Microbially influenced sedimentary structures (MISS) as palaeoenvironment indicators in a storm-wave influenced Proterozoic siliciclastic tidal flat, Kunihar Formation, Simla Group, Lesser Himalaya, Himachal Pradesh

Extensive microbially influenced sedimentary structures (MISS) have been found associated with siliciclastic tidal flat deposits in the upper part of Proterozoic Kunihar Formation, Simla Group, Lesser Himalaya, Himachal... [ view full abstract ]

Extensive microbially influenced sedimentary structures (MISS) have been found associated with siliciclastic tidal flat deposits in the upper part of Proterozoic Kunihar Formation, Simla Group, Lesser Himalaya, Himachal Pradesh. Five dominant facies were identified from MISS bearing siliciclastic horizons- (i) storm dominated sandstone-siltstone facies (FA1), (ii) wave influenced sandstones (FA2), (iii) tide dominated sandstones (FA3), (iv) heteroliths (FA4) and (v) laminated shales (FA5). Storm-wave influenced sedimentary features such as flaser-lenticular bedding, herringbone cross-bedding, mud/silt couplets, hummocky cross stratifications, oscillation and interference ripples connotes episodic wave and storm activities in supratidal to intertidal zones. These siliciclastic tidal flat deposits host microbially mediated structures such as wrinkle structures, kinneyia ripples, palimpsest ripples, sand chips, patchy ripples, gas domes, etc. Wave current, loading/dewatering of microbial mats and gas build-up from mat decay aided appearance of wrinkle marks on sandstone surfaces. Palimpset ripples formed due to low rate of sedimentation and insignificant erosion. Sand chips formed due to wave action. Patchy ripples attribute to reworking of first generation ripples by second generation ripples. Gas domes indicate gas expulsion from decay at mat bottom. Load casts indicate synsedimentary deformations. Limited dessication cracks manifest insignificant synaeresis aided by cohesion due to microbially mediated grains on sediment surface in humid environments. Generation of MISS structures was favoured by development of tidal flat with resistant substratum, low energy conditions, consistent water supply and low terrigeneous input. Limited sedimentation between depositional events gave ample time for growth of microbial mats. Deposits with intervening storm and wave activities and MISS of Kunihar Formation shed light into the conception of processes and energy existent in tidal domains during Proterozoic eon.

Keywords: Proterozoic, Simla Group, Tidal, MISS, Siliciclastics.