FACIES ASSOCIATIONS AND MICROBIALITES OF GIANT LAKE SYSTEMS: ANALOGS FOR THE PRE-SALT OF OFFSHORE BRAZIL AND ANGOLA (Invited Presentation)

Lakes have been pumping out microbialites for over 2.7 billion years. Interest in lacustrine microbialites has skyrocketted with the discovery of significant hydrocarbon reserves in microbialite and microbialite-like tufa-containing lacustrine deposits in the pre-salt of Brazil and Angola. A quest for analogs has ensued. Good recent analogs are lacking. But, ancient analog-candidates exist. Prominent among them is the Eocene Green River Formation (western US). We think the Late Archean Meentheena Member (Tumbiana Formation) of W. Australia is also one. The Meentheena lakes system was a giant, with microbialite-rich strata cropping out over 640 km. Stromatolites are abundant and diverse, and organized as solitary structures, biostromes, and >1-meter-high, elongate mounds. Biostromes are laterally extensive, traceable for 10s of kilometers. The member is ~ 50 m thick, with a lithofacies association of packstone/rudstone – microbialite/rippled grainstone – microbialite/laminated micritic mudstone – shale/siltstone, repeated many times in sections. It was deposited in a balanced-fill lake system with low, topographic gradient.

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.