Geomorphology and Evolution of the Gigantic Murray Canyons on the Australian Southern Margin

The Murray Canyons are a group of deeply-incised submarine canyons on a steep 400-km section of the continental slope off Kangaroo Island, South Australia. Some of the canyons are amongst the largest on Earth. The canyons, some 80 km long, descend from the shelf edge to abyssal plain 5200 m deep. Sprigg Canyon, the deepest and one of the largest, has walls 2 km high. The thalwegs of the larger canyons are concave in profile, steepest on the upper continental slope (15?-30?), with about 4?gradient on the mid slope, then level out on the lower slope to merge with the 1? continental rise. Between canyons, the continental slope is slightly convex to linear with a gradient of about 5?-6?. Canyon walls commonly slope at 15?-22?. The passive continental margin narrows to 65-km at the Murray Canyons and links the Bight and Otway Basins. WNW-trending Jurassic-Cretaceous rift structures control the irregular shape of the central canyons. At the western end, large box canyons 1 km deep are incised into thick sediments of the Ceduna Sub-basin. Formed by headscarp erosion, some of these canyons have coalesced by canyon capture. The upper parts of most canyons are cut into Cretaceous sediments and in some places are floored by basement rocks. Large holes, spaced about 5 km apart and up to several hundred metres deep, along the outlet channels of the larger and steeper canyons were probably gouged by turbidity currents resulting from major slope failures at the shelf edge. Quaternary turbidites were deposited on the abyssal plain more than 100 km from the foot of slope. Canyon down-cutting was episodic since the latest Cretaceous, with peak activity since the Oligocene due to strong glacioeustatic fluctations and cycles, with canyon development occurring during lowstands and early transgressions when sediment input at the shelf edge was usually highest. The timing of canyon development is linked to major unconformities within adjacent basins, with down-cutting events recorded or inferred during early Paleocene, Middle Eocene, Early Oligocene, Oligocene/Miocene transition (~24 Ma), mid Miocene (~14 Ma) and latest Miocene-Pleistocene. The early phases involved only siliciclastic sediments, while post-early Eocene canyon cutting was dominated by biogenic carbonates generated on the shelf and upper continental slope. The Murray River dumped its sediment load directly into Sprigg Canyon during extreme lowstands of the Late Pleistocene when the Lacepede Shelf was dry land.