The Microbialites of Two Giant, Ancient Lake Systems: The Late Archean Meentheena Member and the Eocene Green River Formation
Lacustrine microbialites have captured the attention of the petroleum industry due to the association of microbialites and hydrocarbon reservoirs in the Santos Basin, Brazil. The Brazilian situation suggests big lakes with high primary productivity and the development of large, extensive, microbialite buildups.
Microbialites are known from many recent and ancient lakes. Most occurrences are limited in areal extent and microbialites are not large. But there are a few excellent examples of giant lakes systems with abundant, large microbialites. Two stand out: the Eocene Green River Formation and the Late Archean Meentheena Member, which have fundamental similarities.
The ~2720 Ma Meentheena Member (Tumbiana Formation) lakes system crops out over 640 km in Western Australia. Microbialites are abundant and diverse, consisting of stromatolites, with rare oncoids and no thrombolites. The stromatolites occur in biostromes and are dominated by simple and complex domes, columns, flat-laminated structures, and meter+-high, elongate mounds in a biostrome 4 m thick that is laterally extensive (10s of km). The lithofacies association, ooids/flat-pebble-conglomerate (fpc) - stromatolite - shale, is repeated with each parasequence. The Meentheena Member (MM) was deposited primarily in a balanced-fill lake with low lake-bottom gradient.
Although smaller in extent (~350 km), the ~50 Ma Green River Formation (GRF) contains what may be the richest record of lacustrine microbialites. Stromatolites dominate, and oncoids and thrombolites occur. Bioherms, although uncommon, can be many meters in size. Biostomes are common, dominated by simple and complex domical stromatolites; columns and other shapes are subordinate. The biostromes are characteristic of a Type I Biostrome facies association consisting of ooids/fpc - stromatolite - laminated calcimicrite. This is the same parasequence found in the MM, except siliciclastics substitute for oil shale. Unlike the MM, GRF microbialites occur in three additional facies associations that reflect different lake-bottom gradients and overfilled to underfilled lake-types.
The nearly identical lithofacies association in the Meentheena and Green River of oolite/fpc - stromatolite - clay-to-silt size sediments indicates the biostromes reflect low gradient flooding surfaces in balanced-fill lakes with abundant calcium input. Similarities are a key to understanding the origin and distribution of microbialites in large lake systems.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California