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Abstract

The breakup of Rodinia resulted in the formation of rift basins and passive margins c. 650 Ma. Major palaeoclimatic vents such as Neoproterozoic global glaciation (known as ‘snowball Earth’; Hoffman et al. 1998) followed by global warming have been recorded on different continents, including the Indian Lesser Himalaya (Blaini–Krol Cryogenian–Ediacaran Period). The reconstruction of the Rodinia supercontinent (Powell et al. 1993; Li et al. 2003) and the palaeoposition of India (including Lesser Himalaya–Southern China shelf facies, Tewari 2010) strongly suggest that a connection of the Lesser Himalayan Neoproterozoic sedimentary basins with Rodinia must have existed. Early Earth possibly witnessed its most extreme climatic fluctuations during the mid late Neoproterozoic between 750 and 550 Ma. Palaeoglaciers even reached the equator c. 635 Ma, covering the whole Earth. Evidence from Australia, Africa, Antarctica, South America, South China and the Indian Lesser Himalaya suggest that there may have been three or more palaeoglacial events during this 200 million year interval. The global decline of Meso-Neoproterozoic stromatolites, biotic evolution, diversification, extinctions and the discovery of Ediacaran life following the cold climate are of great significance. Carbon isotopic excursions from all pink cap carbonates from the Lesser Himalaya capping the Blaini glacial diamictites have shown strong negative excursions, whereas the overlying Ediacaran Krol carbonates are characterized by a positive shift in carbon isotope ratios (Tewari 2010). In the Lesser Himalayan Krol belt, extending from the Solan in Himachal to Nainital in Uttarakhand Lesser Himalaya, the Upper Krol Formation (Krol D Member, Auden, 1934, 1937) is a typical Ediacaran (terminal Neoproterozoic) microbial carbonate sedimentation facies. Krol D contains Ediacaran metazoan fossils and abundant microbial structures in the form of columnar, domal and stratified stromatolites. Peloids, oncoids and microphytolites are also microbially formed structures. Carbonate grainstones and packstones are common in Krol D, and the allochemical constituents are intraclasts, ooids, coated grains, peloids, microbial grains and catagraphs. A diagenesis and cathodoluminescence study of the Krol ooids has been done in detail for the first time. The sedimentary structures present in the non-stromatolitic Krol carbonates include bird's eye, cross beddings, symmetrical ripples and shallow channel structures. An intertidal to supratidal carbonate ramp depositional environment with some moderate currents and intermittent periods of exposure has been suggested for the Upper Krol carbonates of the Lesser Himalaya. A moderate to high content of organic matter (kerogen) in the Neoproterozoic microbial fossils indicates the presence of probable hydrocarbon source rocks.

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