Topographic control on distribution of modern microbially induced sedimentary structures (MISS): A case study from Texas coast

Modern back-barrier tidal flats of Galveston Island, Follets Island, and Matagorda Peninsula of the Texas coast are dominated by mud- to fine sand-sized siliciclastic sediments and prolific microbial mats. These microbial mats modify sediment behavior and result in a variety of microbially induced sedimentary structures (MISS). Common structures include: knobby surfaces, reticulated surfaces, gas domes, mat-cracks, sieve-like surfaces, erosional pockets, wrinkles, and mat chips. In general, mat thicknesses increase from ∼ 1 mm in the upper supratidal to ∼ 3 cm (maximum) in the lower supratidal and then decrease to ∼ 2 mm in the lower intertidal areas. This same wedge-shaped pattern is displayed by detailed measurements of mat thicknesses from the rims into the deeper centers of depressions (pools) on the supratidal flats. Measurements of 175 mat-cracks show that height of the curled edges of the mat-crack polygons increases with increase in mat thickness. Similarly, measurements of 150 gas domes reveal that the size of the gas domes also increases with increasing thickness of the associated mats. Because mat thickness varies with elevation on the tidal flat, curl height of the mat-cracks and size of the gas domes are also related to elevation.

Six zones were identified based on the occurrence of MISS within the supratidal (zone-I) to upper subtidal (zone-VI) areas. At the highest elevation, knobby surfaces characterize zone-I whereas zone-II is defined by reticulated surfaces. Along with reticulated surfaces, gas domes and mat-cracks characterize zone-III and zone-IV, respectively. Association of sieve-like surfaces with mat-cracks typifies zone-V whereas mat deformation structures and sieve-like surfaces define zone-VI. Boundaries between the MISS-zones in general are parallel and related to tidal zones. The distribution patterns of the MISS-zones are strongly controlled by local topography of the sediment surface because the degree of inundation is the primary controlling factor for the mat growth and resultant MISS. Therefore, distribution of the microbially induced sedimentary structures in siliciclastics, along with the dimension of the mat-cracks and gas domes, can be potentially helpful in interpretation of topography of paleodepositional surfaces.