MICROBIAL MAT INDUCED SEDIMENTARY STRUCTURES: A COMPARISON BETWEEN SILICICLASTIC AND CARBONATE DEPOSITIONAL SETTINGS

The types and distribution of microbial mat induced sedimentary structures (MMISS) on tidal flats along the Texas Coast are similar for those produced in siliciclastic and carbonate depositional environments. Siliciclastic-MMISS were studied on the wind tidal flats of Christmas Bay and East Matagorda Bay of Texas, whereas tidal flats of Laguna Madre (Texas) were visited to study carbonate-MMISS. The most common types of MMISS in the siliciclastic settings, (e.g., wrinkled surfaces, gas domes, desiccation-cracked mats and sieve-like-surfaces), are also frequent in carbonate tidal flats.

Four MMISS zones were identified based on the MMISS-assemblages, where zone one is the landward extent and zone four is the bayward extent of the MMISS distribution. Since the distribution of microbial mats and their resultant MMISS are controlled by availability of water, the boundaries that separate MMISS zones, although not coincident with, nevertheless, are closely related to the boundaries between the tidal zones.

The first (landward) MMISS-zone is characterized by sparse wrinkled surfaces. Gas domes and wrinkled surfaces are common on the second MMISS zone, whereas the third zone is characterized by desiccation-cracked mats and wrinkled surfaces. Sieve-like-surfaces along with very well-developed, thick desiccation-cracked mat polygons characterizes the fourth zone of MMISS. Mats are rare in wetter areas due to increased grazing. Additionally, desiccation-cracked mats tend to be more common in small depressions replacing an otherwise featureless, gently dipping flat surface.

The importance of this study is two fold. Siliciclastic-MMISS are valuable as are carbonate-MMISS for interpretation of ancient depositional environments. Secondly, regardless of the sediment type, subtle variation of MMISS can be used to detect even small changes in relation to inundation by water.