PREDICTING LACUSTRINE MICROBIALITE DISTRIBUTION AND FACIES ASSOCIATIONS: THE EOCENE GREEN RIVER FORMATION ANALOGUE

The Green River Formation contains one of the most abundant and varied records of microbialites known from lacustrine systems. Microbialites include stromatolites, oncoids, thrombolites, caddisfly-microbialites, tufa and other spring-related deposits. They are commonly organized into biostromes and less commonly into large bioherms. Microbialites are found in all the formation’s members and in a variety of facies, but appear to be restricted to facies deposited in littoral zones during lake expansions and/or where abundant calcium-rich inflow from rivers or springs occurs.

Facies with microbialites include the classic fluctuating profundal facies association composed of dolomicrite, ooids, and flat-pebble conglomerate at the base; followed in turn by stromatolite biostromes, kerogen-rich laminated calcimicrite (oil shale), and kerogen-poor laminated calcimicrite. This association was dependent on low-gradient lake bottoms that were greatly affected by flooding events.

Less common, is an aggradational facies association composed of stacked, 10-30 m-thick successions of oolite and grainstone, stromatolites, wackestone, and carbonate mudstones. This association was deposited in balanced-fill to overfilled lakes with higher lake-bottom gradients. Bioherms, 1-3 m high, occur and may have developed in response to calcium-rich spring inflow.

Oncoidal-conglomerate is an uncommon facies association containing oncoids that occurs within conglomerate in steep, near-shore settings, where the lake flooded a fluvial conglomerate. This association is 3-10 m thick, typically associated with islands or basin sills, and was deposited during overfilled lake phases. This association commonly occurs as thin lacustrine units within fluvial sequences.

A careful study of Green River Formation facies that contain microbialites allows conclusions to be reached about factors that promote microbialite growth and development of microbialite-associated facies. These factors include calcium input, lake, bottom gradient, lake expansions and contractions, low nutrient input, low turbidity, shallow water depth, and relatively high energy. The factors also allow one to predict microbialite distribution and facies associations when exploring for hydrocarbons in deeply buried lacustrine formations.