SEDIMENTARY ROCK

rock formed at or near the Earth's surface by the accumulation and lithification of sediment (detrital rock) or by the precipitation from solution at normal surface temperatures (chemical rock). Sedimentary rocks are the most common rocks exposed on the Earth's surface but are only a minor constituent of the entire crust, which is dominated by igneous and metamorphic rocks. An important characteristic of sedimentary rocks is that they are formed in layers, each of which has features that reflect the conditions during deposition, the nature of the source material, means of transport, and in many cases a record of the organisms present during past geological time. Repeated or continuous formation provides a long-term record, though some modifications may have occurred and deposition may not be continuous at any one location. Detrital (or clastic) rocks are the most common sedimentary rocks, and are formed from material released by the weathering and breakdown of igneous, metamorphic, and preexisting sedimentary rocks. This debris is transported by water, wind, or ice, deposited, and then cemented to form a coherent rock. Shale, breccia, conglomerate, clastic limestone, loess, and the sandstones quartzite, arkose, and graywacke are examples of detrital sedimentary rocks. Certain physical and chemical properties such as grain size, mineralogy, and cementation material provide a basis for their classification. For example, detrital rocks with grains less than 0.06 millimetre (0.002 inch) are classified as shales, those with grains from 0.06 to 2 mm (0.002 to 0.08 in) as sandstones, and those with grains larger than 2 mm as conglomerates. The constitutent minerals of a detrital rock are those of the source rock except in the case of unstable mineral varieties that readily break down, dissolve, or become altered. Whereas clay minerals, feldspars, and quartz will remain as transport distance or time of weathering increases, olivines, pyroxenes, and micas will be removed or altered. Detrital rocks are coherent because of the precipitation of cementing materialprincipally carbonate or quartz. Other distinctive features of detrital rocks are relatively large-scale sedimentary structures such as graded bedding, ripple marks, mud cracks, dune bedding, cross bedding, and swash marks. Each of these structures is characteristic of a particular depositional environment. The second major group of sedimentary rocks, the chemical rocks, are formed by precipitation from solution at the site of deposition as a result of supersaturation or biogenic activity. Variable amounts of detrital material, especially clays and silt, may be present. By far the most common chemical rocks are limestones composed of calcite and aragonite, and dolomite (also known as dolostone) composed of the mineral dolomite. The mineralogy of the chemical rocks indicates depositional conditions; acidity (pH), oxidation-reduction potential Eh, salinity, and temperature, in particular, can be correlated with certain chemical deposits based on present-day observations or laboratory experiments. In addition to these physical factors, many invertebrates precipitate carbonates. Other chemical rocks include evaporites, such as halite, gypsum, and anhydrite, and organic rocks (e.g., peat and coal). The structure of chemical sedimentary rocks results from the segregation of originally dispersed substances, which include oolites, concretions, geodes, nodules, and septaria. These are round to irregular objects formed during or soon after sedimentation. Common nodules that are segregations of material different from the rock are chert, phosphates, and clay ironstones. Fossil remains are particularly common in limestones and dolomites either as original hard parts or as casts. Dolomites are usually recrystallized limestones in which magnesium has replaced calcium during or after deposition; grain size is generally coarse and structures are less apparent. rock formed at or near the Earth's surface by the accumulation and lithification of sediment (detrital rock) or by the precipitation from solution at normal surface temperatures (chemical rock). Sedimentary rocks are the most common rocks exposed on the Earth's surface but are only a minor constituent of the entire crust, which is dominated by igneous and metamorphic rocks. Sedimentary rocks are produced by the weathering of preexisting rocks and the subsequent transportation and deposition of the weathering products. Weathering refers to the various processes of physical disintegration and chemical decomposition that occur when rocks at the Earth's surface are exposed to the atmosphere (mainly in the form of rainfall) and the hydrosphere. These processes produce soil, unconsolidated rock detritus, and components dissolved in groundwater and runoff. Erosion is the process by which weathering products are transported away from the weathering site, either as solid material or as dissolved components, eventually to be deposited as sediment. Any unconsolidated deposit of solid weathered material constitutes sediment. It can form as the result of deposition of grains from moving bodies of water or wind, from the melting of glacial ice, and from the downslope slumping (sliding) of rock and soil masses in response to gravity, as well as by precipitation of the dissolved products of weathering under the conditions of low temperature and pressure that prevail at or near the surface of the Earth. Sedimentary rocks are the lithified equivalents of sediments. They typically are produced by cementing, compacting, and otherwise solidifying preexisting unconsolidated sediments. Some varieties of sedimentary rock, however, are precipitated directly into their solid sedimentary form and exhibit no intervening existence as sediment. Organic reefs and bedded evaporites are examples of such rocks. Because the processes of physical (mechanical) weathering and chemical weathering are significantly different, they generate markedly distinct products and two fundamentally different kinds of sediment and sedimentary rock: (1) terrigenous clastic sedimentary rocks and (2) allochemical and orthochemical sedimentary rocks. Clastic terrigenous sedimentary rocks consist of rock and mineral grains, or clasts, of varying size, ranging from