SUBSURFACE EVIDENCE FOR A RAPID FORMATION OF THE CLARKIA MIOCENE LAKEIN 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 elev ations, and recorded an abrupt lithofacies shift from coarse-grained a lluvial sediments at the lower part to fine-grained lacustrine sedimen ts in up-section. Lake sediments were correlated across the lacustrine deposits in the study area by both physically tracing and geochemical ly comparing distinctive volcanic marker beds. Geophysical data derive d from magnetic and seismic surveys were consistent with the borehole data and indicate that eroded Precambrian metamorphic rocks of the Wal lace Formation (Belt Supergroup) served as an irregular basin floor ab ove which relatively coarse-grained Cenozoic alluvium were initially d eposited in at least two parallel, north-south trending channels which were separated by a prominent divide. The subsurface data reveal a ra pid formation of the Miocene lake environment above the channel deposi ts. 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.