Principles of Sedimentology and Stratigraphy Pearson New International Edition
Sedimentary rocks form through a complex set of processes that begins with weathering, the physical disintegration and chemical decomposition of older rock to produces solid particulate residues (resistant minerals and rock fragments) and dissolved chemical substances. Some solid products of weathering may accumulate in situ to form soils that can be preserved in the geologic record (paleosols). Ultimately, most weathering residues are removed from weathering sites by erosion and subsequently transported, possibly along with fragmental products of explosive volcanism, to more distant depositional sites.
Transport of siliciclastic detritus to depositional basins can involve a variety of processes. Mass-transport processes such as slumps, debris flows, and mud flows are important agents in the initial stages of sediment transport from weathering sites to valley floors. Fluid-flow processes, which include moving water, glacial ice, and wind, move sediment from valley floors to depositional basins at lower elevations. When transport processes are no longer capable of moving sediment, deposition of sand, gravel, and mud takes place, either subaerially (e.g., in desert dune fields) or subaqueously in river systems, lakes, or the marginal ocean. Sediment deposited at the ocean margin may be reentrained and retransported tens to hundreds of kilometers into deeper water by turbidity currents or other transport processes. Sediments deposited in basins are eventually buried and undergo physical and chemical changes (diagenesis) resulting from increased temperature, pressure, and the presence of chemically active fluids. Burial diagenetic processes convert siliciclastic sediments to lithified sedimentary rock: conglomerate, sandstone, shale. Weathering processes also release from source rocks soluble constituents such as calcium, magnesium, and silica that make their way in surface water and groundwater to lakes or the ocean. When concentrations of these chemical elements become sufficiently high, they are removed from water by chemical and biochemical processes to form “chemical” sediments. Subsequent burial and diagenetic alteration of these sediments generates lithified sedimentary rock: limestone, chert, evaporites, and other chemical/biochemical sedimentary rocks.
In summary, the origin of sedimentary rocks involves weathering of older rock to generate the materials that make up sedimentary rock, erosion and transport of weathered debris and soluble constituents to depositional basins, deposition of this material in continental (terrigenous) or marine environments, and diagenetic alteration during burial to ultimately produce lithified sedimentary rock. Because weathering plays such a critical role in generating the solid particles and chemical constituents that make up sedimentary rocks, the text focuses on the physical and chemical processes of weathering, the nature of the resulting weathering products, and a brief discussion of soils. The text continues with a detailed discussion of the various processes by which sediment grains are transported from weathering sites to depositional basins. Other aspects of the origin of sedimentary rocks are introduced and discussed as appropriate.
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