Researchers study deepwater sediment waves at Bounty Fan, New Zealand; turbidity currents dominate with minor contour influence

Researchers study deepwater sediment waves at Bounty Fan, New Zealand; turbidity currents dominate with minor contour influence

Deepwater sediment waves in mixed contouritic-turbiditic setting: The Bounty Fan, New Zealand | EPIC

Deepwater sediment waves in mixed contouritic-turbiditic setting: The Bounty Fan, New Zealand

Sediment waves are widespread deep-marine bedforms whose formative processes and lithological characteristics remain incompletely understood. Their origin is commonly attributed to a range of processes including turbidity currents, contour currents, cascading flows, and internal wave activity, but the relative contribution of these mechanisms in mixed systems is still debated. This study investigates sediment waves developed on the distal Bounty Fan (east of New Zealand’s South Island), where a ~ 400 m thick succession of stacked sediment waves is exposed within the northern levee of the Bounty Channel complex. The integration of high-resolution 2D multichannel seismic data with sedimentological and lithological observations from Ocean Drilling Program (ODP) Leg 181 Site 1122 provides a rare opportunity to directly link bedform morphology with internal stratigraphy in a deep-marine system. Importantly, Site 1122 offers a unique and unprecedented continuous core record of more than 400 m of sediment-wave deposits, representing one of the most complete subsurface archives of deep-water sediment waves yet recovered. The sediment waves display wavelengths of ~1–3 km and heights of ~30 m, and are expressed in seismic data as laterally continuous, aggradational packages within the levee deposits. Results indicate that the evolution of the Bounty Fan is characterized by distinct phases of development, reflecting changing interactions between turbidity currents, alongslope bottom currents, tectonic forcing, and climatic variability. A major reorganization occurred across the late Pliocene–early Pleistocene boundary, marked by the establishment of a persistent channel–levee system and a shift toward sustained turbidite-dominated sedimentation. A second intensification phase during the mid- to late Pleistocene is associated with increased sediment flux, enhanced levee growth, and more frequent turbidity-current activity, likely linked to tectonic uplift of the Southern Alps and climatic reorganization during the Mid-Pleistocene Transition. Lithological analysis of Site 1122 demonstrates that sediment waves are predominantly composed of stacked turbidite deposits, indicating that their formation is mainly governed by gravity-driven processes rather than contour-current deposition. Although contour currents may exert an influence on the Bounty Fan geomorphology, expressed through levee asymmetry and long-term channel migration, the internal architecture and deposits of the sediment waves reflect repeated overspill of turbidity currents. The vertical facies organization and grain-size trends are consistent with depositional cyclic-step dynamics generated by turbidity flows transitioning between supercritical and subcritical regimes, with thicker and coarser-grained turbidite beds and higher sedimentation rates on stoss sides of sediment waves, and a progressive fining and thinning toward lee sides. This study provides one of the most complete stratigraphic records of deep-water sediment waves to date and offers direct field-scale constraints on their internal architecture. More broadly, it demonstrates that, in this setting, sediment waves are primarily the product of turbidity currents, whereas contour currents likely play a secondary role in shaping the geomorphological asymmetry of the channel–levee system.

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gabriele.uenzelmann-neben [ at ] awi.de

PT2:Ocean and Cryosphere in Climate