STROMATOLITES OF THE LACLEDE BED, GREEN RIVER FORMATION (EOCENE): AN ARCHIVE OF PALEOENVIRONMENTS AND CLIMATE

It is proposed that stromatolites that formed on lake margins contain a sensitive and high-resolution record of lake dynamics and climate, not recorded in lake-center sediments. Laminated sediments from lake-center environments are usually used to provide a detailed record of lake chemistry and processes. However, lake-center laminated sediments are insulated from the higher resolution dynamics effecting near-shore sediments and surface conditions on the lake as a whole. Stromatolites are the only laminated record usually preserved in the near-shore environment and thus have the potential to provide unique high-resolution information to aid in paleoclimatic interpretations. Lake margin areas are the first to be effected by changes in lake level, variations in fresh water inflow, and other paleoclimatically induced factors. The stromatolites of the LaClede Bed of the Eocene Green River Formation provide an ideal candidate to study the archive of lake climate and paleoenvironmental dynamics. They are well preserved and well laminated. The strata can be correlated from the lake margin toward the lake center, over distances exceeding 20 kilometers. Individual subsets of laminae within the stromatolites can be correlated as well. Preliminary results have shown that the stromatolites change very little in size and internal laminae structure from nearshore to relatively deeper water environments. These observations suggest that (a) a broad band of relatively shallow water existed at the lake margin or (b) water depth did not affect the internal or gross morphology of these stromatolites. Vertical profiles within the stromatolites record details of lake fluctuations during major transgressions of the lake. Minor regressions are recorded by fragments of stromatolites that are stabilized by subsequent microbial laminae. An understanding of the archive of paleoenvironmental and climatic conditions recorded in the stromatolites of the LaClede Bed will provide a basis for application to other paleoenvironmental studies where stromatolites occur.