The Upper Cretaceous Fox Hills Sandstone on the east flank of the Rock Springs uplift is a regressive sequence of sandstone and siltstone which was deposited in shallow neritic and estuarine environments. It

End_Page 538------------------------------

overlies, and intertongues with the marine Lewis Shale, and is overlain by, and intertongues with the nonmarine Lance Formation.

Exposed Fox Hills strata consist of littoral and shallow neritic, very fine-grained sandstone and siltstone which commonly are overlain, with marked disconformity, by fine- to medium-grained sandstone deposited in estuarine environments. The origin of these widespread erosion surfaces is believed to be related to the lateral migration of estuary and tidal channels behind prograding barrier-island shorelines.

In the subsurface east of the outcrop, the stratigraphic position of the Fox Hills rises, in an east to southeast direction, over 400 ft in a distance of 15 mi. Subsurface shoreline trends in this area are north to northeast, and appear to form a large embayment across the present Wamsutter arch. North and east of this area, in the vicinity of the North Desert Springs field, the typical Fox Hills Sandstone and uppermost Lewis Shale pass laterally into a thick sequence of siltstone and shale interpreted by Asquith to be a major delta.

Small amounts of oil and gas have been produced from the Fox Hills, and numerous shows have been reported from wells on the east flank of the Rock Springs uplift. The optimum targets for future exploration appear to be porous Fox Hills shoreline and estuarine sandstones, where they interfinger in an up-dip direction with shales of the Lance Formation.

End_of_Article - Last_Page 539------------