Age and depositional environment of carbonate exotics associated with the Disang group of Assam–Arakan Basin, Northeast India: constraints from microfossils and geochemistry

Abstract

In this paper, we present the detailed microfacies study and geochemical characteristics of carbonate exotics (olistoliths) associated with the Eocene Disang group of Assam–Arakan Basin, Northeast India to deduct their relative age, and depositional environment. The studied carbonate rocks contain diverse fauna with the dominance of foraminiferal assemblages (planktonic and benthic). The well-preserved planktonic foraminifera, especially Globotruncana spp. and Heterohelix spp. are recorded in carbonates and age has been assigned as late Cretaceous (Maastrichtian in age). Rare earth element (REE) contents (~\u200956 ppm average value) in these carbonates are high compared with the average value of typical marine carbonate (~\u200928 ppm). The Post-Archean Australian Shale (PAAS)-normalized REE patterns of these carbonates exhibit seawater-like REE patterns with light rare earth elements (LREE) depletion and relatively heavy rare earth elements (HREE) enrichment with negative Ce anomalies and positive Eu anomalies. The geochemical data along with microfacies suggest that the investigated carbonate rocks might have been formed in low energy environments deposited in neritic to bathyal palaeoenvironment during Campanian–Maastrichtian age. Our study also confirmed that the investigated Disang group carbonates are similar to the adjoining carbonates exposed in the ophiolitic mélange zone of Manipur ophiolites, NE India. It is likely that the carbonate rocks were influenced by diagenetic activities at different tectonic processes during the evolution of MOC and later emplaced with the Disang group flysch-like sediments during subduction and obduction processes of the Indian plate and Myanmar plate collision.