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Abstract

Parasequence architecture and the nature of parasequence boundaries in marine to nonmarine strata are well illustrated in the Spring Canyon Member of the Upper Cretaceous Blackhawk Formation. Parasequences and parasequence sets are stratal successions which are the building blocks of sequences. In marine strata, parasequences result from basinward progradation of the shoreline, and typically shallow and coarsen upward; in the nonmarine, parasequences show a distinct vertical facies succession which begins with lagoon-fill deposits and ends with freshwater coals. A flooding surface (parasequence boundary), indicating an abrupt increase in water depth, accompanied by minor submarine erosion and nondeposition separates individual parasequences within a parasequence set. The parasequence boundary is a continuous, single surface that can be traced from updip in the coastal plain to downdip in the distal shelf. The parasequence boundary has different physical expressions depending on where it is observed, and enables correlation of nonmarine /marginal marine rocks to coeval marine strata within the same parasequence. Parasequence evolution and deposi- tional reconstruction is dependent on the application of sequence stratigraphic concepts. Outcrop examples from the Spring Canyon Member document parasequence expression. Both wave-dominated shoreface sandstone and river-dominated deltaic sandstone exist laterally in the marine portion of the same parasequence. Both are terminated by a flooding event marked by a rapid landward shift in facies, with no transgressive lag. A number of marginal marine and nonmarine subenvironments exist laterally within the same parasequence. The parasequence boundary provides a temporal framework to link the genetically related subenvironments, allowing reconstruction of the entire depositional system, as opposed to evaluation of outcrops as isolated systems or facies. Continuous coal seams occur immediately beneath parasequence boundaries and are markers used to trace parasequence boundaries from nonmarine sections into the marine.

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