A TALE OF TWO RIVERS: FLUVIAL RECORDS OF COMPETING MOISTURE SOURCES IN A BROKEN EOCENE FORELAND

During the early Eocene, the greater Green River basin occupied the relatively low, dry region between the North American Cordillera to the west and Laramide basement uplifts to the east. Lake Gosiute, at the center of this system, collected water and sediment from the catchments of two regional paleoriver systems draining these respective uplifted areas, as well as from short-headed catchments draining basin-bounding Laramide ranges to the north and south. The Idaho paleoriver entered the greater Green River basin from the northwest, delivering water and sediment from the eastern flank of the Cordillera in central and northern Idaho, while the Aspen paleoriver drained the basement-cored western slope of the Rocky Mountains of northern Colorado and entered the basin from the southeast. Legacy regional climate modeling suggests that the Idaho paleoriver collected atmospheric moisture sourced from the eastern Pacific Ocean, predominantly during winter months, whereas the Aspen paleoriver received a higher proportion of atmospheric moisture that originated in the Gulf of Mexico, predominantly during summer months.

This study compares and contrasts the fluvial architecture and sedimentology of these two fluvial systems to assess differences in allogenic influences on each river and consider how each system influenced the dynamic and evolving hydrology of Lake Gosiute. To quantify these systems from outcrops across the basin, we collected detailed outcrop sedimentologic observations as well as 3D models generated using structure-from-motion photogrammetry from unmanned aerial vehicle imagery. Pairing of these data permits the assessment of fluvial deposits at both the micro (e.g. sedimentary structures) and macro (e.g. channel architectures) scales, enabling a more holistic understanding of the nature of these rivers. Preliminary results indicate larger grainsizes, larger sedimentary structures, a higher degree of amalgamation, and larger fluvial architectures in the deposits of the Idaho paleoriver compared to those of the Aspen paleoriver, suggesting that the Idaho paleoriver was not only notably larger than the Aspen paleoriver, but likely more perennially established as well.