Marine geology of the Quaternary Bass Canyon system, southeast Australia: A cool-water carbonate system

Abstract

The modern Bass Canyon is one of the world's largest submarine canyon systems that is entirely located within a cool-water carbonate environment. Five large shelf-breaching and three slope-confined tributary canyons coalesce on the lower slope and enter the massive deep-water Bass Canyon at 3000 m depth. Normal slope sediments consist predominantly of sandy calcilutite and muddy calcarenite interpreted as mud flow and mud-lubricated sandy debris flow deposits respectively, and hemipelagic foraminiferal calcilutite. Canyon head sediments consist of intraclast-rich calcarenite and calcirudite, probably deposited by mass-wasting events, and fine calcarenite, deposited by cohesionless sandy debris flows. Canyon filling sediments consist of well-sorted fine calcarenite deposited by (long term) semi-continuous cohesionless sandy debris flows, and rarer intraclast-rich mud, interpreted as cohesive sediment gravity flow deposits. Two types of tributary canyons are recognized: V-shaped canyons with steep walls (up to 35°) that are well-defined by backscatter acoustics; and broad U-shaped canyons (walls ≤ 10°) poorly defined by backscatter. It is most likely that tributary canyons developed from down-slope eroding sediment gravity flows triggered at the shelf break, which mature to develop dendritic canyon heads. Dendritic canyon heads entrain coarse-grained shelf sediment, which fuel erosive gravity flows and scour deep V-shaped canyon profiles. Once canyon heads stabilise, shelf-derived erosive sediment flows are reduced and the canyon profile switches from V- to U-shaped. This is aided by continual pelagic sedimentation and sediment gravity flows from the adjacent slope and canyon walls. Mud-free sandy debris flows from the lower slope converge with sandy outflows from tributary canyons and flow down the Bass Canyon floor. Sand cored from the Bass Canyon floor preserves cyclic fluctuations in magnetic susceptibility. High magnetic susceptibility zones are associated with increases in cool-temperature planktonic foraminiferal, and have been tentatively correlated to oxygen isotope stages 6.2–6.8 (135–185 ka) and 8.2–8.4 (245–278 ka). Based on this correlation, sedimentation rates on the slope and within the Bass Canyon were found to be highest during the highstand and regressive systems tracts, and lowest during the lowstand and transgressive systems tracts.