Microbialite "Shrubs" of the Eocene Green River Formation: Analogs for the Cretaceous Pre-Salt Lacustrine Systems of the South Atlantic Conjugate Basins
Awramik, Stanley M.; Buchheim, Paul
Interest in lacustrine microbialites has expanded significantly since reports on the discovery of significant quantities of hydrocarbon in reservoirs associated with microbialites in the pre-salt lacustrine successions of the South Atlantic conjugate basins (Brazil and Angola). What little has been released about the microbialites indicates that mm to cm size, shrub-like carbonate structures seem to be predominant elements in many meters of core. According to Dorobek et al. (2012; Hedberg Conference), Campos Basin (Brazil) shrubs formed on the lake bottom in shallow, high-energy environments. These structures also resemble bacterial shrubs described by Chafetz and others (e.g., Chafetz and Guidry, 1999; Sed. Geol.) from travertines. Such travertines form in continental settings in shallow pools of water, not in large, lake-like settings in which the pre-salt shrubs developed. Analogs, both present-day and ancient, that accommodate the scale and depositional setting for the pre-salt shrubs, have been elusive.
The lacustrine Eocene Green River Formation in Wyoming contains abundant shrub-like structures. The resemblance of Green River shrubs to pre-salt shrubs is striking. Both formed in shallow, high-energy environments in large lake systems. Unlike what has been reported on the pre-salt shrubs, the Green River Formation shrubs formed in a variety of situations. Our preliminary work on these shrubs indicates that some grew on the lake bottom forming laterally extensive biostromes under a meter thick. Others formed as the dominant element in centimeter to multimeter scale microbialite (stromatolite) domes and columns (bioherms), and still others as components of stromatolites encrusting trees and branches. These three modes of occurrence all share some common features: (a) all occurred at the basin margins during transgressions (b) they formed during freshening phases of the lake, and (c) they are usually associated with higher energy environments (coeval sediment rich in grainstones). The Green River Formation shares many important features with the pre-salt of South Atlantic conjugate lacustrine basins: extensive lake deposits that accumulated in rapidly subsiding basins with abundant accommodation space (sag-like conditions), abundant carbonates, large microbialite bioherms, other microbialites, shrubs, and even stevensite. The Green River Formation is probably the best, single analog known.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013