A detailed look at diapir piercement onto the ocean floor: New evidence from Santos Basin, offshore Brazil

Abstract

Diapir piercement through the ocean floor marks the final stages of a dynamic migration path. Once exhumed, a diapir extrudes from the seafloor, placing an obstacle for the flow of ocean bottom currents. While the hydrodynamic response of the flow has been previously studied, the detailed depositional and weathering modifications involved in the piercement process are less understood. To bridge this gap, we gathered already available multibeam bathymetric data, multichannel 2D seismic reflection profiles and collected new single-channel CHIRP profiles, Acoustic Doppler Current Profiler data and sediment samples across Santos Basin, offshore Brazil. In this region, the processes connecting the uppermost subsurface with the lowermost section of the water column are unknown. Data show three main stages of diapir exhumation: pre-, syn- and post-piercement into the seafloor. Extensional faults crown the pre-exposed diapir, before its piercement through the seafloor. Ocean bottom currents rework the top of the faults to form elongated depressions. The bottom currents tightly detour the diapir during and after its exposure at the seafloor. This interaction forms a drift and moat contourite depositional pattern. Our high-resolution data allow relating these morphologies to seafloor processes and distinguishing them from other reflector geometries related to diapir flank deformation, such as outward dipping of reflections. We further use this geometrical distinction to suggest a key for interpreting the exposure versus burial history of other diapirs worldwide.