Search

Paleogeography and process-related facies analysis of the shelf-originated Rohtas Limestone of early Mesoproterozoic age has been attempted. Lithology, structure, microfabric and TOC are the mainstay of facies classification. The Rohtas Limestone consists of seven facies; out of which the organic-rich black shale and the crinkle laminated limestone facies dominates the basal part, reveal distinctive fabric of microbial mat origin and represents outer shelf products. With gradual shoaling, the crinkle laminated limestone facies gains importance and locally turns into the nodular limestone facies through early diagenetic modification. The grey shale facies, poor in organic content, replaces them further upward. The other three facies, viz., the thinly laminated heterolithic facies, typified by close alternations between black shale and limestone, the planar laminated limestone facies and the wavy laminated limestone facies alternate with the grey shale facies, progressively dominating the top part of the succession. While the heterolithic facies records deposition across the paleochemocline, the planar laminated and the wavy laminated limestone facies document deposition in shallow agitated shelf. Confined between a condensed zone below and an unconformity above, the overall coarsening upward Rohtas Limestone records development of a HST.

The Proterozoic Rohtas Limestone succession in the Vindhyan basin, central India depicts a transition from outer shelf to inner shelf depositional setting. The succession comprises of seven facies viz., black shale, crinkly-laminated limestone, nodular limestone, thinly-laminated heterolithic, grey shale, plane-laminated limestone and wavy-laminated limestone. From field and petrographic features the nodular limestone is identified as early diagenetic product of crinkly-laminated limestone. delta C-13 and delta O-18 values of the limestone bear essentially the 'primary' seawater composition of late Paleoproterozoic to early Mesoproterozoic time and rules out any postdepositional resetting. delta C-13 and delta O-18 compositions of the nodular and non-nodular portions when analysed separately reveals that the nodular portions are invariably enriched in heavier oxygen and carbon isotopes. Enrichment in oxygen and carbon isotope values in the same nodular limestone, although negligible, suggest preferential cementation within the nodular portion. Nodules were possibly generated by dissolution of carbonate sediments and subsequent reprecipitation of carbonate cements close to the sea floor.

The early Mesoproterozoic Rohtas Limestone in the Son valley area of central India represents an. overall shallowing-upward carbonate. succession. Detailed facies analysis of the limestone reveals outer- to inner-shelf deposition in an open marine setting. Wave-ripples, hummocky cross stratifications and edgewise conglomerates argue against a deep marine depositional model for the Rohtas Limestone proposed earlier. Stable isotope analysis of the limestone shows that delta(13)C and delta(18)O values are compatible with the early Mesoproterozoic open seawater composition. The ribbon limestone facies in the Rohtas Limestone is characterized by micritic beds, each decoupled in a lower band enriched and an upper band depleted in dissolution seams. Band-wise isotopic analysis reveals systematic short-term variations. Comparative enrichment of the heavier isotopes in the upper bands is attributed to early cementation from sea water and water derived from the lower band undergoing dissolution because of lowering of pH at depth. The short-term positive shifts in isotopic compositions in almost every upward gradational transition from a seamed band to a non-seamed band support the contention that dissolution seams here are of early diagenetic origin, although their formation was accentuated under overburden pressure.