PROVENANCE HISTORY OF NEOGENE SANDSTONES FROM THE ASSAM BASIN, INDIA

The Assam basin, located in northeastern India, is bounded by the Himalayas in the north and the Indo-Burman Ranges to the east and south. Initial sedimentation commenced in the basin during a regional transgression in the Late Cretaceous. Preliminary modal analysis of sand and sandstones from the Neogene Surma and Tipam Groups indicates temporal variations in detrital composition and suggests possible source terranes related to orogenic unroofing.

Polycrystalline quartz grains, many of which are highly tectonized, are conspicuous in the Neogene sandstones from the Assam basin. Argillites are the dominant kind of sedimentary lithic fragments throughout the samples. Many of the constituent plagioclase grains have been altered to sericite and in some cases kaolinite. Potassium feldspar occurs in most samples but not to a high degree. Heavy minerals are present in notable quantities in some samples and have been observed to include orthopyroxene, clinopyroxene, garnet, tourmaline, and mica, including muscovite, biotite, and minor chlorite. Upper-grade metamorphic lithic fragments are also fairly abundant in these sandstones. Potassium feldspar and metamorphic lithic fragments offer insight regarding orogenic activity of the Himalayan/Indo-Burman systems.

These samples plot mostly in “recycled orogenic” provenance field of QtFL (Dickinson, 1985). The presence of orthopyroxene suggests unroofing of deeper orogenic rocks. Compared to the age-equivalent samples from the neighboring Bengal basin, the Neogene sandstones from Assam basin contain a higher percentage of polycrystalline quartz grains. The samples show evidence of deformation resulting from regional tectonics because of the proximity of the basin to the eastern Himalayan syntaxis. During the Neogene, the Assam basin most likely acted as a transient depocenter for the northeasterly-derived sediments that were being transported ultimately toward the topographically lower Bengal basin and the Bengal fan.