VARIATIONS IN MICROBIALITE MORPHOLOGY AND PRESERVATION IN THE EOCENE GREEN RIVER BASIN

The early Eocene Green River Basin of Wyoming preserves records of life in and around geochemically and hydrologically variable paleolakes, including extensive beds of microbialites. We collected samples from microbialite beds and in float which were originally deposited ~52 to <50 Ma, and investigated the diversity in morphology, scale, mineralogy, and diagenetic alteration at three sites to identify variations that may be driven by local or regional conditions. Macroscale features identified in the field ranged in size from millimeter to decimeter, and primarily included domes (sometimes elongated or columnar), digitate structures, and “brainy” surfaces. Mesoscale features included color changes and transitions between continuous layers and columnar/digitate structures. Most were finely laminated, with variations in lamination smoothness and color. While most morphologies were present at all sites, their organization varied. In the <50 Ma Laney Member, Green River Formation, large domes and elongated U shapes contained alternating continuous layers and smaller columns. In a ~50 Ma bed between the Wasatch and Bridger Formations, domes frequently transitioned into branching and digitate structures of highly variable scale. In the 51.6 Ma Rife Bed, some columns and domes were composed of continuous layers and others were built from mm-scale branching columns with rare continuous layers. Diagenetic alteration also varied between and within sites. At the Rife Bed site, recrystallization destroyed lamination in some samples, while it was retained in others. At the Wasatch/Bridger site, similar morphologies were preserved in carbonate and partially or fully silicified samples. We will present μCT scans of representative samples from each site to analyze the 3D morphological variation. While similar microbialite morphologies are preserved at all three sites, the variations in their presentation may be informative of changes in lake conditions.