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Giant Microbialites from the Eocene Green River Formation (Laney Member, Sand Wash Basin), Colorado


The discovery of oil in large microbial carbonate reservoirs in the lacustrine Cretaceous pre-salt of Brazil and Angola has generated great interest in microbialites, lacustrine carbonates, and lacustrine systems in general. Recent and ancient analogs have been sought in order to develop models for the pre-salt to facilitate exploration; however, all analogs known to date lack one or more features reported from the pre-salt. The Eocene Green River Formation of Wyoming, Utah, and Colorado may be the best overall analog. The lacustrine system covered 77,000+ km2, is over 2000 m thick, contains stevensite, has numerous subaqueous spring buildups, is rich in microbialites (many with shrubs), and has large microbialite bioherms. Giant microbialites from the Laney Member in the Sand Wash Basin, Colorado, provide a new example of these large lacustrine microbialites. The microbialites occur as individual and amalgamated structures in an ooid-dominated unit covering an area of ∼2 km2. Some amalgamated structures exceed 5 m in height and 10s of meters in width. Many of the microbialites encrusted logs. Individual and amalgamated microbialites are complex, consisting of cm-thick, successive layers of boundstone, cm-wide columns, shrubs, cm-size laterally linked domical stromatolites, and planar laminated stromatolites. Shrubs, planar laminated stromatolites, and domical stromatolites are often silicified. A number of these microbialites occur as individual columns, 4+ m wide and up to 5.5 m tall, with laminae and layers continuous from base to top indicating a 5.5 m of synoptic relief (sticking up above the lake floor). These are among the greatest synoptic relief lacustrine microbialites known. Like the large bioherms at Little Mesa (Wilkins Peak Member), these large Laney Member microbialites grew in a nearshore environment several kilometers from nearby mountains, were associated with nearby faults, possibly formed on somewhat steeper gradients than more common Green River microbialite biostromes, and likely were supplied with calcium-rich waters from rivers and/or springs in the area.