LACUSTRINE PALEOENVIRONMENTAL CONTROLS ON LAYERING VARIATIONS WITHIN GIANT STROMATOLITES OF THE EOCENE GREEN RIVER FORMATION, SAND WASH BASIN, COLORADO

Lacustrine basins present a sensitive environment with sediments that reflect the regional climate and tectonics. Lacustrine carbonate sediments, especially those from environments in which microbialites form, can provide valuable archives of lake conditions. An extraordinary occurrence of giant stromatolites is found in one, well-defined parasequence of the LaClede Bed (Lower Laney Member, Green River Formation) at Vermillion Creek, in northwestern Colorado. The parasequence records a regional expansion of Eocene Lake Gosiute after the Early Eocene Climatic Optimum (52–50 Ma). These giants, up to 5.5 m tall and 7 m wide, formed over a broad area (16 km²) in the southeast margin of the lake.

We performed aerial surveys using an unmanned aerial vehicle (drone), field mapping, and sampled five giant stromatolites sites during the summers of 2014 and 2015. The giant stromatolites have a peculiar layering on the cm-scale with individual layers composed of a variety of stromatolites, and shrubs (microbialites). Each individual layer from the five stromatolites were collected. Detailed morphological descriptions of the microbialites in each layer was done to provide vertical and horizontal data control. Chemical and mineralogical analyses of the layers include: (1) XRD; (2) REE + Y and (3) C, O, and Sr stable isotopes.

The full range of environments and the imprint of these environmental conditions on stromatolite microstructure, mesostructured, and macrostructure are not well understood. The large size, the layering, and thick accumulation of microbialites in these giant stromatolites present a special case to understand chemical and physical lake basin dynamics that can be recorded in stromatolites. The detailed analysis of the layering patterns and their constituent microbialites of the giants will allow the reconstruction of regional and possibly basin-wide environmental lake conditions.