RELATIONSHIPS AMONG MORPHOLOGY, TEXTURE, AND CHEMISTRY IN STROMATOLITES OF THE GREEN RIVER FORMATION (EOCENE, WYOMING, USA)

Stromatolites are among the earliest records of life on Earth and have been proposed as likely targets for biosignature exploration on other planets. They form in two principal ways: by in situ mineral precipitation and by the trapping and binding of carbonate grains. From these basic building blocks, microbialites express a wide variety of microscale textures and macroscopic forms. However, the relationships among their morphology, texture, and chemistry are poorly understood.

Lakes, especially closed-basin lakes that experience frequent changes in size and salinity, provide a good record of environmental changes because of their sensitivity to precipitation and evaporation. Likewise, the morphology, texture, and chemistry of microbialites are greatly influenced by environmental changes. The Green River Formation is a carbonate and clastic succession deposited in a complex system of interconnected mountain lakes during the Eocene Epoch. The Green River stromatolites display an array of macroscopic morphotypes across basins and even within individual beds. To probe connections between morphology and texture, samples were collected from stromatolite-rich horizons at White Mountain (Greater Green River basin) and Sand Butte (Washakie basin). Stromatolites from both localities are dominated by precipitated and grumeaux microscale textures, with subsidiary trapped-and-bound texture. Preliminary analysis using cathodoluminescence suggests distinct, lamina-scale chemical differences among fabrics, likely indicating changes in lake water conditions; however, the origin of those differences is presently unknown. To probe the relationship between texture and geochemistry, stromatolites from a single biostrome horizon (La Clede bed, Laney Member) were sampled at lamina-scale for trace element composition in conjunction with detailed petrography. This approach provides a unique opportunity to establish detailed relationships among texture, morphology, and chemistry for stromatolites at a single stratigraphic level.