VARYING DEPOSITIONAL ENVIRONMENTS OF THE LATAH FORMATION IN WASHINGTON AND IDAHO

The Latah Formation is a Miocene lacustrine unit exposed in multiple localities in eastern Washington and western Idaho. The Latah Fm is generally described as quiet-water lacustrine facies consisting of mudstones and siltstones that formed between the flows of the Columbia River Flood Basalts. The sediments and fossils contained within have been useful for paleobiologic and paleoclimatic indicators, in addition to providing basic paleoenvironment interpretations for the region. In eastern Washington, the Latah Fm is not laterally extensive and is found in multiple intervals between the Wanapum and Grand Ronde flows. In western Idaho, the exposures of the Latah Fm are more continuous and contain the exceptionally preserved fossils of the Clarkia Lagerstätte. Fossils of this degree of fidelity and abundance are not found within the majority of the Latah exposures. Although many exposures of the Latah Fm may be syndepositional, it is likely that the sedimentary history within this paleolacustrine environment is much more complicated than previously thought and requires a re-description. A well core from Cheney, WA, containing three intervals of the Latah Fm has recently been described, and 10 distinct depositional facies have been identified. These results depict a much more dynamic depositional environment than previously realized, with lacustrine facies ranging from low energy background sedimentation to high energy debris flows. Here we present this data from the Cheney core in addition to revised, detailed facies descriptions from multiple localities within the depositional basin. This study aims to correlate between the core and some of the nearby sites in Washington as well as infer possible taphonomic causes for the localized Clarkia fossils. This will result in a more comprehensive understanding of the complex history of this paleolake.