TEPHROCHRONOLOGY OF MID-MIOCENE CLARKIA LAKE

Mid-Miocene Lake Clarkia formed when the ancestral St. Maries River in northern Idaho was dammed by Columbia River Basalt flows, specifically Priest Rapids Member flows of the Wanapum Basalt. Much of the resulting accumulation of lacustrine sediment occured in anoxic conditions from the time of deposition to the present, resulting in exquisite preservation of abundant leaf fossils in thinly laminated sedimentary deposits. Numerous silicic ash beds are interbedded with the lacustrine sediments. We present a complete ash stratigraphy for the southern end of the lake. Ashes fall into three compositional groups with distinct tectonic affinities that can be attributed to: (1) intraplate sources, derived from volcanoes associated with the Yellowstone hotspot track; (2) volcanic arc signatures, probably derived from the Cascade volcanoes; (3) high-silica rhyolite sources of equivocal affinity. Glass shards of group (1) rhyolitic ashes have high FeO contents, indicating sources from the early western portion of the hotspot track, constraining Miocene Lake Clarkia to the period between 15 Ma and hotspot initiation ~16.7 Ma. Potential chemical correlations exist between Clarkia ashes and tephra layers of known ages in sediments of the Bully Creek Formation, Oregon and the Española Basin, New Mexico, as well as the Cold Springs Tuff, Nevada. These correlations, if correct, constrain the age of Lake Clarkia to 15.4 – 16 Ma. This places the formation of Lake Clarkia at the peak of the mid-Miocene Climatic Optimum, and implies that the bulk of the Columbia River Basalts (Steens through Wanapum Formations) was erupted prior to 15.4 Ma.