4 Dec (GEOL) Lydia Staisch, Structural Geology

Speaker: Lydia Staisch, Geology, Minerals, Energy, and Geophysics Science Center, United States Geological Survey

Title: Miocene-Pleistocene river reorganization across the Pacific Northwest and northern Rocky Mountains: clues from detrital zircon provenance and incision markers

Time: Thu, 4 Dec, 4:30pm

Location: Exploratory 1309 and via Zoom (for link, email lhinnov@gmu.edu)

Host: Linda Hinnov

ABSTRACT: For more than a century, researchers have debated the route of the Snake River across the northern Cordillera and Pacific Northwest, including the associated incision history of Hells Canyon. Here, we use detrital zircon U-Pb provenance analysis of Miocene strata upstream and downstream of Hells Canyon constrain the evolution in fluvial pathways over time. Downstream of Hells Canyon, we show that the ~10-6 Ma Clarkston Heights Gravel was dominantly sourced from the nearby Salmon and Clearwater rivers with substantial contribution from drainages that now flow into the western Snake River Plain (wSRP). These same drainages were also contributing to time-equivalent Lake Idaho strata within the wSRP, upstream of Hells Canyon, suggesting Hells Canyon was an active fluvial connection between the wSRP and Columbia Basin before 6 Ma. Results from Lake Idaho strata suggest a two-phased lacustrine history in the wSRP: before ~6.0 Ma, lake strata were derived from local tributaries, suggesting the wSRP was isolated from the modern upper Snake River system. From ~4.3–2.2 Ma, Lake Idaho expanded to include distal tributaries in conjunction with eastward migration of topography from the Yellowstone Hot Spot. Our results challenge the long-held hypothesis of “capture” of the Snake River through Hells Canyon. We infer that the Hells Canyon was a long-established route for outflow of lakes occupying the wSRP, possibly intermittently, during a ~10-2 Ma phase of slow regional incision, followed by rapid incision in Hells Canyon starting about 2 Ma in conjunction with erosion of the topographic barrier impounding Lake Idaho.