BIOTIC AND TAPHONOMIC RESPONSE TO LAKE LEVEL FLUCTUATION IN THE GREATER GREEN RIVER BASIN, WYOMING

The Green River Formation is one of the most extensively studied lacustrine deposits with respect to its petrology, sedimentology, tectonic features, and fossil fish. Unfortunately few studies have examined the invertebrates, particularly the ostracodes of this world renowned fauna. Stratigraphic sections of the Laney Member from the Greater Green River Basin were measured and sampled for ostracodes at the centimeter scale. Eight species of ostracodes (Hemicyprinotus watsonensis, Potamocypris williamsi, Candona pagei, Cypridea bisulcata, Metacypris paracordata, Erpetocypris sp., Scottia sp., and Procyprois ravenridgensis) were recovered from 24 out of 49 samples within the 136 meter section. Of these eight species, three are plant dwellers, four are mud dwellers and one is nektonic. Their different ecologies reflect the facies in which the ostracodes are preserved, representing changing physical and chemical conditions through time in this balanced and overfilled lake deposit.

In addition to documenting the diversity of the ostracodes throughout the Laney Member, the preservational history of the various ostracode communities were examined. Throughout the section ostracodes commonly occur recrystallized within the kerogen rich micritic layers, with the exception of five ostracode coquinas that occur within the section. In some of the micritic layers, the ostracodes appear to have splayed along bedding planes, indicating possible post burial deformation. In addition, the recrystallized shells often appear flattened and compressed. In the coquinas, the valves are predominantly unaltered and separated with no apparent orientation within the beds, possibly representing deflation surfaces along the lake margin. Their preservational pathways may lead to further information regarding lake stability and changes in lake chemistry through time. Preliminary diversity results show the ostracodes recovered from the Lower Laclede Bed of the Laney Member track lake level fluctuations. The ostracodes in the Upper Laclede Bed represent the biotic response to the transition from a balance to overfilled basin. In addition, a distinct taphonomic signature is likely present between these varying lake types.