Microbially Induced Carbonate Precipitation within the Siliciclastic Tidal Flats on Follets Island, Northeast Texas Gulf Coast

Back barrier flats of Follets Island, northeast Texas Gulf Coast adjacent to Galveston Island, are predominantly composed of fine sand-sized siliciclastic sediments intercalated and thoroughly intermixed with microbial mats. The only carbonates on the mat-sediment surfaces of the tidal flats are whole and broken transported skeletal material (e.g., oysters, molluscs), i.e., micrite is absent on the surface. Most surprisingly, clumps of authigenic micrite occur within the very shallowly buried microbial mats.

The dominant microbial sedimentary structures in the supratidal areas are reticulated surfaces and gas domes. Sections through these microbial structures show that filamentous (predominantly cyanobacteria) microbes dominate the upper ~3 mm of the mat-sediment layer. Abundance of siliciclastic grains increases gradually with depth as the mats decay and the mat-sediment accumulation compresses. Small patches of micritic carbonates are common, particularly in the filamentous microbially dominated areas. These patches are on the order of 1.2 mm in length and 0.8 mm in height. Shapes of these micritic patches vary from convex upward domal structures to layer-like linear accumulations. The clumps of micrites conform in shape as well as in distribution to the clumps of filamentous microbes. Commonly, these micritic patches were observed to incorporate siliciclastic grains and thus, locally, help to lithify the recent siliciclastic sediments. These relationships suggest that these micrites precipitated below the photic zone, where cyanobacteria are beginning to decay, and eubacteria thrive. Numerous previous laboratory experiments have demonstrated the ability of eubacteria to induce the precipitation of calcium carbonate, including laboratory studies using eubacteria from this very same area. Thus, it seems most reasonable that even within this "non-carbonate" environment of the upper Texas Coast, eubacteria can induce the precipitation of carbonates under natural conditions.