Dismantling of an isolated tropical carbonate platform through flank collapse and canyon erosion, Coral Sea, Northeast Australia

Abstract

The steep slopes of carbonate platforms frequently display large-scale sediment destabilization features like rockfalls, mass transport complexes, and slope erosion. The processes and factors triggering such instabilities and how they interact are a matter of ongoing discussion. We use hydroacoustic, sedimentological, and seafloor imaging data to map and characterize slope instabilities and potential controlling factors at the flank of the isolated Tregrosse carbonate bank in the Coral Sea, northeast Australia. Erosion of gullies and submarine valleys is concentrated in slope segments with the platform rim at several 10s of meters of water depth, i.e. where there is potential for sediment transfer from the bank interior to the slope. Gravity core data indicate that most sediment export from the platform occurs during sea-level fall. The toe of slopes neighboring segments with a shallower platform rim are mostly characterized by mass-transport complexes of platform rim and upper slope rocks forming extended block fields. Distal slope areas are dismantled through submarine landslides resulting in scalloped head scarps. The basal detachment surface of these submarine landslides appears to be rooted in several 100 s of meters in the subsurface at a lithological heterogeneity, which is documented by a gamma-ray peak in the downhole logging data from Ocean Drilling Program Site 817. Our findings show that (1) canyon erosion, (2) platform rim and upper slope destabilization as well as (3) lower slope dismantling, largely act independently of each other to destabilize the flanks of the carbonate bank. The complexity of the carbonate platform dismantling processes and the corresponding controlling factors shown in this study should also be considered when interpreting seismic morphological data.