Characteristics of a Tonian reef rimmed shelf before the onset of Cryogenian: Insights from Neoproterozoic Kunihar Formation, Simla Group, Lesser Himalaya

摘要

The Kunihar Formation records evolution of a unique rimmed shelf during the Tonian, before the Cryogenian/Marinoan glaciation of Krol Formation, Lesser Himalaya. The mixed siliciclastic-carbonate system of Neoproterozoic Kunihar Formation provokes interest for outcrop based facies analysis and sequence stratigraphy in order to frame a better understanding on the mechanism and control of its deposition. The formation records a 450 m thick mixed siliciclastic-carbonate rimmed shelf. Five facies associations have been identified- Peritidal-FA1, Lagoonal-FA2, Barrier island/Ooidal shoal-FA3, Reef complex-FA4 and Fore reef-FA5 facies associations. Two systems tracts were delineated - transgressive (TST) and highstand (HST) systems tracts. TST is marked by dominant aggradation of stromatolitic carbonates, while siliciclastic incursion surpassed carbonate deposition during highstand. It conforms to the "lower carbonate-upper clastic" model of mixed siliciclastic-carbonates. During the Precambrian, reef rimmed shelves were considered more abundant in the Archaean and Palaeo-Mesoproterozoic. This study determines that development of rimmed shelves was more common during the Neoproterozoic than previously speculated. Neoproterozoic reef rimmed shelves are concentrated more within the Cryogenian and Ediacaran Period. Stromatolitic reef rimmed shelf of Kunihar Formation is the only rimmed shelf recorded in the Tonian Period till date. Increased microbial stromatolitic accretion was propelled by surge in nutrients due to chemical weathering of underlying tholeiitic basalt (Darla volcanics) associated with the rifting of supercontinent Rodinia. With increasing interest of Tonian-Cryogenian transition as a key period in both planetary and organic evolution, Kunihar Formation represents a terminal reef rimmed shelf system that evolved due to microbial stromatolites in the absence of complex organisms. Subsequent to this, Cryogenian and Ediacaran biota took over as the dominant reef builders.