Mass-Transport Complex Top Surface Morphology Controls Turbidity Current Evolution Over Millennial Timescales

Wenjing Li, Nan Wu, Christopher Aiden-Lee Jackson , Harya Dwi Nugraha , Zhen Guo, Yu Huang

Slope failures and related gravity currents are key mechanisms by which sediment is remobilized, transported, and deposited on most of continental margins. Their deposits, which include mass-transport complexes (MTCs) and turbidites, are commonly deposited in close spatial and temporal association and constitute key depositional elements filling sedimentary basins. Previous studies have revealed that buried relief developed above MTCs can influence the evolution of turbidity currents and the distribution and character of their related deposits. However, the precise mechanisms by which this occurs remain poorly understood. Here we use 3D seismic reflection data from the Taranaki Basin, northwest New Zealand, to investigate how a buried MTC influences the initiation and evolution of subsequent turbidity currents. We interpret ten MTCs (MTC-1 to MTC-10) that constitute ~70% of the total post-Miocene stratigraphy, with turbidites developed intermittently between successive MTCs. We select the near-seabed MTC-9 (c.150 ms below the seabed), which extends over 250 km² and is very well imaged by the 3D seismic data, as the main focus of this study. We reveal that differential compaction driven by rheological contrasts between transported blocks and debrite can produce a rugose top surface with asymmetrical depressions and pronounced variations in slope gradient. When subsequent turbidity currents flow over the MTC-9 top surface, the rugose relief promotes turbidity current acceleration and facilitates transition to a supercritical flow regime, leading to the development of a chain of cyclic steps directly above the transported blocks. This geomorphic influence is preserved up to the modern seabed, indicating that the topographic control of MTC-9 on turbidity-current behavior has persisted for an extended geological time. In turn, hydraulic jumps associated with these turbidity currents can strongly rework the lee sides of transported blocks, thereby reshaping the topography of the buried MTC-9. We also show that retrogressive slope failures, which supplied MTC-9, generated trough-like depressions near its headwall, reaching tens of kilometers in length and tens of meters in depth. These depressions can repeatedly deflect and reflect the incoming turbidity currents, ultimately promoting the formation of a submarine channel. Given that retrogressive slope failure and differential compaction are common processes during and after MTC emplacement, respectively, we suggest that these mechanisms could exert a fundamental control on turbidity-current evolution, thereby influencing deep-water sediment distribution and stratigraphic architecture.