud undergoes recrystallization to form fine-grained limestone or micrite , which is an abbreviation for micr ocrystalline calc ite . Figure 4.14 Calcareous algae from Florida Bay. Note the large holdfast (root ball) at the bottom. Penicillus, the “shaving brush algae,” is on the left. Halimeda is on the right. (Scale in centimeters.) Pamela Gore Pamela Gore Pamela Gore Pamela Gore

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Sedimentary Rocks 98 Textures of Carbonate Rocks Terrigenous rocks are composed of clasts, matrix, and cement. Carbonate rocks have similar textural components: allochems, matrix, and spar. Allochems are particles in carbonate rocks, a term that is analogous to clasts in terrigenous rocks. Types of allochems are intraclasts, oolites, fossils, and pellets. Matrix is microcrystalline calcite or lime mud. Spar is calcite cement. The textures of carbonate rocks are best studied in thin sections (thin slices of rock mounted on glass microscope slides. Classification of Carbonate Rocks There are numerous types of carbonate rocks, and they can be classified by their tex- tures and the allochems that they contain. There are several different classification schemes for carbonate rocks. One of the simpler classifications is based on texture, and uses the following terms: Calcirudite: Limestone dominated by gravel -sized particles Calcarenite: Limestone dominated by sand -sized particles Calcisiltite: Limestone dominated by silt -sized particles Calcilutite: Limestone dominated by mud - or clay -sized particles This classification is fairly general, and it does not specify anything about the types of allochems present. Types of Carbonate Rocks Micrite is fine-grained limestone (Figure 4.15). The color of micrite ranges from gray to tan or other colors. Micrite is an abbreviation for micr ocrystalline calc ite . Micro- crystalline refers to the texture, which consists of clay-sized particles of lime mud. Basically, this is a rock that is all matrix with no allochems (larger particles) or spar (calcite cement). Calcilutite is another name for a limestone dominated by lime mud. Fossiliferous limestone , sometimes called skeletal limestone , contains fossils , or the remains of ancient plants, animals, or algae. Many organisms have calcareous shells or skeletons, and their remains can accumulate in lime mud to form fossiliferous limestone (Figure 4.16). Coquina is a type of fossiliferous limestone made up of fossil shells with little or no matrix (Figure 4.17). It is porous and light-colored, and the shells are commonly broken, abraded, and fairly well-sorted. The shells are gravel sized (>2 mm), and co- quina is a calcirudite. A. Gray micrite with brown dolostone layers. (Scale in centimeters.) Figure 4.15 Micrite B. Outcrop of micrite. (Geology students for scale.) What type of fold is this? Pamela Gore Pamela Gore

Sedimentary Rocks 99 A. Fossiliferous limestone (Ordovician), northwestern Georgia. (Scale in centimeters.) Figure 4.16 Fossiliferous limestone. B. Fossiliferous limestone with brachiopod fossils, western Maryland. (Scale in centimeters.) Figure 4.17 Coquina, St. Augustine, Florida. (Scale in centimeters.) Chalk is a type of fossiliferous limestone made up entirely of microscopic shells (Figure 4.18). These tiny shells are called coccoliths and are the remains of planktonic marine algae called coccolithophores . Coccoliths are typically too small to see using an ordinary light microscope (although they may be seen using an oil immersion lens). They are generally viewed using an electron microscope. The texture of chalk is similar to that of micrite or calcilutite, but chalk is white, less dense, and less compact than micrite. Chalk may be distinguished from other white fine-grained sedimentary rocks (such as kaolinite or diatomite) because it fizzes readily in hydrochloric acid. Figure 4.18 Chalk is white and fine-grained. It can be distinguished from diatomite and kaolinite because chalk fizzes in hydrochloric acid. Specimen is about 10 cm wide. Pamela Gore Pamela Gore Pamela Gore Pamela Gore

