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Conference Abstract 

Grain size distributions of 311 sediment samples from Sites U1450 and U1451 of International Ocean Discovery Program (IODP) Expedition 354 were determined using laser diffraction. Most of the samples were from turbidites, but some hemipelagic beds were also examined. The mean grain size values show that silt-sized particles are the dominant textural class, whereas the grain size values range from clay to coarse-grained sand. An overall upward change in mean grain size value reveals a slight coarsening-upward trend. However, other parameters such as standard deviation, skewness, and kurtosis show no systematic relationship with depth in the holes. The analyzed samples cover the age range from recent to early Miocene. Shepard textural classification plots show the sediments are mostly sandy silts, silty sands, and clayey silts with a few silts and sands also present. Frequency curve plots of samples from individual turbidite beds show inversely graded beds are most common at Site U1450, whereas thicker massive beds are dominant at Site U1451. 

Site U1450 (proposed Site MBF-2A) occupies a central position at 8°0.42′N and 87°40.25′E in the east–west transect across the Bengal Fan at 8°N. It is located at equal distance from Site U1451 on the flank of the Ninetyeast Ridge and Site U1455 on the flank of the 85°E Ridge. The overall thickness of the fan reaches ~4 km at this location (Curray et al., 2003). Neogene sediment thickness decreases toward the two ridges, which is likely the result of ongoing deformation on both ridges during the Neogene (Schwenk and Spiess, 2009). At this central position of the transect, the upper Miocene and Pliocene–Pleistocene sections of the fan appear to be most expanded and are inferred to contain a higher resolution record, as well as accumulating, on average, coarser grained material. The shallow section at this site is one of the seven ~200 m deep sections along the 8°N transect that constrain the Middle Bengal Fan architecture in space, time, and sediment delivery rate during the Pleistocene. The deeper section at this site will document the delivery mechanisms of the fan and the climatically and tectonically influenced sediment supply from the Himalaya during the Neogene. Changes in the source regions in response to tectonic and climatic evolution of the Himalaya are expected to be reflected in the sediment’s mineralogical and geochemical compositions, the geochronological data, and in accumulation rates across the transect.