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Bahamian giant stromatolites: microbial composition of surface mats

Published online by Cambridge University Press:  01 May 2009

Robert Riding ,
Stanley M. Awramik ,
Barbara M. Winsborough ,
Karen M. Griffin  and
Robert F. Dill
Robert Riding
Affiliation:
Department of Geology, University of Wales College of Cardiff, Cardiff CF1 3YE, United Kingdom
Stanley M. Awramik
Affiliation:
Department of Geological Sciences, Preston Cloud Research Laboratory, University of California, Santa Barbara, CA 93106, USA
Barbara M. Winsborough
Affiliation:
Department of Zoology, Patterson Hall, University of Texas, Austin, TX 78712, USA
Karen M. Griffin
Affiliation:
Department of Geological Sciences, Preston Cloud Research Laboratory, University of California, Santa Barbara, CA 93106, USA
Robert F. Dill
Affiliation:
Department of Geology, University of South Carolina, Columbia, S.C. 29208 and Dill GeoMarine Consultants, 610 Tarento Drive, San Diego, CA 92106, USA
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Abstract

Subtidal columnar stromatolites up to 2.5 m high near Lee Stocking Island in the Exuma Cays, Bahamas, have surface mats approximately equally composed of algae and cyanobacteria. The stromatolites are composed of fine–medium oöid and peloid sand. This sediment is supplied to the growing stromatolite surfaces by strong tidal currents which lift grains into suspension and sweep migrating dunes over the columns. The algae include an unidentified filamentous chlorophyte, and numerous diatom species mostly belonging to Mastogloia, Nitzschia and Navicula. The dominant cyanobacteria are two oscillatoriacean species, both probably belonging to Schizothrix. Trapping of sediment is mainly effected by the unidentified chlorophyte which is veneered by epiphytic diatoms. Grains are bound into a mucilaginous mat composed of diatoms and cyanobacteria. Cyanobacteria alone would not be able to trap and bind coarse sediment so effectively in this environment. In being coarse-grained and having a significant eualgal component to their mats, these stromatolites are similar to subtidal columnar stromatolites at Shark Bay, Western Australia. The Lee Stocking stromatolites are physically stressed by high velocity tidal currents and mobile sediment. The Shark Bay stromatolites are stressed by hypersalinity. In both cases stress deters grazers, encrusters and bioeroders. These coarse-grained eualgal stromatolites contrast with micritic and predominantly prokaryotic stromatolites of most Recent marine environments, and are not analogues for most pre-Phanerozoic stromatolites. They appear to be a response to changing stromatolitic mat components in the Cenozoic.

Type
Articles
Information
Geological Magazine , Volume 128 , Issue 3 , May 1991 , pp. 227 - 234
Copyright
Copyright © Cambridge University Press 1991

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