Geomorphic controls on lacustrine isotopic compositions: Evidence from the Laney Member, Green River Formation, Wyoming

摘要

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 balanced-filled in the lower LaClede Bed to overfilled 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 correctable 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