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A Neogene record of Himalayan erosion: the IODP Expedition 354 transect in the Bengal fan at 8° N

Abstract : Recent IODP Expedition 354 in the Bengal fan [1] generated a comprehensive record of Himalayan erosion over the Neogene and Quaternary. It documents the interplay between Himalayan tectonic and the monsoon. The Bengal fan is predominantly composed of detrital turbiditic sediments originating from Himalayan rivers, and transported through the delta and shelf canyon, supplying turbidity currents loaded with a wide spectrum of grain sizes. Turbiditic deposition makes that record at a given site is discontinuous which was the reason for an E-W transect approach. Expedition 354 drilled seven sites along a 320 km E-W transect at 8°N allowing the restitution of an almost complete record of Himalayan erosion at the scale of the Neogene. In spite of the transect extension, a long absence of deposition is observed between 0.6 to 1.2 Ma indicating that turbiditic depocenter was derived more to the West for ca. 600 kyr. Turbiditic sediments have close mineralogical and isotopic analogy with sediments of the modern Ganga-Brahmaputra rivers. Major and trace element geochemistry show stable compositions throughout the Neogene and Quaternary with most variations controlled by the origin of eroded sources ie. the proportions of sediments derived from Himalaya and Transhimalaya. Although relatively stable, source tracers such as Sr-Nd isotopic compositions, and detrital carbonate compositions show organised variations with time. They imply that exposure to erosion of the different Himalayan formations has evolved as a result of the evolution of the thrusting structures. Data suggest that (1) a component derived from Tranhimalayan formation was present even during Lower Miocene, (2) the Tethys Himalaya exposure to erosion was higher during Miocene than during Pliocene and Pleistocene, and (3) that the exhumation of the Lesser Himalaya was initiated around 8 Ma. Coring of large sand samples also allowed to initiate cosmogenic 10 Be tracing of erosion rates at the scale of the Ganga-Brahmaputra basin. Relatively stable paleo-concentrations reveal relatively steady erosion rates in spite of the Pleistocene climatic changes. Clays and major and trace element geochemistry reveal a very weak regime of chemical weathering with no significant variation through time. Concentrations in mobile elements such as Na and K relative to Al are significantly higher than in modern sediments suggesting that weathering or soil erosion is amplified in the modern time. Compositions are indeed controlled by source and weathering processes, but mineral sorting during turbiditic transport may also alter the geochemical signature of the sediments. Close attention has been given to evaluate the possible loss of clay size particles by large scale dispersion compared to silt and sand. It seems, however, that such processes cannot account for the differences observed with modern rive sediments. Low weathering of the sediments at 8°N indicates that erosion was dominated by physical processes and that transport is rapid enough to prevent evolution of particles in the floodplain. In the modern Himalaya, low weathering is achieved primarily by landslides and rapid transfer through the floodplain, i.e. limited recycling of sediment deposited in the floodplain. Both processes are favoured by the seasonality and the intensity of the monsoon. Overall, the low weathering intensity suggests that comparable erosion regime prevailed since at least the Early Miocene.
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https://hal.archives-ouvertes.fr/hal-02328982
Contributor : Christian France-Lanord <>
Submitted on : Wednesday, October 23, 2019 - 1:47:50 PM
Last modification on : Thursday, November 21, 2019 - 4:22:12 PM

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  • HAL Id : hal-02328982, version 1

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C. France-Lanord, V. Spiess, Sébastien Lenard, Albert Galy, Jérôme Lavé. A Neogene record of Himalayan erosion: the IODP Expedition 354 transect in the Bengal fan at 8° N. Working Group on Sediment Generation IV, Jun 2018, Dublin, Ireland. ⟨hal-02328982⟩

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