Sedimentary Rocks A. Oolitic limestone. (Scale in centimeters). Figure 4.19 B. Oolitic limestone with black oolites. (Scale in millimeters). Oolitic limestone is composed of oolites (Figure 4.19). Oolites are tiny concen- tric spheres of calcium carbonate that range between 0.1 and 2.0 mm in diameter. On a cut or broken surface they look circular, and internal concentric laminations may be seen with a hand lens or microscope. Oolites are not fossils! They form by the precipi- tation of aragonite under certain conditions in warm shallow seas (or salt lakes) under the influence of blue-green algae. Because oolites are less than 2 mm in diameter (sand sized), oolitic limestone is calcarenite . Structures that resemble oolites and that are larger than 2 mm in diameter are called pisolites . Intraclastic limestone is a limestone conglomerate or breccia with a lime mud matrix (Figure 4.20). Intraclasts are flat, gravel-sized chips of limestone that form when carbonate tidal flats that are covered by lime mud dry up, experience cracking, and break into flat, gravel-sized chips. These chips of lime mud are redistributed by the tides and currents, and they accumulate to form intraclastic limestone. Intraclasts may be internally layered, reflecting the layering in the tidal flat sediments. Intraclastic limestone is calcirudite . Pelleted limestone is dominated by pellets , which are small (<1 mm) aggregates of microcrystalline calcite, many of which are fecal in origin (excrement of marine in- vertebrates, such as clams and worms). Unlike oolites, they have no internal structure. Pellets are so small that they generally cannot be seen in hand specimens, but they can be seen in thin sections using a microscope. Crystalline limestone generally consists of a coarse mosaic of intergrown calcite crystals, resulting from the post-depositional alteration of some other type of lime- stone. Allochems might or might not be visible. 1 mm Pamela Gore Pamela Gore

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Sedimentary Rocks INTRACLASTS MATRIX Figure 4.20 Intraclastic limestone. (Scale in centimeters.) Travertine is a dense crystalline limestone with tan and white color bands that forms in caves or hot springs (Figure 4.21). It consists of a mosaic of intergrown calcite crystals. In lab, stalactites and stalagmites can be recognized by their cylindrical shape and internal tree ring-like appearance. Texturally, travertine is essentially a carbonate rock made up entirely of spar (calcite cement). Figure 4.21 Travertine in a stalactite. (Scale in millimeters.) Figure 4.22 Interlayered limestone (gray) and dolostone (tan), Conococheague Limestone (Cambrian), Clear Spring, Maryland. (Scale in centimeters.) Dolostone is made up of the mineral dolomite, a calcium-magnesium carbonate (Figure 4.22). Most dolostones form by the chemical replacement of calcium car- bonate through the action of magnesium-rich fluids. Magnesium chemically replaces some of the calcium in calcite to turn a limestone into a dolostone. A dolostone might retain some of the texture of the original limestone, but it is typically dense and com- pact, with a fine-grained texture. Dolomite fizzes weakly in hydrochloric acid, and only after the rock is scratched or powdered. Dolomite is harder than calcite. Pamela Gore Pamela Gore Pamela Gore Pamela Gore

Pamela Gore Pamela Gore Sedimentary Rocks ORGANIC SEDIMENTARY ROCKS: COAL Coals are organic sedimentary rocks composed primarily of carbon-rich organic mate- rial derived from the remains of plants. Unlike the other sedimentary rocks, coals are not mainly composed of minerals. Minerals are by definition in organic, and coal is mostly made of organic matter. Peat is sediment composed of plant fragments. Coal is its lithified equiva- lent. The plant fossils in coal generally indicate deposition in fresh-water swamps. Peat is transformed by burial pressure and temperature to lignite (a soft, black or brownish, coal-like material). Lignite alters to sooty bituminous coal with greater depth and duration of burial, and higher temperatures. With increasing metamorphism (such as proximity to an igneous intrusion, or increasing tempera- tures and pressures associated with burial), bituminous coal alters to anthracite coal , a hard, shiny coal (Chart 4.1). Geologists consider anthracite to be, in fact, a metamorphic rock. Chart 4.1 Transformation of Peat to Anthracite Coal Peat Lignite Bituminous coal Anthracite coal (metamorphi c) Classification of Sedimentary Rocks Sedimentary rocks can be classified by whether or not they have visible grains. Sedi- mentary rocks without visible sedimentary grains can be classified by whether or not they fizz in hydrochloric acid. Table 4.3 lists rocks with visible grains. Table 4.4 lists rocks that do not have visible grains. Instructions: Examine the sedimentary rock and make a series of decisions to determine the rock name. First, determine whether the grains in the rock are clearly visible. If so, use Table 4.3. Next, determine whether the clasts or allo- chems are larger or smaller than 2 mm. Then, read the descriptions to identify the sedimentary rock. If grains are not readily visible use Table 4.4. Test the rock with hydrochloric acid to see if it fizzes readily, if it only fizzes after it has been scratched and powdered, or if it does not react at all with hydrochloric acid. Then, read the descriptions to identify the sedimentary rock. Pamela Gore Pamela Gore

