Prabodha Lakrani Hewage (MSc Thesis 2021)

Sedimentation in the Bengal and Nicobar Fan System.

The study is based on visual interpretation of sediment core data (core images and physical property data) obtained from scientific ocean drilling (ODP and IODP) expeditions. IODP Expedition 362 Site U1480 on the upper distal Nicobar Fan, IODP Expedition 354 sites U1455, U1450 and U1451 on the upper distal part of the Bengal Fan and ODP Leg 116 sites 717, 718 and 719 in the lower distal part of the Bengal Fan. The time period sampled and examined is the Miocene (~ 13 million years ago) to present.

The debrites were classified into four types, based on the proportion of mud clasts (mud clast rich or poor) and bed thickness (> or < 50 cm). Mud-clast-poor debrites were the most common and abundant at all sites and are distributed throughout the depth column i.e., over the last ~13 Ma, except for Leg 116 sites where few debrites occur, limited to the upper 200 m (last ~2 Myr). Thick mud-clast-rich debrites occur in the older sediments at Site U1480, and are present throughout the depth column at Site U1451. These two sites are located closest to the Ninety East Ridge, on the eastern and the western flanks, respectively. On the upper distal Bengal Fan sites (Exp. 354), the proportion of thick mud-clast-rich debrites reduces westward away from the ridge. Thin mud-clast rich debrites occur in all sites except the Leg 116 lower distal fan sites. Thick mud-clast rich deposits are typically deposited close to the source of flow generation whereas mud-clast-poor deposits can be generated by long distance transport.

The observations were then compared with submarine channel abundance (as a means to transport flows), distance from the Ninety East Ridge and distance from the deltaic source (both as potential sources for the flows) to interpret their spatial and temporal distribution. In the upper distal fan sites, at Site U1480 there is a higher frequency of debrites in older sediments (~ 7-10 Ma), but in sites U1455, U1450 and U1451 there are peaks in debrite abundance in the Miocene (~7-13 Ma) and the youngest sediments (last 1-2 Myr). The Miocene debrite peaks correspond to low channel frequency, suggesting channel flow is not the primary source of these debris flows and debrites. In the younger sediments (~0-5 Ma in upper distal part of Bengal Fan), high channel frequencies correlate with high debrite frequencies, suggesting these are deposited from channel flow. Site U1480 (Nicobar Fan) shows the highest number of debrites over time (per million years), followed by Exp. 354 Bengal Fan sites. The lower distal Leg 116 fan sites have the fewest debrites. The Bengal Fan sites highlight the influence of distance from the deltaic source. Mud-clast-rich thick debrites in the Exp. 362 and 354 sites suggest the Ninety East Ridge can be a local source for these debrites, especially for sites close to the ridge. Overall, the influence of multiple combined factors controls the distribution of debrites with both local and distal flow sources likely, and type of flow controlled by the type of source and distance travelled. This study suggests that debrites are common in the upper distal part of the fan system, which is contrary to classic models of debrite distribution on submarine fans. This is something that would be important to consider when siting seafloor infrastructure in the area (e.g., cables and pipelines) for future hydrocarbon exploration in submarine fans.

Keywords: Sediment gravity flows, debris flows, debrites, Bengal Fan, Nicobar Fan, Ninety East Ridge, Indian Ocean