Sur, Sohini1, Juergen Schieber1, Santanu Banerjee2
(1) Indiana University, Bloomington, IN
(2) IIT, Bombay, India
ABSTRACT: The Geochemistry of the Bijaygarh Shale of the Vindhyan Supergroup, India: A Mid- Proterozoic Source Rock
The Mid-Proterozoic Bijaygarh Shale of the Kaimur Formation (Vindhyan Supergroup) is a thick (~50m) succession, deposited during a transgressive-regressive cycle. It is subdivided by a thin pyrite-rich interval into a lower fissile, micaceous unit, interbedded with sandstone, and an upper pyritiferous and carbonaceous unit with interbedded siltstone. These two units show distinctly different geochemical signatures. The lower unit is enriched in clays (Al, Si) and has low contents of Fe, P, Mo, U, Ni, and V. The upper unit is lower in clays (Al, Si) and is enriched in Fe, P, Mo, U, Ni, and V. In contrast to the lower unit, the upper unit has significantly higher Ti/Al, Fe/Al, Mo/Al, U/Th, V/Cr, and has a lower CIA. Ti/Al ratios decline in the lower unit (TST) towards the pyrite interval (MFS) and then sharply increase in the upper unit (HST, shoreline progradation). Combined with declining CIA values, this suggests regression as a consequence of uplift in the source region of the sediments. Various paleo-oxygenation indexes (U/Th, V/Cr) suggest that the water column was oxygenated during deposition of the lower unit, and then switched to being anoxic during deposition of the distinctly more carbonaceous upper unit. The coincidence between anoxic conditions (as suggested by chemical proxies) and a shallowing upward trend (HST) in the upper unit is counterintuitive. This may be explained by enhanced preservation due to benthic microbial mats, or better preservation of organic carbon and organic-sequestered trace elements due to increased sedimentation rates.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.