SUBSURFACE EVIDENCE FOR A RAPID FORMATION OF THE CLARKIA MIOCENE LAKE IN NORTHERN IDAHO

A recent drilling program across the St. Maries River valley near the town of Clarkia in northern Idaho coupled with geophysical surveys and geochemical studies provides new information on the geologic history of the area and the origin of the Miocene lake deposits near Clarkia. The three drill cores reached the same basement rock at different elevations, and recorded an abrupt lithofacies shift from coarse-grained alluvial sediments at the lower part to fine-grained lacustrine sediments in up-section. Lake sediments were correlated across the lacustrine deposits in the study area by both physically tracing and geochemically comparing distinctive volcanic marker beds. Geophysical data derived from magnetic and seismic surveys were consistent with the borehole data and indicate that eroded Precambrian metamorphic rocks of the Wallace Formation (Belt Supergroup) served as an irregular basin floor above which relatively coarse-grained Cenozoic alluvium were initially deposited in at least two parallel, north-south trending channels which were separated by a prominent divide. The subsurface data reveal a rapid formation of the Miocene lake environment above the channel deposits. The new subsurface data gathered in this investigation support the previous findings based on limited surface geologic data and conclude that the Miocene lake was rapidly created by a Columbia River basalt now damming the preexisting proto-St. Maries River drainage.