Giant ooids in a Neoproterozoic carbonate shelf, Simla Group, Lesser Himalaya, India: An analogue related to Neoproterozoic glacial deposits

摘要

This study is the first comprehensive documentation of giant ooids and related facies associations in the Kunihar Formation, Proterozoic Simla Group Lesser Himalaya, Himachal Pradesh, analysing the understanding of pro- cesses and controls on development of giant ooids. Based on field observations, supplemented by outcrop based facies analysis, petrography and delineation of environmental variations, four fades associations have been delineated: (i) Peritidal siliciclastic-carbonate (FA1) (ii) Shelf lagoon (FA2) (iii) Reef complex (FA3) (iv) Fore reef slope (FA4). Deposition of giant ooids and associated facies associations of Kunihar Formation occurred in a carbonate rimmed shelf with high tidal influence. Size of giant ooids from Kunihar Formation is the largest as compared to giant ooids from other geological formations. Kunihar giant ooids developed when normal ooids were washed from ooid shoals (intertidal) into slightly deeper regions (shallow subtidal) resulting in the in creased dimension of ooids in suspension due to higher hydrodynamics. Scanning electron microscopy (SEM) studies support microbial origin of ooids, giant ooids and stromatolites of Kunihar Formation. Increased microbial activity in Kunihar Formation is attributed to increase in nutrients by virtue of weathering of underlying Darla volcanics. Abundant carbonate and Microbially induced sedimentary structures (MISS) deposition in lower part of Simla Group points to increased microbial activity which likely increased the volume of oxygen in Neoproterozoic atmosphere ushering in Ice House conditions during the subsequent deposition of Blaini Group.