GEOCHEMICAL CONSTRAINTS ON PROVENANCE OF TERTIARY SEDIMENTS IN ASSAM, INDIA, AND THE BENGAL BASIN

The Assam and Bengal basins are two separate, but adjacent large depositional sites located at the south of the eastern Himalayas. The Indo-Burman ranges stand to the east, and the Indian craton is to the west. These basins contain kilometers of Cenozoic deposits, and hold considerable potential for recording the erosional history from the uplands around the basin margin. Detrital mineral and whole-rock chemistry of Lower Cenozoic clastics from these basins provide constraints on sediment provenance.

Sediments from Paleogene units in the Bengal basin distinctly contrast from Miocene units in most trace and major element chemistry, supporting differences in modal and heavy-mineral content. The overall concentrations and patters of trace elements, however, roughly follow the trend of the post-Archean average Australian Shale (PAAS). Eocene samples are depleted in most trace elements compared to younger units. Depletion of feldspars in quartzose Eocene samples has produced a negative Europium anomaly. Mudrocks of Miocene units are homogenous in chemical composition, suggesting derivation/mixing of detritus from similar source terranes. A Roser and Korsch (1996) plot of K₂O/Na₂O ratio versus Si0₂ suggests derivation predominantly from an “Active Continental Margin.”

Chromium content in detrital chrome spinels of Paleogene sandstones from both the basins is high, ranging from 20% to 58% in the Bengal basin and from 27% to 54% in Assam. Spinels from the Bengal basin, however, show a wide range in TiO₂ wt% (0.3 to 3%) compared to Assam (0.1 to 0.48%). Other cations abundant in Paleogene spinels of both the basins are Al, Fe³⁺, Mg and Fe²⁺. Assam Paleogene chrome-spinels have strong similarity to podiform chromite, cumulates, and harzburgites common in Alpine-type ophiolitic belts in the Indo-Burman ranges to the east. In addition to this source at the Indo-Burman ranges, Paleogene spinels from the Bengal basin may also have been contributed from the Cretaceous Rajmahal flood basalts to the west.