2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 5
Presentation Time: 1:30 PM-5:30 PM


MURPHY, Megan A., Geology, Univ of California, 1 Shields Ave, Davis, CA 95616-5270 and SUMNER, Dawn Y., Univ of California - Davis, 1 Shields Ave, Davis, CA 95616-5270, megmurphy@geology.ucdavis.edu

2.5-2.6 Ga microbial structures from the Carawine Formation, Western Australia, may shed light on the diversification of early life. The newly identified fenestrate microbialites grew in deeper water than columnar stromatolites and ooid shoals and are not present in previously described shallow subtidal to intertidal facies. Microbialites consist of filmy laminae, supports, and organic clumps. Filmy laminae are often contorted or have a cuspate geometry and commonly drape from only one side of the rare support structures. Supports are vertically oriented microbial growth structures. The organic clumps were most likely ripped from planar laminae during high-energy events. Primary voids are filled with herringbone, bladed, and blocky calcite cement. Thick layers of herringbone and bladed calcite cement coat all of the microbial structures. Micrite drapes some planar microbial laminae. Different microbial communities may have created each of the structures.

Fenestrate microbialites in the 2.5 Ga Gamohaan Formation, South Africa, are similar, but contain little to no micrite, have drapes off both sides of the organic supports, and organic clumps are only present in shallow subtidal to intertidal environments. Herringbone calcite often precipitated on supports before laminated mats in the Gamohaan microbialites. A difference in the timing of precipitation has yet to be identified in the Carawine microbialites. Similar microbial structures have been identified in two other Neoarchean carbonate deposits suggesting that these structures represent typical Neoarchean ecosystems. The variations in textures may be due to different depositional environments or a microbial response to an influx of sediment. Additional research will characterize the origin of these variations.