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Abstract:

Two types of criteria are used to recognize relative changes in sea level in the Cretaceous of the Western Interior (Fig. 1). The first type is a highstand condition identified by: (1) highstand regression of the shoreline, depositing widespread shallow-marine sandstone and shale and shoreline sandstone, sometimese overlain by a widespread coal layer (Fig. 2); (2) deposits that fill incised drainage, reflecting rising sea level and landward movement of the shoreline (transgression) associated with coastal onlap; the incised valley fill may be zoned-more freshwater environments in the lower part and brackish to marine environments in the upper part (Figs. 3, 4); (3) recognition of one or more of the following in a marine condensed section: missing faunal zones; concentrations of phosphate nodules and/or glauconite; organic-rich shale with high total organic content; recrystallized shell debris forming thin lenticular limestone layers or shell hash in shale; residual concentrations of coarse-grained sand with chert pebbles and/or bone and teeth fragments on a transgressive surface of erosion. The second type is a lowstand condition recognized by: (1) lowstand surface of erosion with incised drainage; paleosols and root zones (causing zones of early cementation) may be preserved in marine shale or other deposits under an erosional surface (Fig. 3); this type of erosional surface is a major sequence boundary in analyses related to sequence stratigraphy; (2) more or less uniform depth of erosion by streams over large areas because of lowered base level; (3) missing shoreline and shallow marine sandstone facies; freshwater deposits rest on marine shales; (4) correlation with relative lowering of sea level and unconformities on other continents. Intrabasin fault block movement, creating topographic relief, may influence location of incised drainages (Figs. 3, 4).

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