Monday, September 5, 2016

Review Paper- Proterozoic Basins Of Peninsular India

My friend Vivek Kale has written quite a good review of the status of our knowledge about the Proterozoic basins of Peninsular India. The review covers basin categories, basin initiation, sedimentation patterns, fossil evidence and tectonics with the context of global Proterozoic events.

This compilation is intended to present a snap-shot of the current status of the knowledge on the Proterozoic sediments and tectonic events that are preserved in Peninsular India; on the backdrop of the growing understanding of global events and environmental evolution during that period. Proterozoic sediments in Peninsular India are found in two contrasting categories of basins. Narrow linear intercratonic belts host terrigenous and marine sediments, often interbedded with volcanics and volcaniclastics; that are deformed, metamorphosed and occasionally intruded by granitic bodies. These belts abut with a tectonic contact with wide, unmetamorphosed platform sediments from epicratonic basins with limited igneous activity associated within them; clubbed as the Purana Basins of Peninsular India. Although traditionally the former (mobile belts) were considered to be older and different from the latter, emerging geochronological data demonstrates that they were coeval products of basins evolving adjoining each other in diverse tectonic setting. Available knowledge on these basins is summarised within the framework of the emerging understanding of the Proterozoic geohistory punctuated by assembly and break-up of supercontinents, progressive oxygenation of the atmosphere, changes in the sea-water chemistry; establishment of the continental free-board and the generic environments that laid the foundation of biotic evolution. Although significant advances have been made in the last decade in the knowledge of these sediments, much more is required to achieve the desired precision and resolution.

The paper is copiously referenced; Due to my interest in fossils and evolution I would have added Poornima Srivastava's excellent review of fossil eukaryotes from the Vindhyan Basin. Kale mentions the need to systematically document this fossil record as it has the potential of providing key insights into the Phanerozoic biotic explosion. I think for that the Neoproterozoic record (1000 million years ago to 541 million years ago) in particular will be of great interest. That is the time period that saw great changes in earth continental configuration (breakup of Rodinia) and formation of broader shallow marine environments. The earth warmed from a  deep freeze state - the Cryogenian Period - with perhaps consequential changes in sea water chemistry (increase in continental weathering).  Early animal evolution was triggered by these ecological upheavals.

The sediments from the Vindhyan Basin and from the Jodhpur Group in Rajasthan of Late Neoproterozoic age have yielded tantalizing Ediacaran grade fossils and some evidence of burrowing activity suggestive of a triploblastic grade worm like animals. Although molecular phylogeny show that the origin of animals goes back perhaps to between 800-700 million years ago, unequivocal body fossils of sponges, cnidarians and bilaterans (trace fossils) appear later in the Ediacaran (beginning 635 million years). And the great diversification of animals took place even later in the Early Cambrian fueled by the rise of predators and calcium carbonate biomineralization. Yet, over most of Peninsular India the very latest Neoproterozoic (sediments younger than ~ 600 million years) seems to be missing. Sedimentation had stopped in these basins by then or the section has been eroded. Except for the Rajasthan Basin (Nagaur Group, early Cambrian) there is no record of early Paleozoic sedimentation either. Most of Peninsular India thus lacks the sedimentary record bracketing the crucial 60-80 odd million years (~ 600 mya - 520 mya) over which early animal evolution unfolded.

Attention then, needs to given to the Late Neoproterozoic-Early Cambrian of the Himalayas. Late Neoproterozoic - Early Cambrian sediments from the Lesser Himalayan Krol and Tal Group of sediments not covered in this review contain Ediacaran impressions and putative animal embryos. The late Neoproteozoic Early Cambrian sections from the Tethyan Himalaya section in Spiti and Zanskar ranges also hold promising clues to our understanding of this interesting evolutionary period.

The infographic below summarizes the changes in the Neoproterozoic biosphere against the backdrop of plate tectonic events and the global carbon cycle.

Source: Butterfield N.J. 2015

Open Access

Kale, V. (2016). Proterozoic Basins of Peninsular India: Status within the Global Proterozoic Systems Proceedings of the Indian National Science Academy, 82 (3) DOI: 10.16943/ptinsa/2016/48461

Mathur, V., Shome, S., Nath, S., & Babu, R. (2014). First record of metazoan eggs and embryos from early Cambrian Chert Member of Deo ka Tibba Formation, Tal Group, Uttarakhand Lesser Himalaya Journal of the Geological Society of India, 83 (2), 191-197 DOI: 10.1007/s12594-014-0031-4

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