Predicting Lacustrine Microbialite Distribution and Facies Associations: The Eocene Green River Formation Analogue

The Green River Formation contains one of the richest records of microbialites known from lacustrine systems. The microbialites include stromatolites, oncoids, thrombolites, caddisfly-microbialites, and tufa-microbialite structures.

Four basic facies associations containing microbialites are recognized in the Green River Formation: Type I Biostrome, Type II Bioherm, Type III Oncolite, and Type IV Tufa-microbialite. A fifth facies association without microbialites (Type V Barren) is useful for comparison.

The Type I Biostrome facies association is composed of dolomicrite, ooids, and flat-pebble conglomerate, followed in turn by stromatolite biostromes and oil shale. This association was deposited in balanced-fill lakes and on low-gradient lake bottoms.

The Type II Bioherm facies association is composed a succession of oolite and grainstone, large stromatolite bioherms, wackestone, and carbonate mudstones. Microbialites represent aggradational stacking and are restricted to localized build-ups in a near-shore, littoral setting. This association was deposited in overfilled lakes or balanced-fill lakes with stable shorelines.

The Type III Oncolite facies association is composed of conglomerate followed by oncoids. This facies interfingers with fanconglomerate facies in steep, near-shore settings, where lake margin alluvial fans were transgressed. This association was associated with island margins or basin sills, and was deposited on higher gradient lake bottoms during overfilled-lake phases.

The Type IV Tufa-microbialite facies association is composed of localized intraclastic conglomerate succeeded by porous microbialites that are mostly columnar in shape. They may be associated with spring-inflow features. This association was deposited in overfilled to balanced-fill lakes.

The Type V Barren facies association is composed of grainstone/ flat-pebble conglomerate, dolomicritic oil shale, and evaporites, but lacks microbialites. It was deposited in underfilled lakes that were saline-alkaline and deficient in calcium.

The Green River Formation microbialite facies association types and their relationship to lake types provides an excellent analogue for understanding and hence predicting microbialite distribution and facies associations when exploring for hydrocarbons in deeply buried lacustrine formations.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California