High-resolution calcareous nannofossil biostratigraphy and facies of Nimar Sandstone Formation (Bagh Group), central India and its bearing on Oceanic Anoxic Event 2

Abstract

It is proposed to consider Narmada Valley Late Cretaceous sequences in three sectors with proper selection of stratotype in each sector: Eastern Sector: comprising of Lameta group of rocks with type area in Jabalpur area, essentially laid down in lacustrine-paedogenic milieu and containing a variety of fossils including dinosaur remains of Late Maastrichtian age. No evidence of sea incursion is available in the Eastern Sector either via the Narmada lineament or the K-G Basin. Central Sector: comprising of the Bagh Group in ascending order of: Nimar Sandstone Formation–Nodular Limestone Formation–Coralline-Bryozoan Limestone Formation, laid down in estuarine complex entirely within Turonian without any evidence of Cenomanian and Coniacian age calcareous nannofossils or other fossils being present. In the western part of Central Sector: Siliciclastic rocks and limestones referred to as Nimar Sandstone Formation curiously lack Late Cenomanian marker species, hence could be of Early Turonian age. Western Sector: comprises of Lacustrine sediments associated with Deccan Flood Basalt. The absence of the Cenomanian/Turonian Boundary and Black shales negates the presence of global OAE2 events in the Narmada lineament, but several OAE events are recognised in the Cauvery Basin, southeastern India. Uranium- and Thorium-rich Lower and Upper Nimar Sandstone Formation overlying crystalline basement are interpreted to have been essentially laid down in a fully marine estuarine milieu with reworked plant, and palynofossils including dinosaur remains. Calcareous nannofossils recovered from Upper Nimar Sandstone Formation over four decades ago and assigned Late Turonian age is revised based on modern data incorporating calcareous nannofossil and planktonic foraminifera datum and now suggest an older age of Early Turonian matching a zone with FAD of Quadrum gartneri at the base and FAD of H. helvetica at the top. It must be emphasised that except for the solitary and unique incursion of Early Turonian (ca. 93.9 Ma) sea along the Narmada lineament reaching eastward (>300 km) up to the present town of Indore, no other marine intrusion ever took place on Indian shield except along collision facing Subathu–Dogadda lineament during Late Maastrichtian. It would be erroneous to suggest that the global eustatic rise of sea level caused the entry of the Turonian sea along the Narmada lineament; instead, it was earlier speculated by me that Large Igneous Province activity of Mahajanga Flood Basalt of Madagascar (ca. 93.9 Ma) could have most probably caused a short-lived Turonian sea to enter Narmada lineament from the west around C/T boundary.