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Abstract

Microbially induced sedimentary structures (MISS) were studied on siliciclastic and carbonate tidal flats of Texas to investigate the influence of sediment on the types and distribution of these biosedimentary structures. Carbonate MISS were studied on the wind tidal flats adjacent to Laguna Madre, whereas tidal flats next to Christmas Bay and East Matagorda Bay were visited to study siliciclastic MISS. In both depositional settings, microbial mats characteristically occur on the supratidal to intertidal flats. Reticulated surfaces, gas domes, mat desiccation cracks, sieve-like surfaces, and a variety of mat deformation structures are common in both siliciclastic and carbonate settings. However, the knobby surfaces that were common in the upper supratidal areas of the siliciclastic setting were not seen on the carbonate flats.

The distribution of these structures is similar on the two types of sediments. Six MISS zones were identified in the siliciclastic settings based on the assemblages of these structures; however, because of the absence of the “knobby surfaces,” which define Zone I, only Zones II through VI were recognized in the carbonate settings. Zones I, II, and III coincide with the upper supratidal areas and thus occur at the shallower topographic levels, whereas most of Zone VI is located in the lower intertidal and subtidal areas.

Zones II through IVare characterized by reticulated surfaces, whereas Zones III and IVare distinguished from Zone II by the additional occurrence of the gas domes and mat desiccation cracks, respectively. Mat desiccation cracks also occur in Zone Vin association with sieve-like surfaces. Finally, Zone VI is characterized by a dominance of mat deformation structures and the occasional existence of sieve-like surfaces.

This study highlights the similarity between the siliciclastic settings and the carbonate environments in terms of the interaction between the microbial world and the sedimentary processes. This results in similar types of sedimentary structures and similar distribution profiles. Therefore, this study shows that compared to carbonate MISS, siliciclastic MISS can be equally important in terms of environmental interpretation. The results of this study also demonstrate that subtle variations in MISS and their facies changes can be used to indicate small variations in water depth, regardless of the sediment type.

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