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60 4.1. Photographs of clastic dikes. (A) Multiple crosscutting clastic dikes on Highway 240 near Pinnacles Overlook. (B) Clastic dike along the Old Northeast Road showing sheetlike morphology. (C), (D), (E) Close-up of clastic dikes showing crenulated texture. (F) Mud cracks from strata near Door and Window Overlook. Note size difference and fracture pattern compared to clastic dikes. Photos by the authors. 61 4 Postdepositional Processes and Erosion of the White River Badlands The Badlands that we see today are the result of both depositional and postdepositional processes. Our understanding of these processes and their role in the formation of the Badlands is only possible as a result of the significant erosion that characterizes this region. We discuss postdepositional features first, with the understanding that it is erosion that has allowed us to study these features. Burial and Lithification The rock strata that comprise the Badlands were once loose sand, silt, and clay, with the occasional wind-deposited tephra and freshwater limestone. Upon burial, these sediments were turned to rock by compaction and cementation during the process of lithification. Compaction is due to the enormous overlying weight of additional sediments that are brought into the basin. As a result, the thicker the accumulation of sediments, the greater the reduction of porosity (void space) between the grains. Cementation is the process by which individual grains of sediment are bound together by the precipitation of secondary minerals, commonly calcite , quartz, or iron oxide, out of groundwaters that have moved through the sediment. Geologists refer to lithification as just one form of diagenesis, the sum of physical and chemical changes that can be induced in sediment upon burial. Additional chemical diagenetic changes include the wholesale change of certain minerals into new phases, such as the dewatering of iron oxyhydroxides to hematite and the conversion of smectite (swelling) clays to illite clay by the addition of potassium. Along with cementation, entirely new minerals can grow in available pore spaces. In the White River Group, the common secondary minerals include calcite , gypsum, quartz, chalcedony, barite, various uraniumbearing minerals, and zeolites (Retallack, 1983b; Terry and Evans, 1994). Many of these diagenetic changes require microscopes to detect small-scale mineralogical changes and specialized instruments to measure the chemical makeup of rock samples. Diagenetic changes that can be seen with the naked eye include the overall reddish appearance (due to the recrystallization of various yellowish-brown iron oxyhydroxides into hematite) that are seen in many of the stripes that cut across the Badlands, and secondary mineralization within fractures caused by tectonic forces that compressed and stretched rock units of the Badlands. Clastic Dikes Some of the more prominent postdepositional features of the Badlands are the numerous nearly vertical sheets of resistant material that crosscut the horizontal layering of the Badlands (Fig. 4.1). These features, referred to as either clastic dikes or chalcedony veins, depending on the type of material that is present, vary in size and in regional and stratigraphic distribution , and are formed by different processes. Clastic dikes are composed of small particles of sediment that have been cemented to form ridgelike structures that cut across the Badlands. Their resistance to erosion is greater than that of typical Badlands rocks, which results in “fins” of clastic dike material that stand out in erosional relief and commonly help to support less resistant Badlands materials (Fig. 4.1). The appearance of these dikes can vary from light beige colors that are similar to typical Badlands strata to those that are stained with a greenish color. When viewed from the edge, these features range from paper-thin bodies to tens of centimeters thick that cut across tens of meters of solid rock. Some are relatively smooth along their surfaces, whereas others appear to be crenulated, similar to a wavy potato chip (Fig. 4.1). These dikes sound a glassy ring when pieces are struck together. When viewed along the surface of the ground or from the air, these sheets of clastic material are sometimes straight lines, but in other instances they form curvilinear features that seem to randomly run across the surface for tens to hundreds of meters. Models describing the formation of these dikes have included large-scale desiccation cracks that were filled with sediments (Retallack, 1983b), fractures that were formed (and later filled) in response to tectonic and structural strain (Smith, 1952), and injection of liquefied sediments from deeper in the Badlands strata (Whelan, Hamre, and Hardy 1996). 62 The White River Badlands Modern-day...


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