Presented by Uddin, Ashraf .
Authors:
Uddin, Ashraf,
Sarma, J. N.,
Kher, Suvrat,
Lundberg, Neil.
Key words: Assam, Himalaya, Provenance, Sandstone petrology
In Session 81 Patterns and Processes of Siliciclastic Basin Fill Tuesday, 27-Oct-98 PM in Room: 701B at 4:00 PM for 15 minutes.
Abstract: Thick Eo-Oligocene sequences, exposed near Digboi, northeast Assam, represent detritus derived from the early Himalayan and Indo-Burman orogenic belts. Sandstones from these sequences contrast strongly in composition with Eo-Oligocene sands from the Bengal basin of Bangladesh, indicating quite distinct provenance histories for these two eastern Himalayan foreland basins. Sandstones from the Eocene Disang Series (up to 3300 m) and the lower Oligocene Naogaon Series (2200 m) are texturally immature and are composed mainly of quartz (both mono- and polycrystalline), sedimentary and low-grade metamorphic lithic fragments (including abundant chert), and plagioclase. Sandstones of the overlying middle and upper Oligocene Borogolai and Tikak Parbat Series (3300 and 600 m) are similar but also contain significant amounts of volcanic and higher-grade metamorphic detritus as well. These sandstones are clearly derived from an orogenic source, exposing and eroding sedimentary and low-grade metamorphic units to form the older sandstones, followed by increasing contributions from volcanic and higher-grade metamorphic rocks during deposition of the younger sandstones. In contrast, Eo-Oligocene strata from the neighboring Bengal basin contain mature quartzose sands that probably represent a cratonic source. The Bengal basin was apparently protected from orogenic sedimentation during the Eo-Oligocene, but clearly orogenesis had begun to the north. More regionally, Eo-Oligocene sands from the western Himalaya are more similar in thickness and composition to the sequences in Assam, suggesting that the initial collision of both NE and NW India with Asia were not strongly diachronous.
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