Eustatic and environmental implications of a microgastropod shell bed in the Cenomanian–Turonian boundary interval in the Narmada Basin (India)

The Cenomanian–Turonian boundary (CTB; ~94 Ma) marks a period of extreme climate warming, the highest sea-levels of the Phanerozoic, and a turnover in marine microfossils and macroinvertebrates. This boundary interval is marked by black shales that mark a global carbon cycle perturbation and the Oceanic Anoxic Event-2. However, the identification of OAE-2 is complicated by the frequent absence of black shales and/or age diagnostic fossil species, and diagenetic overprinting of δ¹³C data. This warrants a systematic investigation of other biostratigraphic indicators of the OAE-2 crisis. This study investigates the stratigraphic utility of a microgastropod-dominated shell bed as a key marker within the OAE-2 interval.

A regional lensoidal microgastropod-dominated shell bed (2 to 10 cm thick) is observed in the shallow marine carbonate sequence the of Bagh group of sediments (Narmada basin) in the Karondia, Soyla, Jeerabad, and Rampura outcrops near Manawar, India. Previous reports suggest a Turonian age based on ammonite biostratigraphy. We test the hypothesis that the microgastropods indicate biotic stress in shallow marine environments due to OAE-2. The age diagnostic planktic foraminifera are absent. However, low-diversity benthic and planktic foraminifera assemblages with low oxygen-tolerant species confirm biotic stress in Nodular Limestone Formation. Microfacies studies indicate a low-energy supratidal to upper intertidal environment of deposition for the Nodular Limestone Formation, which bears the microgastropod shell bed. The occurrence of microgastropods in association with opportunist planktic foraminifera (e.g., Muricohedbergella, Planoheterohelix) indicate a pioneering palaeocommunity of generalists that colonized new ecospace on the shelf created by the late Cenomanian-early Turonian transgression in the Eastern Narmada Basin. Similar and coeval microgastropod shell beds have been reported from the Tethyan marginal sites of the Western Saharan Atlas of Algeria (Whiteinella archeocretacea zone, CTB interval), Eastern Desert of Egypt (Vascoceras proprium zones, lowermost Turonian) and Upper Benue Trough, Northeastern Nigeria (Turonian). The diachronous occurrence of microgastropod shell beds at various Tethyan marginal sites may be due to a regional offset in the timing of marine incursion.