Abstract

Upper Cretaceous rocks of the Mesaverde Formation, exposed between Hudson and Muskrat Creek on the south flank of the Wind River basin, accumulated in an ascending succession of delta-front, tidal-flat, and delta-plain environments. Rocks of the lower two environments grade east and northeast into strata deposited in an offshore marine environment, whereas rocks of the youngest environment grade east into deposits of a delta-front platform. During early Campanian time, the delta-front facies accumulated across a 14-km-wide belt near Alkali Butte. Repeated fluctuations of strand position produced interbedded units of siltstone and sandstone in the facies, and intermittent development of low-relief bars permitted accumulation of estuarine or swamp sediments across subjacent deposits of the delta front.

Sandstone units of the delta-front facies show an upward gradation from siltstone and sandy siltstone beds to silty sandstone and fine sandstone beds characterized by slump and massflow structures. Slumping was triggered by instability of sediments on the delta-front slope and by recurrent tectonic movements along the trend of the ancestral Alkali Butte anticline. Succeeding the slump-dominated sandstone in each unit is well-sorted sandstone, deposited in a variety of littoral marine, beach dune, and estuarine channel environments. Depositional strike of the delta-front sediments, determined from primary sedimentary structures, ranged from N8°W to N30°E, and the slope declined east-southeast.

Sandstone units of the delta-front facies are saturated with oil near Alkali Butte. Variations of porosity and permeability of the units along depositional strike in the subsurface, or an association of the units with reversals of dip along faults in the south-central part of the Wind River basin may form traps for petroleum, and make the units worthwhile exploration objectives.

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