New Insights into Sediment Provenance and Climate along the IndoBurma Ranges, Eastern Bangladesh
Abstract
The Ganges and Brahmaputra rivers drain the Himalayas depositing >1Gton of sediment each year. These mighty rivers formed the Ganges-Brahmaputra Delta (GBD) that has prograded for 300-400 km since the Eocene. Subduction beneath the Burma sliver along the eastern margin of the Indian plate, and ~20 km thick GBD sediments has lead to the formation of the IndoBurma Ranges. These Ranges consisting of folds and thrust faults are part of the subaerial accretionary prism that is now being exhumed and is shedding a second-generation recycled material. New insights into the Neogene evolution of the Chittagong-Tripura Fold Belt (CTBT outer belt of the IndoBurma Ranges) are provided by measurements of Sr and Nd isotopes taken in the Dolai River Valley, a synclinal structure of the CTBT, and the Jaflong area, south of the thrust-controlled southern flank of the Shillong Massif. The rivers along the CTBT drain towards the north joining the Meghna River system. Thirteen, 5-50 m deep wells sampled the Dolai Valley and sediment aprons south of the Dauki Fault and Shillong Massif.
A large number of the samples in the Dolai Valley have Sr and Nd values that, as expected, reveal a provenance similar to modern Brahmaputra sources, which include the TransHimalaya batholiths exposed to erosion along the Tsangpo. We identified a second population with high eNd values and low Sr isotopes. These values are similar to Brahmaputra tributaries in eastern Assam that tap igneous rocks near the syntaxis. During the early deposition of the delta the early Brahmaputra had access to different sediment sources in addition to the known sources. In the Jaflong area, we find Holocene sediment more enriched in eNd than Goodbred et al. (2014) found along the axial region of Meghna drainage. Direct drainage in Jaflong from the Sylhet Traps may account for this difference. The Dolai River responds to sea level and late Pleistocene-Holocene climate changes. Radiocarbon ages revealed that the valley was eroded during the late Pleistocene culminating in Last Glacial Maximum river entrenchment. Soon after, sediment began accumulating in the Dolai flood plains, reaching a maximum in the early Holocene and decreasing in the late Holocene. This thick (up to 30m) sediment accumulation is consistent with the monsoon being most pronounced during the early interglacial.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T11B..06M
- Keywords:
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- 8110 Continental tectonics: general;
- TECTONOPHYSICS;
- 8122 Dynamics: gravity and tectonics;
- TECTONOPHYSICS;
- 8169 Sedimentary basin processes;
- TECTONOPHYSICS;
- 8177 Tectonics and climatic interactions;
- TECTONOPHYSICS