Weathering and Source Rock Characteristics of the Upper Disang Sedimentary Rock of the Indo-Myanmar Ranges, NE India

Abstract

The Eocene Disang Group occupies a vast area of Manipur and extends in parts of Tista and Tirap valley in Arunachal Pradesh, Nagaland Hills, and small portions of North Cachar Hills (Assam) and continues up to Chin Ranges of Myanmar Ranges. The Disang Group is characterized by a group of monotonous sequences of dark grey to black splintery shales and has intercalation of siltstones and light to brownish grey, fine- to medium-grained sandstones of especially in the upper horizons, occasionally giving rise to rhythmic character. ICV vs. CIA, K2O/Na2O vs. Fe2O3+MgO and TiO2 vs. Al2O3 diagrams reveal that sediments from the Tista and Tirap river valleys in Arunachal Pradesh and towards the central portion of the Naga Hills in Nagaland are more weathered and recycled than sediments from the study area (most of Imphal valley or Manipur valley in Manipur). The sediments for the study area were dominantly derived from the unweathered rising Indo-Myanmar Ranges. The pre-Himalayan rocks might have been supplied sediments for the Disang Group. Sediments were also possibly derived from the uplifted fold thrust belt of Myanmar’s landmass comprising of igneous and metamorphic basement complex with older sedimentary sequences with minor contributions of detritus from Mishmi Hills region lying to the NE of the Arunachal Pradesh. The Disang sediments were deposited in different sedimentary environments from tidal flat to nearshore lagoon and neritic shallow marine environment and different in composition might have been related to variation in source rock for these sediments which was deposited in the Indo-Myanmar basin formed by rifting and crustal stretching on the continental margin of the Myanmar landmass.