IDENTIFICATION AND TIMING OF EVENTS IN MIOCENE BASINS OF THE GROUSE CREEK MOUNTAINS REGION OF UTAH, NEVADA, AND IDAHO – INPUT FROM TEPHROCHRONOLOGY

The Utah Geological Survey is mapping the 1:100,000 Grouse Creek Quadrangle (GCQ) in NW Utah as part of a larger a larger project to generate geologic maps for all the 1:100,000 quadrangle maps of Utah. An important facet of the GCQ project is determining the age of widespread Miocene basin fill of the Salt Lake Group using tephrochronology. In all 160 tephra samples within the GCQ have been analyzed by electron microprobe at the Univ. of Utah. These analyses, along with over 100 tephra analyses in adjacent areas, were compared with the analyses of ~3200 samples in the University of Utah tephra database. These comparisons provide numerous correlations to dated tephra and the age of these tephra reveal details on: 1) the timing of initiation of basins throughout the region; 2) timing of unconformities within and between basins: 3) timing of flux of sediment into the basins from sources in the metamorphic rocks of the Raft River Range and Middle Mountain as well as sources within superjacent unmetamorphosed Paleozoic and early Cenozoic units; 4) age of extensive rhyolite lava flows and rare rhyolitic welded tuffs along the western margin of the GCQ.