Effects of the Mid-Miocene Climatic Optimum and the Eruption of the Columbia River Basalt on Paleo-sedimentation Processes in Clarkia Lake, Idaho: Evidence from Tephrochronology

Abstract:

Miocene Lake Clarkia formed when lava flows of the Priest Rapids Member of the Columbia River Basalt dammed the ancestral St. Maries River, in northern Idaho. The lake deposits are well known for their exceptional preservation of leaf fossils. Chemically correlating lacustrine tephra layers to volcanic centers, of known compositions and ages, can be used to help better constrain the timing and rate of deposition in the lake. Early work attempted to constrain Clarkia Lake’s depositional age from index fossil flora correlations. More recently, electron microprobe analyses of Clarkia Lake’s ash deposits demonstrate that several ash layers may correlate with Nevada’s Yellowstone hot spot Cold Springs Tuff of the Santa-Rosa Calico volcanic field (15.5-15.9 Ma; Ladderud et al., in press). In the present study, further microprobe analysis has uncovered additional ash layers with the same range of chemical compositions. The apparent identical compositions of multiple tephra layers at different stratigraphic positions may be attributed to the eruption of multiple, chemically similar magmas, perhaps mapped as a single unit in the source area, or to post-emplacement re-working of these layers by sedimentary processes. The latter is considered less likely because many of the ashes contain little detrital material. Despite the ambiguity, these chemically identical tephra layers have the potential to provide information about the unique tephra composition and eruptive frequency in the source area, and/or unusual hydrological reworking processes associated with the mid-Miocene lacustrine environment.

Reference: Ladderud et al., Northwest Science, in press, 2015.