MICROBIAL MATS IN SILICICLASTIC COASTAL SEDIMENTS OF THE MODERATE CLIMATE ZONE : A CASE STUDY FROM FISHERMANS ISLAND, VIRGINIA

Microbial mats are dense, organic layers that are formed by benthic bacteria and cyanobacteria. In carbonate coastal environments, the metabolism of the microbenthos induces the formation of stromatolites. Conversely, in siliciclastic marine milieus, the microbial mats interact with the prevailing physical sedimentary dynamics and cause ‘microbially induced sedimentary structures – MISS’. MISS are known from siliciclastic deposits from the Upper Archean to the Holocene, and the structures have the same significance for paleontology as their carbonate relatives, the stromatolites.

Today, microbial mats colonize the lower supratidal areas of coastal zones worldwide. They occur abundantly in the moderate climate zone as well, where no or minor mineral precipitation takes place. This study investigates mat communities that colonize the sandy tidal flats of Fishermans Island, a barrier island situated at the coast of Virginia.

The microbial mats on Fishermans Island develop during the spring and summer months. They colonize areas within the lower supratidal zone, which are protected by a morphological barrier against high-energetic wave action and strong tidal currents. The microorganisms preferentially settle on quartz-rich sands of fine grain size composition. By light microscopy, three cyanobacterial species were identified as the dominant mat constructors: Oscillatoria limosa (39.6%), Microcoleus chthonoplastes (17.4%), and Merismopedia punctata (<1%).

The microbial mats interact with the sedimentary dynamics of tidal currents by “biostabilization”, “baffling, trapping, and binding”, or “pushing”. This interaction gives rise to MISS such as “multidirected ripple marks”, “erosional remnants and pockets” and many others. The knowledge of the modes of formations of MISS permits the interpretation of those structures in the fossil record.

Acknowledgements: The study is funded by Old Dominion University.