PALAEOENVIRONMENTAL SIGNIFICANCE OF MICROBIAL MAT-RELATED STRUCTURES ON SILICICLASTICS: EXAMPLES FROM THE MODERN COASTAL PLAINS OF THE GULF OF CAMBAY AND THE PROTEROZOIC VINDHYAN BASIN, INDIA

The distribution of microbial mat-related structures (MRS) was investigated on siliciclastic sediments in the modern intertidal and supratidal zones of the Gulf of Cambay in western India to compare with those in the ~1.6 Ga Chorhat Sandstone the of Vindhyan Supergroup in central India for high-resolution palaeoenvironmental interpretation. The abundance of MRS in modern environment increases from the lower intertidal zone to the upper supratidal zone, accompanied by a systematic change in the type of MRS. While the lower intertidal zone exhibits occasional wrinkle structures, patchy ripples and sieve-like surfaces, the upper intertidal zone exhibits diverse MRS related to intermittent exposure and reduced current activity on the mat layer, which include wrinkle structures, Kinneyia, multi-directional ripples, patchy ripples, sieve-like surfaces, setulfs, reticulated surfaces, rolled-up mat fragments and occasional gas domes, petee ridges and cracked mat surfaces. The lower supratidal zone is characterized by the increased occurrence of gas domes, petee ridges and cracked mat surfaces compared to the upper intertidal zone. The upper supratidal zone is distinguished by the presence of abundant petee ridges, gas domes, cracked mat surfaces and wrinkle structures. Cm-scale disc-shaped microbial colonies (DMC) with a variety of internal structures are abundant in the lower and upper supratidal zones. While setulfs occur close to the boundary between the upper intertidal and lower supratidal zones, wrinkle structures occur in all the coastal zones. The upper part of the intertidal segment of the predominantly shallow marine-originated Precambrian Chorhat Sandstone is distinguished from its lower part by the presence of abundant petee ridges, cracked mat surfaces and gas domes in the former, while wrinkle structures, rolled-up mat fragments, Kinneyia, patchy ripples and multi-directional ripples are equally abundant in both parts. The lower part of the intertidal segment of the Chorhat Sandstone is thus analogous to the upper intertidal zone of the modern Gulf of Cambay environment while the upper part of the intertidal segment indicates lower to upper supratidal zones. The bottom-most part of the intertidal segment of the Chorhat Sandstone with fewer MRS is analogous to the lower intertidal zone of modern.