Sedimentology, detrital zircon geochronology, and stable isotope geochemistry of the lower Eocene strata in the Wind River Basin, central Wyoming

摘要

Shallow subduction of the Farallon plate beneath the western United States has been commonly accepted as the tectonic cause for the Laramide deformation during Late Cretaceous through Eocene time. However, it remains unclear how shallow subduction would produce the individual Laramide structures. Critical information about the timing of individual Laramide uplifts, their paleoeleva-tions at the time of uplift, and the temporal relationships among Laramide uplifts have yet to be documented at regional scale to address the question and evaluate competing tectonic models. The Wind River Basin in central Wyoming is filled with sedimentary strata that record changes of paleogeography and paleoelevation during Laramide deformation. We conducted a multidisciplinary study of the sedimentology, detrital zircon geochronology, and stable isotopic geochemistry of the lower Eocene Indian Meadows and Wind River formations in the northwestern corner of the Wind River Basin in order to improve understanding of the timing and process of basin evolution, source terrane unroofing, and changes in paleoelevation and paleoclimate. Depositional environments changed from alluvial fans during deposition of the Indian Meadows Formation to low-sinuosity braided river systems during deposition of the Wind River Formation. Paleocurrent - directions changed from southwestward to mainly eastward through time. Conglomerate and sandstone compositions suggest that the Washakie and/or western Owl Creek ranges to the north of the basin experienced rapid unroofing ca. 55.5-54.5 Ma, producing a trend of predominantly Mesozoic clasts giving way to Precambrian basement clasts upsection. Rapid source terrane unroofing is also suggested by the upsection changes in detrital zircon U-Pb ages. Detrital zircons in the upper Wind River Formation show age distributions similar to those of modern sands derived from the Wind River Range, with up to ~20% of zircons derived from the Archean basement rocks in the Wind River Range, indicating that the range was largely exhumed by ca. 53-51 Ma. The rise of these ranges by 51 Ma formed a confined valley in the northwestern part of the basin, and promoted development of a meandering fluvial system in the center of the basin. The modern paleodrainage configuration was essentially established by early Eocene time. Carbon isotope data from paleosols and modern soil carbonate show that the soil C02 respiration rate during the early Eocene was higher than at present, from which a more humid Eocene paleoclimate is inferred. Atmosphere pC02 estimated from paleosol carbon isotope values decreased from 2050 ± 450 ppmV to 900 ± 450 ppmV in the early Eocene, consistent with results from previous studies. Oxygen isotope data from paleosol and fluvial cement carbonates show that the paleoelevation of the Wind River Basin was comparable to that of the modern Great Plains (~500 m above sea level), and that local relief between the Washakie and Wind River ranges and the basin floor was 2.3 ± 0.8 km. Up to 1 km of post-Laramide regional net uplift is required to form the present landscape in central Wyoming.