Provenance of the Tullock Member of the Fort Union Formation, Powder River Basin, Wyoming and Montana: evidence for early Paleocene Laramide uplift

A petrologic and provenance study indicates that Laramide uplifts to the west and south of the Powder River Basin (PRB) were emergent and shedding detritus by early Paleocene time. This conclusion is based largely on the presence of abundant first-cycle carbonate clasts in the northwestern PRB, and metamorphic and igneous clasts and labile heavy-mineral grains in the Tullock throughout the basin. The proximity and composition of the north end of the Bighorn uplift strongly suggest that it was the source for carbonate, igneous, and metamorphic rock fragments in northwestern Tullock outcrops. The conclusions are supported by recent fission-track, palynological, and sedimentological studies that indicate that Laramide-style foreland deformation in southwestern Montana began in late Cenomanian to Turonian time and migrated through central Wyoming to the Colorado Front Range by late Maastrichtian time. -from Authors