clay-, silt-, and sand- up to pebble-, cobble-, and boulder-size materials. These clasts are transported by gravity, mudflows, running water, glaciers, and wind and eventually are deposited in various settings (e.g., in desert dunes, on alluvial fans, across continental shelves, and in river deltas). Because the agents of transportation commonly sort out discrete particles by clast size, terrigenous clastic sedimentary rocks are further subdivided on the basis of average clast diameter. Coarse pebbles, cobbles, and boulder-size gravels lithify to form conglomerate and breccia; sand becomes sandstone; and silt and clay form siltstone, claystone, mudrock, and shale. Chemical sedimentary rocks form by chemical and organic reprecipitation of the dissolved products of chemical weathering that are removed from the weathering site. Allochemical sedimentary rocks, such as many limestones and cherts, consist of solid precipitated nondetrital fragments (allochems) that undergo a brief history of transport and abrasion prior to deposition as nonterrigenous clasts. Examples are calcareous or siliceous shell fragments and oids, which are concentrically layered spherical grains of calcium carbonate. Orthochemical sedimentary rocks, on the other hand, consist of dissolved constituents that are directly precipitated as solid sedimentary rock and thus do not undergo transportation. Orthochemical sedimentary rocks include some limestones, bedded evaporite deposits of halite, gypsum, and anhydrite, and banded iron formations. Sediments and sedimentary rocks are confined to the Earth's crust, which is the thin, light outer solid skin of the Earth ranging in thickness from 40100 kilometres (25 to 62 miles) in the continental blocks to 410 kilometres in the ocean basins. Igneous and metamorphic rocks constitute the bulk of the crust. The total volume of sediment and sedimentary rocks can be either directly measured using exposed rock sequences, drill-hole data, and seismic profiles or indirectly estimated by comparing the chemistry of major sedimentary rock types to the overall chemistry of the crust from which they are weathered. Both methods indicate that the Earth's sediment-sedimentary rock shell forms only about 5 percent by volume of the terrestrial crust, which in turn accounts for less than 1 percent of the Earth's total volume. On the other hand, the area of outcrop and exposure of sediment and sedimentary rock comprises 75 percent of the land surface and well over 90 percent of the ocean basins and continental margins. In other words, 8090 percent of the surface area of the Earth is mantled with sediment or sedimentary rocks rather than with igneous or metamorphic varieties. The sediment-sedimentary rock shell forms only a thin superficial layer. The mean shell thickness in continental areas is 1.8 kilometres; the sediment shell in the ocean basins is roughly 0.3 kilometre. Rearranging this shell as a globally encircling layer (and depending on the raw estimates incorporated into the model), the shell thickness would be roughly 13 kilometres. Despite the relatively insignificant volume of the sedimentary rock shell, not only are most rocks exposed at the terrestrial surface of the sedimentary variety, but many of the significant events in Earth history are most accurately dated and documented by analyzing and interpreting the sedimentary rock record instead of the more voluminous igneous and metamorphic rock record. When properly understood and interpreted, sedimentary rocks provide information on ancient geography, termed paleogeography. A map of the distribution of sediments that formed in shallow oceans along alluvial fans bordering rising mountains or in deep, subsiding ocean trenches will indicate past relationships between seas and landmasses. An accurate interpretion of paleogeography and depositional settings allows conclusions to be made about the evolution of mountain systems, continental blocks, and ocean basins, as well as about the origin and evolution of the atmosphere and hydrosphere. Sedimentary rocks contain the fossil record of ancient life-forms that enables the documentation of the evolutionary advancement from simple to complex organisms in the plant and animal kingdoms. Also, the study of the various folds or bends and breaks or faults in the strata of sedimentary rocks permits the structural geology or history of deformation to be ascertained. Finally, it is appropriate to underscore the economic importance of sedimentary rocks. For example, they contain essentially the world's entire store of oil and natural gas, coal, phosphates, salt deposits, groundwater, and other natural resources. Several subdisciplines of geology deal specifically with the analysis, interpretation, and origin of sediments and sedimentary rocks. Sedimentary petrology is the study of their occurrence, composition, texture, and other overall characteristics, while sedimentology emphasizes the processes by which sediments are transported and deposited. Sedimentary petrography involves the classification and study of sedimentary rocks using the petrographic microscope. Stratigraphy covers all aspects of sedimentary rocks, particularly from the perspective of their age and regional relationships as well as the correlation of sedimentary rocks in one region with sedimentary rock sequences elsewhere. (For further information about these fields, see geologic sciences.) Additional reading Sam Boggs, Jr., Principles of Sedimentology and Stratigraphy (1987); Rhodes W. Fairbridge and Joanne Bourgeois, The Encyclopedia of Sedimentology (1978); Andrew D. Miall, Principles of Sedimentary Basin Analysis, 2nd ed. (1990); F.J. Pettijohn, Sedimentary Rocks, 3rd ed. (1975); F.J. Pettijohn, P.E. Potter, and R. Siever, Sand and Sandstone, 2nd ed. (1987); P.E. Potter, J. Barry Maynard, and Wayne A. Pryor, Sedimentology of Shale (1980); Donald R. Prothero, Interpreting the Stratigraphic Record (1989); J.G. Reading (ed.), Sedimentary Environments and Facies, 2nd ed. (1985); Stuart R. Taylor and Scott M. McLennan, The Continental Crust: Its Composition and Evolution (1985); Maurice E. Tucker, Sedimentary Petrology: An Introduction to the Origin of Sedimentary Rocks, 2nd ed. (1991); James L. Wilson, Carbonate Facies in Geologic History (1975). Frederick L. Schwab