Contrasting Sediment Transport Modes in the Holocene Levee of the Middle Bengal Fan, IODP Site U1454

By providing paleocurrent information in deep-sea cores where other paleoflow indicators are missing, anisotropy of magnetic susceptibility (AMS) can connect turbidity current dynamics to the architecture of submarine sedimentary systems. We use the AMS fabric of turbidites from the Holocene channel-levee complex of the Bengal Fan at 8°N to infer the speed and direction of turbidity currents, likely associated with spillover events. Within the levee, the general trend in magnetic fabrics is characterized by a shift from low to high degree of anisotropy. Oblate to triaxial fabrics with dipping magnetic foliation planes are common. Dip directions of magnetic foliation and statistically significant AMS maxima are interpreted as paleocurrent indicators. We observe a shift from flow-parallel lineation to a lineation perpendicular to the flow direction. Ferromagnetic minerals, mainly (Titano-)magnetite, account for ~25-95% of the magnetic susceptibility, with the highest ferromagnetic fractions associated with silty and sandy lithologies. Different shapes and densities of ferromagnetic and paramagnetic particles therefore complicate our interpretation of AMS fabrics. However, we interpret the AMS fabric to records an early levee-building regime of moderate-speed currents directed toward the west-northwest. AMS fabrics in this unit are consistent with deposition from suspension. These currents weakened and changed direction prior to shutdown of flow in the channel (evidenced by a hemipelagic unit deposited between ca. 14 and 19 cal ka BP). The channel reactivated later, characterized by much more intense and directionally consistent flow toward the southwest, which again weakened and changed direction prior to more recent shutdown. We interpret the AMS fabric in this unit to reflect orientation of rolling grains. AMS paleocurrents are broadly consistent with seismic data from the area.