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Cyclones and tides as feeders of a submarine canyon off Bangladesh

¹ Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover, Germany
² Geowissenschaftliche Gemeinschaftsaufgaben, Stilleweg 2, D-30655 Hannover, Germany

Extremely high annual sedimentation rates of about 50 cm/yr at the head of the submarine canyon "Swatch of No Ground," which connects the submarine delta of the Ganges-Brahmaputra to the Bengal deep-sea fan were estimated by ²¹⁰Pb and ¹³⁷Cs gamma spectrometry. The sediment in the canyon at 228 m water depth consists of graded sand and silt layers and laminated mud. The sand and silt layers are assumed to have been deposited by tropical cyclones, whereas the intercalated laminated mud is thought to have been deposited by daily variations of tidal currents. The graded layers correlate well with the historical record of cyclones and allow an annual fine tuning. The rapid infill of the canyon head is caused by trapping of suspended sediment that has been mobilized by storms and tides on the delta topset. The sediment is stored only temporarily in the canyon head: slides, slumps, and earthquakes occasionally remove and transfer it by turbidity currents to the deep-sea fan. The focusing effect of the canyon and the subsequent stepwise export of sediment, probably combined with erosion of the canyon floor, explain the long-term regional stability of this canyon and probably other shelf canyons.

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