242 Geobiology & Geomicrobiology (Posters)

Wednesday, 12 October 2011
9:00 AM - 6:00 PM
Hall C

EVALUATION OF BIOGENICITY AND BRANCHING IN STROMATOLITES FROM THE TIPTON MEMBER, GREEN RIVER FORMATION

Paper number: 242-11
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Stromatolite biogenicity is often difficult to confirm because of diagenetic alteration, an absence of microbial fossils, and the potential for abiogenic processes to form a variety of stromatolite morphologies. This study uses two approaches, magnetic susceptibility and microfacies analysis, to assess the biogenicity of stromatolites from the Boar’s Tusk Section of the Tipton Member, Green River Formation (Early Eocene), Wyoming. Magnetic susceptibility measurements reflect the abundance of detrital magnetic grains within the carbonate stromatolite. Relatively high and even values across a single layer at all depositional angles suggest microbial trapping and binding of sediment, whereas relatively low values -- especially at high versus low angles -- indicate that microbial mats were likely absent. Several microfacies alternated as the stromatolites grew, including microdigitate columns and stratiform laminae composed of peloidal micrite and microdigitate columns composed of calcite crystal fans. Magnetic susceptibility of the calcite fan microfacies is low (1.3-1.7), whereas magnetic susceptibility of the micritic microfacies is high (3.0-4.2). These values indicate that the calcite fan microfacies incorporated fewer detrital grains than the micritic microfacies, consistent with a predominantly abiogenic origin of the fans, and the micritic microfacies incorporated more detrital magnetic material, consistent with trapping and binding by microbial mats.

The micritic microfacies consists of microdigitate columns at low depositional angles which grade into stratiform laminae at high angles. This morphological transition is consistent with the established hypothesis, implicit in some stromatolite morphogenetic computer models, that stromatolite branching and column formation is controlled by the interplay of sedimentation and upward growth of mats and/or crystals. However, branching in the calcite fan microfacies occurs at high angles, and even in overhung areas, where vertical sedimentation could not initiate branching. This inconsistency with conventional theory indicates limitations in the parametrization of some morphogenetic computer models.

This research was undertaken at the International Geobiology Course 2011.