NEAR SHORE OR OFF SHORE? RECONSTRUCTING THE DEPOSITIONAL SETTING OF AN ANCIENT LAKE ASSEMBLAGE IN STEWART VALLEY (MIOCENE), NEVADA

A model for reconstructing ancient lake environments using taphonomic and taxonomic variables for fish, insect and plant compression fossils was proposed by Wilson (1980, 1988). This model suggests that specific patterns of preservation are exhibited in lake assemblages, and those preservational patterns, when recognized, can point to certain features about the environment, such as proximity to the shoreline, relational depth of water, and types of microenvironments present. The lacustrine shales of Stewart Valley, Nevada (Miocene, 11.7 Ma) produce exquisite compression fossils of insects, plants, fish, and mollusks. The assemblage at the Pacific Union Site has been interpreted previously as an off-shore, deep water environment. Sedimentological, taphonomic, and taxonomic data will be compared to the Wilson model for reconstruction of ancient lake environments to determine whether the model can be applied to other ancient lake assemblage containing compression fossils. Fossils studied include 1,150 insects, 65 leaves, over 70 plant reproductive parts, approximately 15 mollusks, 5 articulated fish, uncounted amounts of disarticulated fish parts, and coprolites. Taphonomic data include articulation, size, preservation level and specimen orientation. Specimens are identified at least to Order and, in the case of some insects, to the Family level. Sedimentological data include a stratigraphic column and detailed descriptions of lithologies based on oriented thin sections from within the column. This in depth study of the depositional environment, taphonomy, and taxonomy of the Pacific Union Site will be an important basis for future studies in the Stewart Valley Lake Basin, and a test to the applicability of the Wilson model to lacustrine environments of different ages.