MICROBIAL STRUCTURES IN SILICICLASTICS: AN INDICATOR OF DEPOSITONAL ENVIRONMENT

Siliciclastic sediments of the tidal flats at the back of the barrier islands along the Texas coast are dominated by fine sand and silt. These flats are commonly covered by thick microbial mats. Salinity of the bay water averages ~32 ppm whereas, some restricted water bodies in the lower supratidal areas show very high salinities (e.g., 60 ppm) due to evaporation. Microbial mats proliferate preferentially in those confined water bodies probably because of the 1) restriction of grazers by high salinity, and 2) availability of water. Mats are also common outside those pools but are generally consumed by grazing at these sites. Abundant microbial mats in this area result in various types of microbially induced structures in the siliciclastic sediments. These include wrinkled-surface, mat-crack, gas dome, and sieve structures. Wrinkled-surfaces are produced by the networking of the cyanobacterial filaments under stressful growth conditions, e.g., aridity, sediment deposition, and salt precipitation. Mat-cracks are resulted by desiccation of mat covered sediments. Polygons of mat-cracks have curled up peripheries and thus differ from the mud cracks. Mat stabilized sediment surfaces bulge up due to the gas trapped below the surfaces. Escaping gas also produces sieve structures subjacent to the gas bulges. Preservation potential of these delicate structures greatly increases when associated with salt precipitation.

A strong correlation between the microbially induced sedimentary structures and their depositional environment is noted in field. Observations indicate that wrinkle structure characterizes the supratidal areas, whereas the mat cracks and gas bulges grow preferentially in the lower supratidal zones and upper supratidal zones, respectively. This is probably because the lower supratidal areas are relatively more moist (and thus produce thicker mats) than the upper supratidal areas. Curl height of mat-crack increases with the thickness of the mat. Similarly, bigger gas domes are produced in thicker mats. Considering that the occurrences of mats as well as the type and morphology of the microbially induced sedimentary structures are controlled by the depositional environment, these structures when preserved in rocks, can be potentially useful in environmental interpretation.