STATISTICAL ANALYSIS OF THE THICKNESS OF DEEPWATER SEDIMENTARY DEPOSITS FROM IODP EXPEDITION 354: CLIMATIC INSIGHTS GAINED USING A DATA-DRIVEN APPROACH

The deep-sea Bengal Fan and linked Ganges-Brahmaputra River system forms Earth’s largest source-to-sink sediment-dispersal system, offering a unique opportunity to examine large-scale sediment transfer from the Himalaya to the Indian Ocean. International Ocean Discovery Program (IODP) Expedition 354 (2015) drilled a seven-site transect at 8^oN in the Lower Bengal Fan, recovering abundant Pleistocene-aged sands. Medium to coarse sands at such a distal point (~1400 km from the shelf edge) raises questions about the drivers and processes that transported these sands.

To investigate these deepwater depositional mechanisms, we did centimeter-scale lithological descriptions of cores from all seven IODP 354 sites, focusing on grain size and sedimentary structures to identify the preserved thickness of individual events. We noted sedimentary structures indicative of sediment gravity flows (SGFs), including normal grading, dewatering and load structures, and convolute bedding suggestive of syn-depositional seismic events.

The thickness distribution of the observed beds (< 790 ka) follows an asymmetrical lognormal pattern (n = 1271, mean = 31 cm, std = 82 cm, max = 917 cm, skewness = 5.28), with most values centered around thin beds composed of very fine sands. Coarse-grained outsized SGFs (thickness > 2 m, ~mean + 2 std), although outliers, account for 47% of the total thickness of all clastic deposits in the seven cores as a whole. There is a strong correlation between grain size and increasing bed thickness for sandy deposits. However, silty beds exhibit an inconsistent trend, possibly due to the rework by bottom currents. Further facies analysis is required to fully understand the complexity of these finer-grained beds.

The lognormal distribution of sandy SGFs in Bengal Fan cores may be linked to the frequency and magnitude of floods from the terrestrial source area, with outsized SGFs likely resulting from very infrequent, high-magnitude floods generated in the Himalaya. This data-driven approach employs grain size and thickness statistics for depositional facies analysis, providing insights into the forces that transporting clastic sediments recovered during IODP 354 to the deepwater sink. These results quantify the preservation of rare but substantial events in the stratigraphic architecture and explore signal propagation from the Himalaya to the distal Bengal Fan.