Erosion of Laramide-style uplifts in the western United States exerted an important first-order influence on Paleogene sedimentation by controlling sediment supply rates to adjacent closed basins. During the latest Cretaceous through Paleocene, these uplifts exposed thick intervals of mud-rich Upper Cretaceous foreland basin fill, which was quickly eroded and redeposited. Cretaceous sedimentary lithologies dominate Paleocene conglomerate clast compositions, and the volume of eroded foreland basin strata is approximately twice the volume of preserved Paleocene basin fill. As a result of this sediment oversupply, clastic alluvial and paludal facies dominate Paleocene strata, and are associated with relatively shallow and ephemeral freshwater lake facies. In contrast, large, long-lived, carbonate-producing lakes occupied several of the basins during the Eocene. Basementderived clasts (granite, quartzite, and other metamorphic rocks) simultaneously became abundant in lower Eocene conglomerate. We propose that Eocene lakes developed primarily due to exposure of erosion-resistant lithologies within cores of Laramide uplifts. The resultant decrease in erosion rate starved adjacent basins of sediment, allowing the widespread and prolonged deposition of organic-rich lacustrine mudstone. These observations suggest that geomorphic evolution of the surrounding landscape should be considered as a potentially important influence on sedimentation in many other interior basins, in addition to more conventionally interpreted tectonic and climatic controls.
Oxygen isotope values from lacustrine carbonate in the Laney Member of the Green River Formation (Wyoming) exhibit a sudden, basinwide, ~6‰ upsection decrease in δ 18 O at ca. 49 Ma. 40 Ar/ 39 Ar geochronology constrains the duration of the isotopic shift to ≤~200,000 years. This change coincides with a sudden change in lake type, from balancedfi lled in the lower LaClede Bed to overfi lled in the upper LaClede Bed, as well as an increase in the proportion of calcitic (>80% calcite out of total carbonate by X-ray diffraction [XRD]) samples from 32% to 73%. The δ 18 O shift is correlatable through several locations across the Greater Green River Basin, and also coincides with a previously observed shift to less radiogenic 87 Sr/ 86 Sr. Minimum δ 18 O values observed are the same as values previously reported in aragonitic bivalves from the same unit, indicating that low δ 18 O in this record is not diagenetic. A simultaneous shift to evaporative conditions in the Uinta Basin to the south indicates that the δ 18 O shift and lake-type change are not driven by regional climatic cooling and/or humidity increase. We propose that all of these observations resulted from the capture by Lake Gosiute of a river that drained higher elevations in central or north-central Idaho. Mass-balance modeling of Eocene Lake Gosiute indicates that capture of a river with an annual average discharge of ~20 billion m 3 /a (slightly larger than the modern Snake River) and δ 18 O of -24‰ standard mean ocean water (SMOW) or lower would be capable of producing the observed change. A more likely alternative is a river with less negative δ 18 O and greater discharge. For example, if river waters had a δ 18 O composition of ~−16‰, an estimated river discharge of ≥50 billion m 3 /a would produce the same effect.
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