Fluid expulsion features associated with sand waveson Australia's central North West Shelf

摘要

Multibeam swath bathymetric data collected in95-120 m water depth on Australia's North West Shelfrevealed two distinct populations of sand waves: a laterallyextensive, low-amplitude composite form comprising super-imposed dunes and ripples, and a laterally restricted formwhich has unusually high bedform heights and slopes. Theselarge subaqueous sand waves comprise bioclastic ooid/peloidsand. Significantly, evidence of seabed fluid flow wasdetected 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 andmassive carbonate concretions dredged from the seabed, andpotential active venting of a fluid plume from the seabedobserved during an underwater camera tow. Molecular andisotopic analyses of carbonate concretions collected fromwithin pockmarks associated with the high-amplitude sandwaves indicate that the fluids from which they precipitatedcomprise modem seawater and are not related to thermo-genic fluids or microbial gases. The fluid flow is interpretedto be driven by macrotidal currents flowing over therelatively steep slopes of the high-amplitude sand waves.Pockmarks and carbonate concretions then develop wherethe interstitial flows are confined and focused by subsurface`mounds' in a shallow seismic reflector.