Microbially induced sedimentary structures (MISS) in siliciclastic sediments

Modern back-barrier tidal flats of Galveston Island, Follets Island, and Matagorda Peninsula of the Texas coast are dominated by fine sand-sized siliciclastic sediments and prolific microbial mats. Mat thickness increases from ∼1 mm in the upper supratidal areas to ∼3 cm (maximum) at the lower supratidal areas, and then decreases to ∼2 mm in the lower intertidal areas. These mats modify sediment behavior and result in various microbially induced sedimentary structures (MISS). MISS, classified based on their genesis, include the following: gas domes, sieve-like surfaces, wrinkles, mat chips, erosional pockets, reticulated surfaces, mat-cracks, and knobby surfaces. Thin section studies show that in association with these mats micritic cements have been precipitated within the aphotic zones of these predominantly siliciclastic settings.;Six zones were identified based on the abundance of MISS within the supratidal to upper subtidal areas. Zone-I and zone-VI are the landward and bayward extents, respectively. 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. Sieve-like surfaces with mat-cracks typifies zone-V, whereas sieve-like surfaces and mat deformation structures define zone-VI. Transitions between the MISS-zones are generally parallel and related to the tidal zones. Additionally, reticulated surfaces are typically associated with the lowest degree of inundation and initial stages of mat growth.;Height of the curled edges of the mat-cracks and size of the crack-polygons are controlled directly by mat thickness. Mat thickness also varies with slight changes in elevation, and thus, fine scale MISS features (e.g., type and morphology) exhibit effects of topographic variations. The type and distribution of the MISS in carbonates is similar to that in the siliciclastics, which indicates that the overall microbial influence on sediment is similar irrespective of the type of sediment. Therefore, the strong relationship between the topography and MISS can be potentially helpful in interpretation of paleodepositional surfaces in both siliciclastic and carbonate deposits.