UC Santa Barbara

The Eocene Green River Formation is the best studied ancient lacustrine deposit. It also contains the most diverse and abundant microbialites of any lake deposit. Unfortunately, there have been few studies of these microbialites. Microbial bioherms and biostromes formed considerable primary carbonate and rare Mg-clay, deposits in the littoral to sublittoral environments of Green River lakes. These deposits were studied in three areas: Vermillion Creek (Colorado), Evacuation Creek (Utah), and Sanpete Valley (Utah). The microbialites were formed mostly under balanced fill, saline-alkaline lake conditions and their morphology was influenced by fluctuating lake dynamics.1. The upper LaClede Bed, Laney Member of the Eocene Green River Formation at Vermillion Creek, Colorado, contains an unusual, unique microbialite: giant columns, over 5 m tall, composed of 3-10 cm thick layers, that in turn are composed of columnar, columnar branching, domical, and shrubby stromatolites. These stromatolites are largest known lacustrine columnar stromatolites. Large coniform stromatolites also occur as do stromatolite encrusted logs. The layering patterns and their contained microbialite types resulted from lake-water fluctuations due to changing climatic conditions (primarily arid vs. humid) and provide critical indicators for paleoenvironmental analysis. 2. Two microbialite-bearing carbonate short-term cycles were examined from the upper Douglas Creek Member of the Eocene Green River Formation at Evacuation Creek, Utah. The carbonates are notable because of the number of distinctive microbialites occurring as biostromes and bioherms that are traceable over several kilometers. Nine microbialite facies were established. Unique arborescent and branched minicolumnar stromatolites, which resembles shrubs, developed within the bioherms. Each distinctive microbialite facies reflect high-frequency fluctuations of lacustrine depositional conditions. All carbonate facies successions are associated with two short-term rhythmic cycles that record periods of fluvial quiescence (aridity) represented by littoral carbonate deposition under an evaporative regime. The presence of dolomicrite mineralization within the carbonate beds and analcime within siliciclastic beds indicate a microbial and evaporative shallow depositional environment under saline alkaline conditions. Most of the identified diagenetic phases took place in near-surface and shallow diagenetic settings. The lateral extent and facies changes of the microbialite carbonate beds allow a reconstruction of a NE-SW lake shoreline at the western shoulder of the Douglas Creek Arch, and how these carbonates beds evolved. 3. In Sanpete Valley, Utah, in the upper Green River Formation, in addition to microbialites, the Mg-clay mineral stevensite (and other Mg-clay minerals) is found, in both carbonate-chert microbialites and claystone. Also, this area was subjected to volcanoclastics. Various species of Mg-clay minerals (stevensite, mixed-layer kerolite-stevensite, and sepiolite), along with calcite and dolomite, were found in four Mg-clay-bearing lithofacies: 1) Mg-Clay-bearing Stromatolites; 2) Mg-Clay Arenites; 3) Mg-Claystone with Calcite Aggregates; and 4) Intraclastic Hybrid Arenites and Conglomerates. Mg-clay bearing rocks and the microbialites from the Sanpete Valley were deposited in a southwestern, shallow embayment of Eocene Lake Uinta along the margin of the Sevier fold and thrust belt. All the findings provide a comprehensive framework for the lithostratigraphic and chemostratigraphic evolution of the lake system, as well as texture classification, providing a unique analog for detailed correlation and comparison to other basins containing similar Mg-clay and microbialite-bearing deposits, in particular the Cretaceous pre-salt deposits in off-shore Brazil and Angola.