Sedimentary Rocks TABLE 4.3 SEDIMENTARY ROCKS WITH VISIBLE GRAINS Grain Size Description Sedimentary Rock Name Grain s visibl e Clasts or allochem s larger than 2 mm Grains are all shell fragments; no mud; fizzes in hydrochloric acid Coquina Clasts and matrix are fine grained; clasts are limestone and may be ﬂat and laminated; fizzes in hydrochloric acid Intraclastic limestone Matrix color variable; multiple clast lithologies; clasts differ from matrix in color or composition Breccia (angular clasts) Conglomerate (rounded clasts) Clasts or allochems smaller than 2 mm White or colorless grains, mostly quartz Quartz sandstone Contains pink, gray, or white feldspar (look for cleavage); feldspar grains may be weathered to white kaolinite Arkose Contains rock fragment grains, mostly dark green or gray grains (such as basalt or shale fragments) Litharenite or lithic sandstone or graywacke Round grains with concentric laminations, fizzes in acid Oolitic limestone Dark red to brown, red-brown streak, may contain replaced oolites or fossils, may fizz in hydrochloric acid, may be dense and heavy Oolitic ironstone or fossiliferous ironstone or iron oxide concretions TABLE 4.4 SEDIMENTARY ROCKS GRAINS THAT ARE NOT READILY VISIBLE Grain Size Reaction to Acid Description Sedimentary Rock Name Grain s not visibl e Fizzes in HCl acid White, soft, and powdery Chalk Gray, black, brown or tan; compact; dense; very fine grained (clay-sized) Micrite or calcilutite Fossils in lime mud matrix Fossiliferous limestone Coarse crystalline mosaic; brown and white color bands; may be cylindrical (stalactite or stalagmite) Travertine Fine to coarse crystalline mosaic; compact, dense, massive Crystalline limestone Fizzes in acid when scratched and Gray or black, weathers yellowish gray to brown; compact, dense, massive; dolomite Dolostone

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Sedimentary Rocks powdered (Continued)

Sedimentary Rocks TABLE 4.4 (Continued) Grain Size Reaction to Acid Description Sedimentary Rock Name Grain s not visibl e Does not fizz in acid Fissile (breaks into thin layers); may be softer than fingernail; clay-sized texture; commonly gray, black, brown or red Shale Feels gritty to the fingernails; commonly gray, black, brown, or red Siltstone Salty taste, may feel slippery; often clear and transparent; cleavage Rock salt Softer than fingernail; white, pink, clear; may be fibrous, fine- grained, or crystalline Rock gypsum Hard - scratches glass; opaque; color variable; smooth feel; may have conchoidal fracture Chert White; looks like chalk but does not fizz; very low density (may ﬂoat) Diatomite White; looks like chalk but does not fizz; dense (does not ﬂoat); may stick to moistened finger Kaolinite Black, bright and shiny (may almost look metallic in luster), compact, low density Anthracite coal Black, may leave sooty marks on fingers or paper, may have layers Bituminous coal Brown to black, crumbly, very soft; porous Lignite Brown, porous, soft; pieces of plants may be visible Peat

Sedimentary Rocks Summary Here is a list of the major rock types mentioned in this lab . Terrigenous (or Clastic, or Detrital) Sedimentary Rocks 1. Conglomerate 2. Breccia 3. Sandstone r Quartz sandstone or quartz arenite r Arkose r Litharenite or lithic sandstone or graywacke 4. Siltstone 5. Shale 6. Claystone 7. Mudstone 8. Mudshale Chemical and Biochemical Sedimentary Rocks EVAPORITES 1. Travertine 2. Rock gypsum 3. Rock salt SILICEOUS SEDIMENTARY ROCKS 1. Diatomite 2. Chert SEDIMENTARY IRONSTONES 1. Banded iron formation 2. Oolitic ironstone 3. Iron oxide concretions CARBONATE ROCKS: LIMESTONE AND DOLOSTONE 1. Micrite 2. Fossiliferous limestone 3. Coquina 4. Chalk 5. Oolitic limestone 6. Intraclastic limestone 7. Pelleted limestone 8. Crystalline limestone 9. Travertine 10. Dolostone Organic Sedimentary Rocks COAL 1. Peat 2. Lignite 3. Bituminous coal 4. Anthracite coal (metamorphic)

