Precambrian sedimentary sequences of India: Their peculiarities and comparison with modern sediments

Abstract

The Indian peninsular shield and the Himalayan area are characterized by aerially extensive Proterozoic sequences often several thousand metres thick, made up of a mature sandstone (orthoquartzite)—shale—carbonate association. Crustal stability, reworking of the sediments in coastal areas, and probably extensive chemical weathering on land led to the development of mostly mature sandstones; graywacke and arkose are rare. Deposition of these sediments took place essentially in the coastal complex of a tidal sea, viz., lagoon, tidal flat, beach-shoal complexes under very stable conditions. Thick slate successions are predominantly rhythmites, and represent deposits of a low-energy coastal complex including extensive supratidal zone — alluvial plains, with an increased supply of fine-grained sediments. Orthoquartzite (shallow-marine deposits) interbedded with basic lavas makes a characteristic association in some of these Proterozoic sequences. Shallow marine red beds, stromatolitic carbonates associated with phosphorite and magnesite, and glauconitic tidal flat deposits are often encountered. The land—sea transition area, i.e., supratidal zone—alluvial plain, played a significant role in coastal sedimentation, where supratidal facies may appear similar to the intertidal facies. Carbonate sequences mostly represent laterally and vertically extensive algal mat deposits. It is postulated that many of these algal-mat carbonates represent deposits of extensive supratidal zones, affected only by storm surges. The Rann of Kutch, India, provides a good modern analogue of extensive supratidal zone sedimentation. Large-scale inhabitation of the supratidal and coastal terrestrial region by algae during the Proterozoic, associated with high production of oxygen in restricted supratidal areas probably helped in the evolution of metazoans.