Fluid expulsion features associated with sand waves on Australia’s central North West Shelf

Multibeam swath bathymetric data collected in 95–120 m water depth on Australia’s North West Shelf revealed two distinct populations of sand waves: a laterally extensive, low-amplitude composite form comprising superimposed dunes and ripples, and a laterally restricted form which has unusually high bedform heights and slopes. These large subaqueous sand waves comprise bioclastic ooid/peloid sand. Significantly, evidence of seabed fluid flow was detected in association with the high-amplitude sand waves. This evidence includes seabed pockmarks approximately 2–15 m in diameter imaged with side-scan sonar, tubular and massive carbonate concretions dredged from the seabed, and potential active venting of a fluid plume from the seabed observed during an underwater camera tow. Molecular and isotopic analyses of carbonate concretions collected from within pockmarks associated with the high-amplitude sand waves indicate that the fluids from which they precipitated comprise modern seawater and are not related to thermogenic fluids or microbial gases. The fluid flow is interpreted to be driven by macrotidal currents flowing over the relatively steep slopes of the high-amplitude sand waves. Pockmarks and carbonate concretions then develop where the interstitial flows are confined and focused by subsurface ‘mounds’ in a shallow seismic reflector.