Documentation and redefinition of the "Nongkulang hill series" and hitherto unreported Eocene-Oligocene transition sediments, Meghalaya, India

Abstract

Richly fossiliferous calc-argillaceous sediments interbedded between the carbonates of the 'Nummulitic series' and the sandstone-shale of the 'new series' around Nongkulang in the West Khasi Hills, Meghalaya, India, were originally designated as the Nongkulang hill series. The series was not considered as a formal unit due to the very thin succession and local nature of the sediments. In the Garo Hills and Khasi Hills, this lithosuccession was previously correlated with the Kopili Stage,whose stratotype is non calcareous and largely unfossiliferous. Successive occurrences of late Eocene and early Oligocene foraminifera in the uninterrupted calc-argillaceous sediments in the Therria section suggest the presence of an Eocene-Oligocene boundary succession. Since the calc-argillaceous sediments closely resembled the Nongkulang series these were examined to determine whether the Nongkulang qualifies for an independent lithounit or a part of the Kopili Stage. The Nongkulang series was mapped and its stratigraphic sections measured via river and road outcrops in a stretch from the Someswari (= Simsang) River in the Garo Hills to the Therria nala in Khasi Hills.With definite lithocontacts, lateral continuity, and variation in lithology and fauna, the series is justified as a formal lithounit characterized by small size Nummulites, ooids and thin impure limestone beds, and divisible into lower, middle and uppermembers. It is redefined and described as the Nongkulang Formation and assigned late Eocene to early Oligocene age based on integrated larger and planktonic foraminiferal biostratigraphy. Atentative Eocene-Oligocene boundary is documented in the middle member. The associated Prang and Kopili formations are also refined and revised. The Nongkulang Formation corresponds to the Eocene-Oligocene transition and contains terminal Eocene biostratigraphic events. These new sites documenting the Eocene to Oligocene succession may be potentially useful to study sea level changes in NE India and adjoining Bangladesh.