TEPHROCHRONOLOGY OF MID-MIOCENE CLARKIA LAKE: BETTER CONSTRAINING THE LAKE’S AGE AND INSIGHT INTO MID-MIOCENE LACUSTRINE DEPOSITIONAL PROCESSES

The Clarkia lake beds, part of the Latah Formation of northern Idaho, formed when the ancestral St. Maries River was dammed by Priest Rapids Member flows of the Wanapum Basalt. Finely laminated lacustrine sediments accumulated at the lake bottom, preserving both suspension-settled plant macrofossils and tephra strata, the latter derived from a variety of sources (Yang et al., 1995, Nash & Perkins, 2012, Ladderud et al., 2015). Continuing excavation and sampling of core from Clarkia Lake sediments is revealing more ash beds; this contribution is a status report on ash geochemistry, building on the work of previous authors. Ash layers consist of angular glass shards or finely vesicular pumice fragments mixed with varying amounts of detrital material. Tephras fall into three compositional groups with distinct tectonic affinities that can be attributed to: (1) intraplate sources, derived from volcanoes of the Yellowstone hotspot track; (2) volcanic arc signatures, derived from the Cascades; (3) high-silica rhyolites of equivocal affinity. High Fe contents of group (1) rhyolitic ashes indicate sources in the early, pre-15 Ma, western portion of the hotspot track. One set of ashes in group (1), representing repeated eruptions of very similar compositions, may correlate with the complex ~15.5 Ma Cold Springs Tuff of the Santa Rosa-Calico volcanic field, northern Nevada; another ash in this group matches a ~15.7 Ma ash described by Nash & Perkins (2012). High-silica rhyolites include a chemically variable tephra layer, likely the distal equivalent of a compositionally zoned ignimbrite. Sources for high-silica rhyolites may lie in the southern Great basin or, possibly, calderas buried beneath the modern Cascades. These results suggest that the Priest Rapids lavas are up to 1 million years older than the currently accepted age of ~15 Ma.

References: Ladderud et al. (2015) NW Science 89, 309-323; Nash & Perkins (2012) PLOS ONE 7:e44205; Yang et al. (1995) NW Science 69, 52-59.