SEDIMENTATION AND TECTONICS OF CENOZOIC SEQUENCES FROM ASSAM AND BENGAL FORELAND BASINS OF EASTERN HIMALAYAS

There are at least two separate but large foreland basins located south of the eastern Himalayas. The Assam basin is to the east and the Bengal basin is adjacent to the Indian craton. Kilometers of Tertiary sediments are exposed and drilled in these two basins. Modal analysis and heavy-mineral studies shed light into provenance of Cenozoic sequences from these two basins.

Modal analyses document that almost all sandstone samples from the Assam basin are qartzolithic. Sandstones from the Eocene Disang Group (Qt₅₆F₅L₄₀), having very low feldspar content. The Oligocene Barail Group (Qt₅₉F₇L₃₄) has also low feldspar. Sandstone of Surma Group, which marks the Oligocene-Miocene transition in Assam basin, is more quartzose (Qt₆₈F₃L₂₉) than other units.  The Mio-Pliocene Tipam Sandstone (Qt₅₃F₉L₃₈) is higher in feldspar and lower in quartz contents. The Oligocene Barail Group (Qt₈₃F₃L₁₄) sandstones from Bengal basin are generally quartzolithic, with very low feldspar content. The Miocene Surma sandstone (Qt₅₉F₁₈L₂₃), the Mio-Pliocene Tipam Sandstone (Qt₅₇F₁₄L₂₉), and the Pliocene DupiTila Formation (Qt₅₄F₂₁L₂₅) all are also qaurtzolithic to quartzofeldspathic. The Oligocene sandstones from the Bengal basin contain relatively more ZTR minerals than Miocene and younger sandstones. Sandstones of Assam and Bengal basin include abundant garnet, muscovite, biotie, spinel, chromite, chlorite, amphibole, epidote, sillimanite, kyanite, andalusite, and pyroxene, indicating a predominant metamorphic provenance.

Modal composition of sandstones from two basins plots in the “recycled orogenic” provenance field (Dickinson, 1985) suggesting an orogenic source, probably the Himalayas to the north and/or Indo-Burman ranges to the south and east. Sandstones from Assam generally contain relatively more lithic grains than those of the Bengal basin, indicating a proximal orogenic source for the Assam sequence.  Sandstone composition of Pre-Miocene sequences from two basins reveal two separate sources and level of weathering process. Neogene sequences from both basins show similar composition, suggesting a common source and/or uniform weathering activity. Presence of spinels in most sequences of two basins suggests input from ophiolitic belts in the Himalayas and/or the Indo-Burman ranges.