Himalayan Uplift, Sea Level, and the Record of Bengal Fan Sedimentation at the ODP Leg 116 Sites

Cochran, James R.

Three closely spaced sites located 800 km south of Sri Lanka on the distal Bengal Fan were drilled during ODP Leg 116. Two of these sites, 717 and 718, together penetrated over 1300 of the 2000 m of stratigraphic section present at that location and provide a complete record of sedimentation since the lower Miocene. The entire section consists of turbidites, primarily derived from the Ganges-Brahmaputra delta. The main controls on sedimentation appear to be the uplift and erosion history of the Himalayas, along with the position of sea level relative to the shelf edge. Although the base of the fan was not penetrated, a number of lines of evidence suggest that it was approached and that fan sedimentation began in the lower Miocene. This corresponds in time with the oldest known molasse sedimentation in India and Pakistan and probably represents the time of beginning of major uplift and erosion in the Himalayas. Sedimentation continued at a substantial rate from the lower Miocene to the upper Pliocene indicating continued erosion and thus presumably continuing uplift and significant relief in the Himalayas during that time. The unconformity in seismic sections corresponding to the beginning of intraplate deformation can be dated as occurring between 7.5 and 8.0 Ma. It does not correspond to any change in the nature of the sediments. A change from the gray, silt-mud turbidites that had predominated throughout the Miocene to finer muddy, black organic-rich turbidites occurred in the uppermost Miocene. However, this occurred at 5.6-6.7 Ma in the Messinian, 1-2 m.y. after the beginning of the intraplate deformation. The Messinian change in the character of the sediments appears to result from a rise in mean sea level which occurred at that time. The beginning of intraplate deformation is probably due to a change in the plate boundary forces, perhaps resulting from the beginning of east-west extension in southern Tibet. A return to the deposition of coarser, silty turbidites occurred in the upper Pleistocene at about 800 ka. These sediments accumulated at very high rates until the Holocene. The beginning of the deposition of this unit corresponds in time to the marked intensification of the Pliocene-Pleistocene glaciations. The greater sea level variations resulting from the more intense glacial cycles may have resulted in completely exposing the shelf during glacial maximum with the sediment load of the rivers delivered directly to the continental slope.

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Proceedings of the Ocean Drilling Program, 116 Scientific Results
Texas A&M University

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Academic Units
Lamont-Doherty Earth Observatory
Marine Geology and Geophysics
Texas A&M University
Published Here
June 12, 2019