Depositional architecture of a Neopreoterozoic distally-steepened carbonate ramp from the Bilara Group, Marwar Supergroup, Rajasthan, India and a few clues on Supercontinent and ocean- atmosphere interaction

The late Neoproterozoic–early Cambrian signify a time bracket when the Indian craton got separated from the supercontinent Rodinia and was in a process of becoming an integral part of supercontinent Gondwana.  The late Neoproterozoic–early Cambrian Bilara Group (BG) of the Marwar Supergroup provides a scope for study of depositional processes and spatio-temporal evolution of a distally-steepened carbonate ramp that developed on westward dipping greater Indian shelf; arguably formed in Indo-Arabian geological province during the time period. Deposited within an intracratonic rift/sag set up fringing the northern margin of the Aravalli craton, the BG succession, is subdivided under three Formations viz. Dhanapa, Gotan and Pondlu, in order of superposition.

Six different facies are identified within BL succession with their depositional environment spanning from supra-peritidal to intertidal to shallow and deep subtidal. The cabbage-headed stromatolites and crinkly laminites with tepee structure represent the shallowest supratidal-peritial setting whereas the LLH-type stromatolite in alternation with algal laminite and plane-laminated carbonates represent products of intertidal to shallow subtidal set-up. The limestone-shale heterolithics having signature of storm action is interpreted as deposit of subtidal shelf above storm wave base. The occurrence of calaclastite, intraclastic conglomerate with carbonate mass flows origin are indicative of steep slope at the distal part of the ramp. A distally-steepened ramp geometry is visualized for the Bilara carbonate platform. Additionally, metres-thick soft sediment deformation (SSD) structure layers including disharmonic folds, low-angle thrusts, distorted laminae, fluidisation pipes, slump and load structures, homogeneities, diapirs, etc. at different stratigraphic levels through the BG succession, traceable over hundreds of metres in outcrop, bear indication of basin-scale instability in course of Bilara carbonate platform development.  

From delineation of facies succession and documentation of facies stacking pattern, two cycles of deposition inferred from the Bilara lithopackage; DC1 and DC2. While the DC-1 is transgressive and represented by superimposition of facies types of increasing bathymetry, the second cycle DC-II is progradational, shallowing upward and represented by progressively shallow water facies types.

Stable isotope (C ) study on Bilara carbonate with systematic sampling from different facies associations reveal four major intervals of negative isotope excursion (EN1, EN2, EN3 and EN4) of medium to long duration and three positive excursions (EP1, EP2, EP3) of short duration. The EN1- records highest negative value; where δ¹³Ccarb value as low as -10.4 ‰ but for significantly of short duration. The other three negative excursions (i.e. EN2, EN3 and EN4) δ¹³C values are quite similar and ranges in between –8 to -6.5‰; in terms of stratigraphic interval (i.e time duration) the EN3 records the longest.

In most of the cases , high negative δ13C values are in close association and succeeded upward by SSD horizons. The presence of SSD structures, presence of Bitumen in pore spaces and large scale negative carbon isotope suggest, destabilization of methane clatharate as possible cause of carbon isotope excursion.