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Abstract

The upper Campanian Grassy Member of the Blackhawk Formation, Mesa Verde Group, Utah, consists of two high-frequency, unconformity- bounded sequences within the upper part of the Blackhawk Formation highstand sequence set. Incised valley-fill deposits overlie the sequence boundaries and are laterally separated by subaerially exposed interfluve areas with minor pod-like channel tributaries. There is indirect evidence of an emerging structural high which controlled the position of incised valleys and associated interfluves. Evidence for shelf erosion and subaerial exposure together with a significant basinward shift in facies tracts is indicative of Type 1 sequence boundaries. Within the Grassy Member, the degree of shelf erosion in outcrop appears far less significant than that associated with type 1 sequence boundaries described from the stratigraphically higher Desert Member and Castlegate Formation. There appears to be a hierarchy of unconformities bounding sequences within the Blackhawk Formation recording progressively larger basinward shifts of facies tracts and greater amounts of shelf incision with time. Similarly, several orders of stratal surfaces define the internal architecture of each sequence and constituent parasequences.

Valley-fill deposits consist of multistory fluvial-estuarine complexes representing lowstand and early transgressive systems tracts. Transgressive facies preserved between the subaerially exposed interfluve and maximum flooding surface usually consist of a downdip heavily bioturbated lag deposit correctable updip with the top of a coal or swamp facies. The internal architecture of Grassy shoreface parasequences consists of stacked bedset shingles. The lateral shift in facies tracts across bedset boundaries is an order of scale smaller than that across parasequence boundaries. The style of bedset stacking is a function of the rate of change of accommodaton space during the evolution of a shoreface parasequence, and thus by the interaction of sediment supply, climate, and relative sea level. Within the Grassy Member, lagoonal and swamp deposits (coals) are interpreted to have developed relatively late in the evolution of a coeval shoreface. They are correlated with aggradational to retrogradational bedsets in the upper part of a shoreface parasequence recording initial flooding and base level rise which maintained sufficient accommodation space and groundwater level for formation and preservation of these deposits. The shoreface thus evolved from a prograding strand plain into a barrier island prior to formation of the parasequence flooding surface.

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