Evolution of submarine gullies on a prograding slope: Insights from 3D seismic reflection data

Submarine gullies are ubiquitous on the modern seafloor of both passive and active continental margins. However, the processes dictating gully formation and the role of gullies in deep-water sediment transport are topics of debate. In this study, through 3D seismic reflection interpretation of Pliocene-Pleistocene continental slope strata in the Taranaki Basin, New Zealand, we present results constraining the evolution of gullies over a roughly 2 million year timespan. We document gullies ranging from 20-400 m wide, ~ 10-60 m deep, and 2-22 km long, on slopes ranging from 2° to 5°. Gully width to depth ratios (3.5-5) are distinct from aspect ratios of submarine and fluvial channels (29.5 and 17), suggesting a process-based control on this morphometric relationship that could aid in predicting sediment transport styles from first order observations in datasets of modern or ancient seafloors. Gullies in this study commonly form nested complexes hundreds of meters thick, preserving patterns of aggradation, partial burial, and re-incision that highlight the long-lived nature of the gullies and their strong influence on subsequent turbidity flows. Gully longitudinal profiles are morphologically similar to slope interfluve profiles, suggesting that gullies are integral to slope sedimentation processes and that gully-forming flows are also the dominant drivers of continental slope sedimentation and outbuilding in this setting. We interpret these flows to be large, dilute, sheet-like turbidity currents, which produce net-aggradational, shallowly-incised gullies due to autogenic instabilities that promote regularly spaced variations in erosion and deposition. From the size and spacing of the gully arrays and the scale of sediment waves in the study area, we estimate these flows to have been at least tens of kilometers wide and up to 40 m thick.