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Turbidites have been recognized increasingly in the Cretaceous Interior seaway. Most are described as thin, economically unimportant sandstone beds isolated in prodelta mudstone. This study documents the occurrence of southerly prograding, sand-dominated turbidite fans of sufficient size to be considered economically viable hydrocarbon reservoirs.
Two fans intercalate with the Upper Cretaceous Pierre Shale and form cliffs over more than 10 mi (16 km) of continuous outcrop in the Eagle basin, north of Walcott, Colorado. Both units exhibit progradational sequences typical of turbidite fans. A common vertical succession of sedimentary structures consists of starved ripples, flat-bottomed ripple beds, thin flat beds grading into ripples of climbing ripples, and amalgamated flat beds. Massive to graded beds are rare and occur only in the upper part of each sandstone body. Associated sedimentary features include parting lineation, grooves, prod marks, mud chips, contorted bedding, and flute casts. Broad, low-relief channels occur at the top of the lower, more well-developed sequence.
The sedimentary structures described correlate well with accepted models for turbidite-fan sedimentation. The upward-thickening and coarsening character of the sandstone bodies, the abundance of incomplete Bouma sequences, and the presence of broad low-relief channels are typical of the outer-fan lithofacies associations of E. Mutti and F. Ricci-Lucchi.
Alternative interpretations of these laterally continuous, progradational sandstone bodies might include deposition in a distal shoreface or offshore bar environment. Hummocky cross-stratification and large-scale cross-stratified bed forms are not common in the sequence, as would be expected in a shoreface or marine-bar environment.
Turbidite-fan deposits similar to those studied could be economically significant because of their extreme lateral continuity, updip seals, intercalation with hydrocarbon source rock,and possible overpressuring. In addition, the apparently "distal" nature of these sandstone bodies suggests the possibility of thicker, better developed turbiditic sandstone bodies to the north. The presence of submarine fans within the Cretaceous Western Interior seaway may increase significantly the hydrocarbon potential of previously unexplored, "shaly" portions of the basin.
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