MICROBIALITE AND ASSOCIATED FACIES IN THE EOCENE GREEN RIVER FORMATION: COMPARISON WITH SOUTH ATLANTIC APTIAN PRE-SALT LACUSTRINE SYSTEMS

Details concerning the spatial and stratigraphic relations of the Green River Formation (GRF) microbialites are being studied with an eye toward developing models that can apply to South Atlantic Aptian, pre-salt, lacustrine systems. The pre-salt microbialite facies host abundant hydrocarbons. This has motivated a quest for modern and ancient analogs for these microbialites and associated facies. However, little is known about the spatial and stratigraphic distribution of microbialites in large, long-lived, lacustrine basins in general, primarily because there are few such basins known. The GRF of Wyoming, Utah, and Colorado is an exception, and provides probably the best analog. The excellent exposures of GRF stratigraphic sequences contain microbialites including bioherms, biostromes, oncolites, microbialite-associated tufa, and spring deposits. These can be abundant and record basin margin to center facies transitions.

The GRF provides insight into the development of: (a) large, multimeter-size bioherms, (b) smaller, patch-reef-like bioherms, (c) inter-biohermal facies, (d) facies transitions toward lake depocenter of bioherm-bearing successions, (e) biostromes, (f) spring deposits (tufa) associated with microbialites, (g) microbialites composed of shrubs, and (h) successions with stevensite, microbialites, and oolites. The facies relations of these insights can be used to recognize patterns and develop depositional models.

We propose models that explain the facies patterns that developed as the lakes expanded and contracted in response to climate, precipitation/evaporation, basin subsidence (accommodation space), and rates of sedimentation. These models have direct application toward understanding pre-salt lacustrine systems. In addition, they can be related directly to the under-filled, balance-filled, and over-filled model of Carroll and Bohacs.

Although no perfect analog exists for the pre-salt, the Green River Formation provides an excellent analog that compares very well with many of the features of the pre-salt lacustrine systems.