THE PALEOGENE SEQUENCE OF ASSAM, INDIA: RESULTS OF COLLISION OR CONVERGENCE?

The Assam basin of eastern India, located along the present Brahmaputra River valley at the eastern syntaxial area of the Himalayas, preserves 6+ km of Tertiary sediments near the site of convergence between India and Asia. Sediment composition suggests conflicting ideas on the provenance history and plate convergence.

Ongoing heavy-mineral studies of pre-Miocene sediments from northeast Assam reveals the presence of diversified species of heavy minerals including garnet, apatite, epidote, hornblende, zircon, tourmaline, rutile, zoisite, chloritoid, kyanite, staurolite, spinel and opaque minerals, suggesting derivation from an orogenic source. This confirms modal analyses of sandstones from the Assam sequences, which are quartzolithic and apparently orogenic. These contrast strongly with Eo-Oligocene sands from the Bengal basin of Bangladesh, which are highly quartzose, indicating quite distinct provenance histories for these two eastern Himalayan foreland basins. Pre-Miocene strata of the Bengal basin are apparently not orogenic, consistent with their heavy-mineral suites, which are dominated by zircon, tourmaline and rutile, in addition to opaque minerals. Strata in the Bengal basin apparently accumulated on crust of the Indian plate prior to arrival of the clastic wedge shed from the approaching orogen. The Bengal basin was protected from orogenic sedimentation during pre-Miocene time, either by a barrier to sediment transport (a peripheral forebulge or a marine basin?) or simply by distance.

Assuming that the Assam sequences analyzed were deposited on continental crust of the Indian plate, the Himalayan collision was not strongly diachronous, with initial collision of both syntaxial areas of northeast (Namche Barwa) and northwest India (Kohistan arc) in the Eocene. Another possibility is that the sequences in Assam were deposited on continental crust that was part of southern Asia. If this was the case, the orogenic source of the sandstones analyzed could have been Andean-type mountains reflecting simple plate convergence, rather than a continent-continent collision.