Middle to Late Pleistocene Architecture and Stratigraphy of the Lower Bengal Fan—Integrating Multichannel Seismic Data and IODP Expedition 354 Results

Abstract

Utilizing a novel data set of integrated high-resolution multichannel seismic data with IODP Expedition 354 drilling results, a Middle to Late Pleistocene stratigraphy for the lower Bengal Fan is developed. The study reveals a high lateral and temporal variability of deposition expressed by lateral shifts (often exceeding 100 km) between successive channel-levee systems (CLSs), which occurred on average every~15 kyr independent from sea-level changes. The CLSs are embedded in sheeted sediments deposited out of unchannelized turbidity currents, which represent almost two thirds of the lower Bengal Fan sediments. On 100-kyr timescales, CLSs and sheeted/unchannelized sediments build up subfans, which alternately occupied the western and the eastern Bengal Fan, while the remaining area was draped by~10 to 20 m-thick layers of background/hemipelagic sediments. Three subfans have been reconstructed: Subfan B (1.24-0.68 Ma) formed concurrently with the Middle Pleistocene Hemipelagic Layer, Subfan C (0.68-0.25 Ma) covered the entire study area, and Subfan D (0.25 Ma to recent) deposited concomitant with the Late Pleistocene Hemipelagic Layer. The continuous succession of subfans indicates an uninterrupted fan activity independent from sea-level cycles at least since the Middle Pleistocene. This remarkable independent behavior in terms of sediment supply has not been observed at the Amazon Fan but is in agreement with observations from the Congo Fan. Finally, the analysis of a complete cross section through the lower Bengal Fan reveals that almost half of the sediment represents sands, indicating that the lower Bengal Fan may not generally be classified as "mud rich" (≤30% sand). Plain Language SummaryThe northern Indian Ocean receives large amounts of material eroded by wind and rain onshore. Most of the sediment comes from the Himalayan mountain range and contains important information about the evolution of the Himalaya as well as the past Asian climate. After erosion, it is transported by large rivers, such as the Ganges and Brahmaputra, to the coast. Underwater, the sediment moves even farther southwards within river-like features, the so-called channel-levee systems. Eventually, the material is deposited within the submarine Bengal Fan, a large sediment body covering most parts of the Bay of Bengal. In this study, we combine the results of a large underwater drilling campaign (IODP Expedition 354) with acoustic images of the subsurface from the lower Bengal Fan in order to develop a profound understanding of the timing and location of channel-levee activity and sediment deposition. This knowledge will eventually help scientists to understand the past climate in the Himalaya region. We show that the location of sediment deposition is highly variable and can move more than 100 km laterally. These variations are investigated in detail for the last 1 million years.

Middle to Late Pleistocene Evolution of the Bengal Fan: Integrating Core and Seismic Observations for Chronostratigraphic Modeling of the IODP Expedition 354 8° North Transect

“…Seismic line GeoB97‐020/027 with interpretation of Bergmann et al (2020), highlighting the sediments studied here. IODP Expedition 354 Sites U1449–U1455 are indicated with vertical black lines.…”

“…Light blue shading indicates the regionally extensive thick middle and upper Pleistocene hemipelagic units. Seismic estimates of Bergmann et al (2020) for the depth ranges of Brunhes‐Aged Reflectors (BAR) (1 = purple; 2 = blue; 3 = green), which are generally associated with thin hemipelagic units where recovery is good, are also indicated. Question marks indicate uncertainty in tracing the BAR reflectors, including multiple possibilities of where BAR 3 intersects with IODP Sites U1453 and U1449.…”

“…IODP Expedition 354 Sites U1449–U1455 are indicated with vertical black lines. Channel‐levee systems are numbered after Bergmann et al (2020) according to their stacking pattern (1 = oldest; 40 = youngest). Regionally extensive reflectors are highlighted, from oldest to youngest: the middle Pleistocene hemipelagic layer (MPHL; red shading); the Brunhes‐Aged Reflectors: BAR 1 (magenta line), BAR 2 (dark blue line), and BAR 3 (green line); approximate reflector for the E. huxleyi datums (light blue line); and the base of the upper Pleistocene hemipelagic layer (UPHL; blue line).…”

“…Vertical exaggeration (VE) is 300. We refer the reader to Bergmann et al (2020) for the uninterpreted seismic data.…”

“…Sediments deposited by gravity flow or hemipelagic depositional processes are objectively identified based on physical properties, and each lithology is modeled using different strategies. The law of superposition provides additional constraints in turbiditic deposits with few, if any, site‐based constraints, by solving for all seven sites simultaneously and only accepting solutions that respect the channel‐levee stacking pattern defined by Bergmann et al (2020) (Figure 2). Here we analyze and discuss these model results.…”