Late Cretaceous and Cenozoic History of Laramie Basin Region, Southeast Wyoming

Earliest deformation is recorded by conglomerates of late Campanian-earliest Maestrichtian age derived from distant sources. Early Paleocene deformation defined the northern Front Range, Medicine Bow Mountains, and the Sierra Madre-Park Range, which attained limited height as erosion matched uplift. Late Paleocene gravels unconformably overlapped the margins of the uplifts. By early Wasatch time major northwest-trending structural features were outlined, the Paleocene conglomerates had been folded, and locally derived sediments again overlapped uplift margins. During middle Eocene time, sediments were deposited in the Shirley basin area. A mature landscape was produced in late Eocene time, and northeast-trending structural features developed. During Oligocene time, volcanic debris intertongued with local conglomerates and was unconformably deposited on older rocks. Aggradation continued until deposition extended across the northern Laramie Mountains and central Medicine Bow Mountains. Miocene rocks in the Saratoga valley, Hanna basin, and near Cheyenne suggest areas of nondeposition lying south of these localities. Present high-level erosion surfaces probably coincide with the level of basin fill in early Pliocene time. Volcanic activity near Specimen Mountain, Colorado, in late Oligocene time (28 m.y.) may have continued into Pliocene(?) time. Post-Miocene normal faulting and folding were active in the Laramie and Medicine Bow Mountains, Saratoga valley, North Park basin, Rawlins uplift, and along the Wheatland fault zone. Pliocene(?) and Pleistocene boulder deposits reflect late structural and climatic changes.