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Large sedimentation rate in the Bengal delta: Magnetostratigraphic dating of Cenozoic sediments from northeastern Bangladesh

¹ Institut für Geophysik, Universität Göttingen, Herzberger Landstrasse 180, 37075 Göttingen, Germany
² Bangladesh Petroleum Exploration Company (BAPEX), Dhaka, Bangladesh
³ Geologisches Institut, Universität zu Köln, Germany
⁴ Federal Institute for Geosciences & Natural Resources (BGR), Hannover, Germany

A 4.3-km-thick section of clastic sediments of the Surma basin, northeastern Bangladesh, that is exposed along the Shari River was sampled for a magnetostratigraphic study. Miocene through Pleistocene ages have been discussed previously; however, there has been no firm dating through biostratigraphy, or radiometric or other methods. Primary paleomagnetic remanent magnetizations of normal or reversed polarity were determined for most of the 300 collected samples. On the basis of assumptions of (1) constant sedimentation rates, on average, throughout the deposition of the sedimentary sequence, (2) no major gaps in sedimentation, and (3) a fairly young (< 2 Ma) age for the topmost Dupi Tila deposits, we find good agreement between the measured profile and the geomagnetic polarity time scale if the entire sedimentary sequence is between 4.9 and 1.4 Ma in age. The Upper Marine Shales, an important seismic marker horizon, are dated as 3.5 Ma and are therefore 1.5 m.y. younger than previously assigned by biostratigraphic results. The Upper Marine Shales presumably correspond to the last pre-Pleistocene sea-level highstand. The dating of rock units below the Upper Marine Shales is tentative because of a large gap in exposure. The inferred sedimentation rate—and thus the rate of delta subsidence—of 1.2 m/k.y. is much larger than previously assumed; it appears to be one of the highest sedimentation rates in Earth history that was sustained for millions of years.