Microbial communities were of critical importance in the development of Late Devonian and other Paleozoic reefs. The microbial communities filled pores and probably aided in the cementation of the reefs. In some cases metazoan framework organisms were entirely absent. Due to the Late Devonian mass extinction, Famennian reef platforms in the Canning basin, Australia, were constructed solely of microbial communities. Previous studies of Famennian reefs have classified the microorganisms but have not determined the paleoecology, which is critical for understanding platform construction.
Ten to fifty meter, allochthonous, Famennian reef blocks in the fore-reef facies contain well preserved microbial fabrics. Primary porosity of some of the blocks was filled with quartz sand prior to tumbling down the reef slope. The in-filling sand terminated reef growth and captured the progression of both growth and encrustation by microbial communities. Reef paleoecology and the roles of various microbial communities in reef construction can be evaluated in the reef blocks.
The stratigraphic tops of the allochthonous blocks contain thrombolites lacking encrusting organisms. Thrombolites consist of roughly 0.5 cm wide ribbons of clotted, laminated, and structureless micrite with rare shell fragments. Rare fossil algal filaments are also present within the thrombolite, but other microbes have not been identified. The thrombolites formed the primary reef structure. Deeper in the allochthonous blocks, the thrombolite framework is encrusted by microbial communities, such as Girvanella and Renalcis. These microbial communities cemented the porous thombolite. Microstratigraphic observations suggest that Girvanella grew before Renalcis in the reef pores, and the encrustation of these organisms was depth dependent. Girvanella is probably a photosynthetic cyanobacteria, which encrusted the reef at shallow depths. Renalcis is a problematic organism, which may be a cryptic bacterial community. The combination of encrusting organisms within a thrombolite framework helped produce a prograding microbial reef platform in the Famennian of the Canning basin.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616