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Unexcelled exposures of the coal-bearing Blackhawk Formation near Castle Gate, Utah, provide a cross section of sediments deposited by wave-dominated deltas along the western shoreline of the Cretaceous Interior seaway. Four sandstone tongues resulted from deltaic sedimentation, each overlain by thick coal. A clear genetic relationship exists between the occurrence of coal and geometries of paleoshorelines and fluvial channels. Coals are thickest where underlain by thin shoreface sandstones, and they pinch out abruptly against beach-ridge sandstones responsible for swamp proliferation. Fluvial channels subsequently cut wide swaths in swamp deposits normal to shoreline trends. Commonly, thick coals of different seams occur together, as the compaction of vegetation controll d subsequent swamp accumulation. Excellent exposures and considerable subsurface data provide the details necessary to construct a predictive exploration model useful in the Cretaceous coals of the central Rockies. Cretaceous deltaic deposits also create hydrocarbon potential, as three facies associated with Blackhawk deposition produce ideal stratigraphic relationships for hydrocarbon accumulation. Porous delta-front sandstones interfinger with the underlying organic-rich marine shale of the Mancos formation. Shale and siltstone of the flood plain then cap the sandstone. Hydrocarbons derived from the marine shale or from associated coal may accumulate in porous sands of stream channels or in mouth-bar or beach-ridge deposits of the delta front. A clear understanding of deltaic sedimenta ion, provided by analysis of the Blackhawk model, could aid in predicting the occurrences of similar subsurface sandstones.
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