Neogene and Pleistocene landslides along the Grand Valley fault in eastern Idaho and western Wyoming, USA, and their relationship to caldera-forming eruptions of the Yellowstone hotspot

Abstract Caldera-forming eruptions from the Yellowstone hotspot (western USA) have been some of the most destructive geologic events in Earth’s history. These large-scale eruptions appear to be coincident with numerous large mass-wasting deposits that contribute to valley-fill sequences adjacent to the Yellowstone hotspot. Within the Mio-Pliocene Salt Lake Formation and the Pleistocene Long Spring Formation, at least 20–21 landslide deposits are exposed over a distance of 65 km from Irwin, Idaho, to Bedford, Wyoming. The majority of these deposits are remarkably similar and exhibit characteristics of either rock-avalanche or brecciated slide-block deposits that form as a result of catastrophic, large-volume (>1 Mm3) rock falls and slides. A stratigraphic framework was constructed in the area of Grand Valley to determine the relative position of the landslides and estimate their ages. Interbedded tephra deposits originating from the volcanic fields to the northwest provide key marker beds. Five new 40Ar/39Ar ages were determined, including the first age for a tephra deposit in Star Valley, to aid the correlation of outcrops in Grand Valley and between Grand Valley and Star Valley. The age of these ash deposits better defines the formation and evolution of the Grand Valley–Star Valley fault. Our detailed correlations and mapping have resulted in the identification of 11 different stratigraphic horizons containing landslide deposits; intervening ash beds allow us to approximate the timing of these landslides. The oldest landslide in Grand Valley occurred ca. 9 Ma and coincides with eruptions from the Twin Falls volcanic field. We identify six additional landslide horizons between 6.66 Ma and 4.0 Ma, one horizon between 4.0 Ma and 2.1 Ma, two horizons between 2.1 Ma and 0.6 Ma, and one landslide in the Late Pleistocene (<130 ka). The correlation of the various landslide deposits and the broader stratigraphic framework suggest that most of these landslides are coincident with the caldera-forming ignimbrite eruptions associated with the volcanic fields of the Yellowstone hotspot.