The Bengal Fan stratigraphy as a function of tectonic and climate - Correlation of IODP Expedition 354 results and available seismic data from the Bay of Bengal
Geophysics
Final Report Abstract
Within this project, echosounder and multichannel seismic datasets were integrated with drilling results from the IODP Expedition 354 to investigate the architecture and evolution of the lower Bengal Fan at different scales. The Bengal Fan is the main sink for Himalayan sediments and, thus, represents the primary recorder of the Himalayan erosion as well as Asian Monsoon history. During the project the spatial and temporal variability of sediment delivery and depositional processes since the Middle Pleistocene at the lower Bengal Fan (8°N) have been examined in order to enhance our understanding of the fan evolution as well as our knowledge on the link between source and sink. The project focused on (i) the impact of an actively meandering channel on levee formation as well as the Late Quaternary to Holocene activity of the Bengal Fan; (ii) the temporal and spatial dynamics of depocenter migration; (iii) the interplay of different depositional processes (channelized and unchannelized turbidity currents vs. hemipelagic deposition); as well as (iv) accumulation-rate variability and their potential link to climate changes in the source area. High-resolution Parasound data linked with levee lithology and radiocarbon ages derived from IODP Site U1454 were used to assess the spatial and temporal changes in deposition along the youngest channel of the Bengal Fan, the so-called Active Channel. The onset of overspill deposition at 8°N occurred around ~26.8 ka BP. Active overspill lasted for only 3.2 ka, interrupted twice by times of hemipelagic deposition at ~25.5-11.5 ka BP and 9.6 ka BP – recent. Our results suggest that both times of interruption were triggered by a decrease in turbiditycurrent height or upfan avulsion, and that no general link between hemipelagic deposition at 8°N and sea level exists. Spatial mapping of individual levee units revealed a strong influence of channel sinuosity on the overspilling deposition resulting in a patchy deposition. This highlights that, on short terms, sedimentation rate cannot be directly transferred to flux. Mass accumulation rates during the investigated time intervals, however, stay relatively constant despite the locally uneven deposition, demonstrating a constant sediment flux unaffected by sealevel variations. On longer terms, the patchy deposition is equalized in response to the dynamic channel geometry. The architecture and stratigraphy of the lower Bengal Fan since the Middle Pleistocene were studied utilizing a 420-km long multichannel seismic profile across all seven IODP Expedition 354 drill sites. On millennial time-scales, deposition is focused along the channel-levee system with average lifetimes of ~15 ka. Moreover, sedimentation is organized in subfans with lifetimes of ~200-400 ka (Subfan B: 1.24-0.68 Ma; Subfan C: 0.68-0.25 Ma; Subfan D: 0.25 Ma – recent). While a subfan dominates parts of the Bengal Fan, the adjacent areas are draped by hemipelagic deposition. Integrating the seismic stratigraphy into a cores-seismic integrated age model revealed constantly increasing accumulation rates and overall sediment load throughout the lMiddle Pleistocene. The increasing input seems to reflect denudation rate changes in the source area, potentially linked to Middle Pleistocene climate variations. Subfan C fan sedimentation is interrupted thrice by decimeter-scale, regionally extending hemipelagic deposition. Their deposition does not appear to be systematically linked to sea-level variations but may be driven autocyclic. This finding not only suggests that the Bengal Fan behaves contradictory to most other large submarine fans, which are starved during times of sea-level highstand, it also hints towards a relatively constant sediment de-livery to the fan, since the Middle Pleistocene. Overall, the outcomes highlight the importance of a comprehensive approach to be able to understand the Bengal Fan’s complex depositional dynamics and to ultimately connect changes in the sedimentary sink with changes in the sedimentary source. It also delivers a new stratigraphy crucial for the interpretation for IODP Expedition 354 drilling results.
Publications
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IODP 354 Scientific Party, Evolution of the Middle Bengal Fan at 8°N in the Oligocene to Pliocene - Preliminary Results from IODP Expedition 354. EGU, 17-22 April 2016, Vienna, Austria
Spieß, V., Schwenk, T., Bergmann, F., France-Lanord, C., Klaus, A.
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IODP 354 Scientific Party. The Himalaya-Bengal Fan source to sink system – new insights by correlation of reprocessed seismic data and IODP Expedition 354 results. EGU, 17-22 April 2016, Vienna, Austria.
Bergmann, F., Spiess, V., Schwenk, T., France-Lanord, C., Klaus, A.,
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2017. Channel-levee evolution in the Himalaya-Bengal Fan source-to-sink system. Workshop Land-Ocean Interactions Across the Indian Ocean. 10-12 July 2017, Narragansett, RI, US
Bergmann, F., Spiess, V., Schwenk, T., Lantzsch, H., Dekens, P., Galy, V., France-Lanord, C.
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2017. Sinuosity-controlled deposition along a channel-levee system at the middle Bengal Fan – a high-resolution study integrating seismoacoustic and IODP Expedition 354 data. IMS, 10-12 October 2017, Toulouse, France
Bergmann, F., Lantzsch, H., Spiess, V., Schwenk, T., Dekens, P., Galy, V., France-Lanord, C.
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IODP 354 Scientists, 2017. Magnetic Fabric and Channel Migration in the Active Levee of the Bengal Fan, IODP Site U1454. GSA Annual Meeting, 22-25 October 2017, Seattle, Washington, USA
Selkin, P., Bergmann, F., Brainard, M., Dekens, P., Galy, V., Lantzsch, H., Manzueta Jr., M., Meynadier, L., Reilly, B., Ruiz III, V., Savian, J., France-Lanord, C., Spiess, V., Klaus, A.
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2018. The Bengal Fan evolution and stratigraphy at 8°N – Integrating seismic data and IODP 354 drilling results. ISC, 13-17 July 2018, Quebec, Canada
Bergmann, F., Reilly, B., Schwenk, T., Spiess, V., France-Lanord, C.
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2018. The Bengal Fan on different temporal and spatial scales - Integrating seismoacoustic and IODP Expedition 354 data to examine internal and external controls on depositional processes. Dissertation, University of Bremen
Bergmann, F.
