PROVENANCE HISTORY OF MIDDLE OLIGOCENE BARAGOLAI FORMATION OF BARAIL GROUP FROM MARGHERITA AREA OF ASSAM, NORTHEAST INDIA

The Assam basin is located near the eastern syntaxis of the Himalayas. The Indo-Burman ranges lie to the east and south, and several imbricate thrust slices make up the Schuppen belt in the basin. Kilometers of Tertiary sediments are exposed and drilled in the basin. Modal analysis and heavy-mineral studies shed light into provenance of middle Oligocene Baragolai sandstones of the Barail Group.

Modal analysis indicates that the Baragolai sandstones are quartzolithic, composed of monocrystalline quartz (dominantly undulatory) and polycrystalline quartz (both massive and foliated), chert and microcrystalline quartz; both potassium and plagioclase feldspar; and sedimentary, volcanic and low- to intermediate-grade metamorphic lithic fragments. Total quartz (Qt), feldspar (F), and lithic fragment (L) percentages range from 44 to 68, 13 to 26, and 17 to 36, respectively, with an average Qt-F-L value of 54-19-27. Heavy-mineral percentages in the Baragolai Formation range from negligible to 1.3%. Opaque minerals are dominant over the non-opaque varieties. The non-opaque minerals identified are zircon, rutile, tourmaline, biotite, chloritoid, epidote, garnet, hornblende, kyanite, staurolite, zoisite, apatite, and spinel.

The modal composition of sandstones plots in the “recycled orogenic” provenance field of Dickinson, suggesting an orogenic source, probably the Himalayas to the north and/or Indo-Burman ranges to the south and east. Presence of spinels in the study area suggests derivation from ophiolitic belts in the Himalayas and/or the Indo-Burman ranges. The heavy-mineral suites of the Barail Group reported from the Haflong-Dalu-Damchara area (~ 200 km SW of the study area) and Naginimora-Kongam area in Mon district in Nagaland (~100 km south of the study area) show nearly similar assemblages indicating uniform provenance for these areas of northeastern India during the Oligocene. Sediment composition (modal analysis and heavy-mineral data) of the Baragolai Formation are however much different from the equivalent Barail sediments in the neighboring Bengal basin suggesting a different provenance history.