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10 6 Sedimentary Rocks Sedimentary Rocks Beach sand and shells, St. Augustine, Florida. Shells average about 1 cm in diameter. Pamela Gore

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PRE-LAB EXERCISES Answer these questions before looking at the sedimentary rock specimens to be sure that you have mastered the material in the lab. 1. Which of the following are terrigenous sedimentary rocks? Draw a circle around them. Black shale Arkose Intraclastic limestone Diatomite Rock gypsu m Quartz conglomerate Oolitic limeston e Red sandstone Lignite Graywacke Chert Bituminous coal 2. There are three white, fine-grained sedimentary rocks. What distinguishing characteristics might you use to tell them apart? Chalk Kaolin Diatomite 3. What is the main difference between shale and claystone? 4. For each of the rock types in the table below, give the name of the dominant mineral, the chemical formula of that mineral, and tell whether or not the mineral (or rock) will effervesce (fizz) in hydrochloric acid. Rock Type Mineral Name Chemical Formula Reaction to HCl Limestone Dolostone Rock gypsum Rock salt Chert Travertine Sedimentary Rocks Exercises 107 Sedimentary Rocks Exercises EXERCISES

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Sedimentary Rocks 5. Classify the following terrigenous rocks according to whether they are: (a) rudites, (b) arenites, or (c) argillites. Put the letter in the blank. Lithic sandstone Conglomerate with quartz clasts Arkose Gray silty shale Breccia with iron oxide cement Kaolinite 6. Classify the following carbonate rocks according to whether they are (a) calcirudites, (b) calcarenites, or (c) calcilutites. Put the letter in the blank. Intraclastic limestone Chalk Oolitic limestone Coquina Micrite 7. Match the rock type with the sediment from which it is formed. Put the letter in the blank. micrite a. rounded gravel shale b. plant fragments graywacke c. microscopic skeletons of silica chalk d. feldspar grains diatomite e. angular gravel lignite f. sand-sized rock fragments coquina g. clay arkose h. lime mud conglomerate i. broken shells breccia j. coccoliths 8. List three allochems you might find in limestone. LAB EXERCISES Sedimentary Rock Identification Instructions Examine the sedimentary rock samples provided by your instructor. Fill in the chart below, and use Table 4.3 and Table 4.4 in the section Classification of Sedimentary Rocks to help you identify the samples.

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Sedimentary Rocks Instructions for Filling in the Sedimentary Rock Identification Table Column 1: Grain Size (If Visible) There are three ways to measure grain size: r Measure the size of the grains in millimeters with a ruler. This works well for gravel-sized clasts, but it may be difficult for finer-grained samples. r Compare the grain size to that of reference sediment textures on a grain-size comparator or sand gauge. This works well for distinguishing sandstone from siltstone. r List whether the sample consists of gravel, sand, silt, or clay particles based on a simple visual inspection. r If you cannot see grains, write “Grains not visible.” Column 2: Minerals or Grain Type r Identify the minerals present (calcite, dolomite, gypsum, halite, quartz, feldspar, clay minerals, etc.) r If grains (clasts or allochems) are present, what are they? (Possibilities include fossils, oolites, intraclasts, and mineral grains or rock fragments.) Column 3: Fizz in HCl? r Use dilute hydrochloric acid to test the sample to determine whether calcium carbonate (calcite or aragonite) or dolomite is present. Calcite and aragonite readily fizz in acid. Dolomite fizzes only if scratched or powdered. r If it fizzes readily, write “Yes.” r If it fizzes only when scratched and powdered, write “Only if powdered.” r If it does not fizz, write “No.” Column 4: Describe r Use this column to describe the sample. For example, the color, any layers, or any other distinguishing features. Column 5: Sedimentary Rock Name r Identify the rock. Use Table 4.3 and Table 4.4 in the section Classification of Sedimentary Rocks. r If you are in the lab, and if there are examples of “known” samples, you may also use them for comparison. Grain Size (if visible) Minerals or Grain Type Fizz in HCl? Describe Sedimentary Rock Name 1 2 3 4 5

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Sedimentary Rocks Grain Size (if visible) Minerals or Grain Type Fizz in HCl? Describe Sedimentary Rock Name 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

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Sedimentary Rocks 25

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LABORATORY 4

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