PROVENANCE OF THE MIOCENE CHALK HILLS FORMATION AND RELATION TO HOTSPOT LANDSCAPE EVOLUTION, WESTERN SNAKE RIVER PLAIN, IDAHO

During deposition of the Miocene fluvio-lacustrine Chalk Hills Formation, Yellowstone hotspot activity on the western Snake River Plain produced a dynamic topographic landscape. However, the timing and magnitude of landscape response and changes in drainage patterns to hotspot departure is not well constrained. The prevailing drainage model indicates throughflow from the east was not established until 4 Ma due to the presence of a regional drainage barrier. Yet, flow indicators measured in volcanic deposits from the Twin Falls eruptive center published in a more recent study suggest a topographic high was not present between 10 and 6 Ma. To better characterize the landscape response to the passing of the hotspot and provide more resolution to the debate, our study uses the absence or presence of Grenville-aged recycled zircon grains (sourced from Neoproterozoic to Paleozoic strata in eastern Idaho) to test for a drainage connection between eastern and western Idaho by the late Miocene.

For this study, we collected 6 sedimentary samples from the western Snake River Plain and obtained detrital zircon U-Pb ages with a laser-ablation mass spectrometer at the University of Arizona LaserChron Lab. The dominant detrital zircon populations identified in the Chalk Hills samples include: Eocene Challis Volcanic Group, Cretaceous-Paleogene Idaho Batholith, and Miocene Yellowstone volcanic units. Two peaks centered at 110 Ma and 130 Ma are hypothesized to be sourced from accreted terrains to the west. Grenville-aged zircons were observed only in a sample taken from the Pliocene Glenns Ferry Formation unconformably overlying the Chalk Hills. Given the presence of Grenville-aged grains within the distinct detrital zircon signature of modern Snake River Plain sediments, the absence of Grenville grains in the Chalk Hills supports a hypothesis promoting a tumescent landscape preventing throughflow of the Snake River from the east through the Miocene.