ABSTRACT

Upper Eocene to Neogene fill of the Bengal basin provides an earlier unroofing history of the eastern Himalaya and Indo-Burman ranges than that provided by drilling of the Bengal fan, and analysis of heavy minerals in these sequences provides useful provenance constraints. Quartzose sandstones of the Eocene Kopili and Oligocene Barail Formations contain only 0.2% heavy minerals, comprising abundant opaque minerals and stable minerals such as tourmaline, garnet, rutile, and zircon. This assemblage suggests sediment sources dominated by low- to intermediate-grade metamorphic, silicic igneous, and metasedimentary rocks; the low heavy-mineral content and abundance of stable minerals suggest intense chemical weathering. These sediments may have been derived in part from incipient uplifts of the proto-Himalaya and/or the Indo-Burman ranges, but a more likely source is the Indian craton immediately to the west.

Miocene sandstones of the Surma Group contain more abundant and more diverse heavy-mineral assemblages than Oligocene sandstones. These include abundant opaque minerals and garnet, mostly almandine, and moderate to minor amounts of tourmaline, kyanite, zircon, calcic amphibole, rutile, chlorite, zoisite, staurolite, epidote, sillimanite, and clinopyroxene, indicating a broad range of mostly metamorphic source rocks. Upper strata of the Surma Group also contain abundant blue-green amphibole, orthopyroxene, and sparse chromite, suggesting exhumation of arc and ophiolitic rocks. Sands and sandstones from the upper Miocene to Pliocene Tipam Group and the Pliocene to Pleistocene Dupi Tila Formation contain assemblages similar to those of the underlying Miocene sandstones, along with more abundant orthopyroxene and sillimanite in Dupi Tila sands.

Temporal variations in heavy-mineral assemblages in the Bengal basin suggest that orogenic detritus first appears in the eastern Himalayan foredeep much later (early Miocene) than in the western part of the foredeep (Eocene). The appearance of high-pressure phases in upper Miocene strata reflects continued unroofing of deeper-crustal metamorphic and silicic to ultramafic plutonic rocks.