A SEDIMENTOLOGIC ANALYSIS OF THE LATAH FORMATION OF LAKE CLARKIA (MIOCENE), IDAHO

Lake Clarkia is a Miocene Lagerstätte near Clarkia, Idaho, which was deposited ~15 Ma when the ancestral St. Marie’s River was dammed by the Wanapum flow of the Columbia River Basalt Group. This Lagerstätte is world famous for its exceptionally well-preserved plant and insect fossils. These fossils are preserved within the Latah Fm. which has been described as Middle Miocene lacustrine and fluvially deposited finely laminated siltstone and, claystone, with minor amounts of sandstone. Although detailed paleontological studies at Lake Clarkia are numerous, sedimentologic analyses of similar detail have yet to be conducted. Here we present a detailed description of the sedimentary facies present in the Latah Fm. at Lake Clarkia to give a more thorough understanding of the depositional and environmental conditions during formation. Samples from two locations were collected, dried, cut, preserved in resin, and polished perpendicular to laminae for detailed sedimentological analyses. These samples were analyzed on a micro-scale (mm) to identify specific sedimentary facies and features present, and to provide detailed interpretations of the depositional environment of Lake Clarkia. Thus far, six discrete sedimentary facies have been identified, including background sedimentation, event beds showing rapid sedimentation influx, and volcanic ash. Background sedimentation is represented by darker laminae containing the highest concentrations of organic matter of all facies. Differing rates of background sedimentation demonstrate possible seasonal variation of deposition. The volcanic ash is the lightest colored facies and contains very little organic material. Multiple event facies were identified; determination of mechanism of emplacement of different events is ongoing. Event facies are overall lighter in color, slightly coarser than the ash layers, and contain small concentrations of relatively large fragments of organic matter. Mineralogic analysis will help constrain these facies and clarify the depositional environments, and potential provenance, of the sediments at Lake Clarkia.