FACIES ASSOCIATIONS AND MICROBIALITES OF GIANT LAKE SYSTEMS: ANALOGS FOR THE PRE-SALT OF OFFSHORE BRAZIL AND ANGOLA
The Green River Formation (GRF), although smaller (~400 km across), is thicker (>1000 m), more complex, and contains the richest record of lacustrine microbialites known. Stromatolites dominate, but oncoids, thrombolites, and tufa-like microbialites occur. Laterally extensive biostromes predominate in a facies association of oolitic packstone/rudstone – microbialite – kerogenous mudstone (oil shale) – dolomitic mudstone. These formed on low gradient lake bottoms. Some biostromes can be traced over 40 km. Large bioherms also occur. These contain clusters of meter-size microbialites. Bioherms are in a facies association of stacked, 10-30 m-thick successions of oolite-grainstone, microbialite, wackestone, and carbonate mudstone deposited on relatively higher topographic gradients. The GRF‘s complexity is due largely to its accumulation in overfilled, balanced-fill, and underfilled lake systems. The GRF contains stevensite, “arborescent” and “arbustiform” stromatolites similar to those from the Brazilan pre-salt (Terra et al., 2010), and radiating crystals shrubs among other structures, which make it the excellent analog to pre-salt lacustrine systems.