EXPOSURE DATING OF MAJOR LANDSLIDES ALONG THE TETON FAULT, WY: PRELIMINARY RESULTS AND IMPLICATIONS FOR PALEOSEISMICITY

Although the Teton fault (northwest Wyoming, USA) has been seismically quiescent in historic times, evidence points to the occurrence of large earthquakes during the Holocene: well-preserved fault scarp exposures along the east side of the range indicate tens of meters of displacement since deglaciation (~15 ka). The potential for strong earthquakes and related slope failure events presents significant hazards to park visitors, yet an incomplete record of postglacial seismicity impedes our ability to characterize the magnitude of these hazards. Past seismically-induced landslide activity is evident in the presence of large head scarps near the fault trace and distinct slope failure deposits in adjacent lake basins, identified in both seismic stratigraphy and lacustrine sediment cores. Here we present preliminary results from the first effort to directly date a large landslide scarp on the southwest side of Jenny Lake, Grand Teton National Park, using the cosmogenic radionuclide ¹⁰Be. Excluding one anomalously young age, one head scarp and two boulder samples yield a mean ¹⁰Be age of 8.27 ± 0.23 ka. This mean age is consistent with a prominent disturbance layer observed in sediment cores from Jenny Lake, where radiocarbon dating of terrestrial plant material above the layer provides a minimum limiting age of 8.3 ± 0.7 ka for the event. A smaller landslide scarp on the northwest side of Jenny Lake provides the opportunity to date another earthquake-triggered slope failure event using ¹⁰Be. A cluster of mature trees submerged in ~80 feet of water directly below the scarp dates to 1352 ± 60 AD based on multiple radiocarbon ages, indicating that this landslide is correlative to a more recent earthquake. Ongoing efforts to date these landslides using ¹⁰Be offer the opportunity to not only improve the paleoseismic record for the Teton fault, but also to further refine the calibration of local ¹⁰Be production using independent age control for two significant tectonic events spanning much of the Holocene.