FALSE INDICATORS OF A CLIMATIC SHIFT IN THE GREEN RIVER FORMATION OF FOSSIL BASIN AND THE IMPORTANCE OF RECOGNIZING STREAM CAPTURE IN REGIONAL CORRELATION OF KEROGEN-RICH LACUSTRINE SEDIMENTS

Previous studies have suggested that the kerogen-rich evaporitic facies of the Angelo Member of the Green River Formation in Fossil Basin are temporally equivalent to similar kerogen-rich evaporitic facies of the Wilkins Peak Member of the Green River Formation in the Green River Basin 40 km to the east. Problematically, palynomorphs from the Angelo Member reveal the same subtropical flora as the underlying freshwater Fossil Butte Member, suggesting that climate change was not responsible for the dramatic shift in lacustrine sedimentation. During the Eocene, Fossil Basin was bounded by hogsback ridges of the Wyoming thrust belt to the east and west, producing a narrow piggyback basin. Lithologic data, including the presence of extensive deltaic facies, suggests that most of the water within the basin was derived from the western end of the Uinta Mountains. The lower and middle members of the Green River Formation in Fossil Basin reflect relatively freshwater conditions driven by this major southern river system, resulting in the ultimate breach of the eastern hogsback ridges just after deposition of the famous fossil fish bearing layers in the center of the basin. At this most expansive over-filled phase of the lake, lacustrine sediments can be physically traced across the hogsback and into the greater Green River Basin. The overlying Angelo Member of the Green River Formation in Fossil Basin is characterized by evaporitic facies reflecting a hypersaline-alkaline lake that changed dramatically in size in response to inflow. There is no evidence that the major southern river system fed the lake during Angelo Member time. We suggest that basin infilling and overflow at the southern end of Fossil Basin led to stream capture through either lobe switching or down-cutting, effectively starving the basin. This set up conditions favorable for the formation of kerogen rich sediments in the stranded, water-starved basin. This scenario is consistent with palynologic data suggesting there was no major climatically driven shift in the flora. We also suggest that correlations based on lithologic similarities between Fossil Basin and the Greater Green River Basin are doomed to failure, as the Angelo Member may be more closely related temporally to fresher facies within the Tipton Shale Member of the Green River Formation.