The Fossil basin of southwestern Wyoming and adjacent north-central Utah is a Late Cretaceous-early Tertiary depositional basin formed largely on the hanging wall of the Absaroka thrust system. The basin is divided into the northern Fossil basin and the southern Fossil basin by the cross-basinal, northwest-southeast-trending Little Muddy Creek transverse ramp, which appears to be related to a lateral change in the stratigraphic position of the Absaroka thrust fault in both hanging wall and footwall rocks. The Absaroka thrust sheet is characterized by distinctly different structural styles north and south of this transverse ramp.

North of the ramp the Late Cretaceous-early Tertiary northern Fossil basin lies between the toe of the Absaroka thrust on the east and the Rock Creek anticline on the west. The basin was created by movement on, and erosion, of the Absaroka thrust sheet in pre-late Campanian-Maestrichtian time. Exploratory drilling has not as yet found significant oil and gas reserves in the northern Fossil basin even though Ordovician Bighorn Dolomite on the hanging wall of the Absaroka thrust has been juxtaposed with Cretaceous source beds in the footwall.

South of the transverse ramp the Late Cretaceous-early Tertiary southern Fossil basin lies between the toe of the Absaroka thrust system on the east and structure created on the hanging wall of the Medicine Butte thrust on the west. Within the southern Fossil basin, Cambrian through lower Upper Cretaceous rocks within the Absaroka thrust sheet are in fault contact with organic-rich Lower Cretaceous (on the west) and lower Upper Cretaceous (on the east) source rocks in the footwall. Essentially all oil and gas production established to date has been found in the southern Fossil basin in three lines of folding in the Absaroka thrust hanging wall. The westerly two lines of folding produce from Paleozoic and, locally, Mesozoic objectives, and the easterly folding produces from Mesozoic ob ectives. Exploratory and development drilling permits better interpretation of timing of thrust motion and subsurface structural geometry in the Fossil basin area.

End_of_Article - Last_Page 853------------