Evolution of an Eocene landscape: The effects of tectonics, climate, and volcanism in the Lake Gosiute watershed

Deposition of lacustrine sediment in the greater Green River Basin in southwestern Wyoming was progressively terminated in the middle Eocene as volcaniclastic detritus prograded across the basin. The termination of lacustrine deposition marked both a reversal in the polarity of drainage across the greater Green River Basin which occurred between 49.6 Ma and 48.9 Ma, and a change in the composition of the clastic sediment that accumulated in the basin from locally sourced arkosic sediment to volcaniclastic sediment sourced from a volcanic system external to the greater Green River Basin.The Pb isotope composition of volcaniclastic sandstone in the greater Green River Basin is consistent with the composition of the Challis Volcanic Field of central Idaho, not the Absaroka Volcanic Province of northwest Wyoming and southern Montana that has long been assumed to have been the source. 40Ar/39Ar ages for volcaniclastic detritus are also consistent with both the eruptive history of the Challis Volcanic Field which was active between 49.8-45.5 Ma and also with cooling ages for Mesozoic and Proterozoic rocks that were exposed in the Idaho segment of the North American Cordillera during the Eocene. The appearance of Challis-derived sediment in the greater Green River Basin indicates that a major river, here termed the Idaho River, connected the interior of the cordillera to the greater Green River Basin.The Eocene Idaho River delivered approximately 18,000 km3 of Challis-derived volcaniclastic sediment to downstream depocenters in the greater Green River Basin between 49.9 Ma and 47 Ma at a rate of 7.2 x 10 -3 km3/yr. During this interval the greater Green River Basin collected a unique and detailed history of Challis volcanism. When interpreted as a minimum eruptive flux for the Challis Volcanic Field, the sediment flux of the Idaho River is consistent with the eruptive fluxes of the most rapidly erupted silicic and continental igneous provinces. The flux of sediment from the Challis Volcanic Field to the greater Green River Basin varied in relation to volcanic events upstream. During intervals when eruptive flux is interpreted to have been high, the youngest eruptive ages in volcaniclastic sandstones are in agreement with the estimated depositional age of the sample. Lack of agreement between eruptive and depositional age is observed only in the sample that was deposited between the episodes when eruptive flux in the Challis Volcanic Field is interpreted to have been high. The restricted nature of the floodplain of the Idaho River corresponds to a low buffering capacity and instantaneous deposition of volcaniclastic sediment in the greater Green River Basin at the resolution of the methods employed in this study.