Nature and origin of the submarine Albany canyons off southwest Australia

Abstract

The Albany canyons complex off southwest Australia extends 700 km from Cape Leeuwin to east of Esperance. The submarine canyons head on the uppermost continental slope and extend from there up to 90 km offshore, to the lowermost slope and onto the abyssal plain. Distributaries have transported shelf carbonate grains at least 150 km onto the abyssal plain. The largest canyons have cut down 1500-2000 m in places. In general, on the upper slope they have cut down into harder, older rocks: canyon walls are steep, canyon axes slope at up to 20°, and ancient structures control their orientation. On the lower slope the canyons generally have not eroded down into harder rocks: canyon walls are less steep, canyon axis slopes are lower, and the canyons are generally oriented downslope. The canyons have exposed Jurassic and younger sedimentary rocks: their nature, canyon morphology and information from seismic reflection profiles have helped us build an understanding of canyon history. Floodplain deposition rather than erosion occurred during Australia-Antarctic rifting in the Late Jurassic, so river canyons (possible precursors of marine canyons) were unlikely to have been cut. A transition from non-marine to shallow-marine sedimentation characterised Early Cretaceous deposition in the slowly developing rift of the Australo-Antarctic Gulf. Gradients were low and canyon cutting unlikely. Deep river canyons were probably cut during uplift and erosion immediately before the Santonian breakup from Antarctica and their paths probably controlled later marine canyons. Only with the onset of rapid sea-floor spreading and subsidence in the Middle Eocene (ca 43 Ma) did gradients steepen and major marine canyon cutting become possible. The major sea-level fall at the Middle/Late Eocene boundary (ca 40 Ma) may perhaps have accelerated canyon formation. Carbonate sedimentation started to replace siliciclastic sedimentation in the late Middle Eocene, and became completely dominant in the Oligocene as the Antarctic Circumpolar Current started to scour the outer shelf of fines. However, carbonate grains displaced from the outer shelf could continue to cut the canyons, largely during periods of low sea level.