Evolution and Stratigraphic Architecture of the Wonoka Submarine Canyons in Salt-Withdrawal Mini-Basins, Northern Flinders Ranges, South Australia

Dyson, Ian A., Salt Tectonics Australia Research, Aldgate, Australia

Neoproterozoic submarine canyons of the Wonoka Formation outcrop in the northern Flinders Ranges. Individual canyons are exposed in synclines overlying or contained within salt-withdrawal mini-basins of the Adelaide Geosyncline.

The 1500 meter-deep Patsy Springs canyon was cut during salt-withdrawal adjacent to the Christmas-tree diapir at Pinda Springs. Its base is a deep-water sequence boundary with no shallow-water equivalent. The canyon fill contains high-frequency, fining-upward sequences of the forced transgressive systems tract, punctuated with unconformity-based channel complexes passing laterally into mudflows off the canyon wall. The basal, several meter-thick carbonate breccia of each channel complex is overlain by vertically amalgamat­ed, laterally interconnected, narrow and sinuous channel sands. The sinuous Fortress Hill canyon displays similar architecture. Mudflows generated from instability on the outside of each bend were reworked into channel breccias at the base of successive channel complex­es, with fine-grained overbank facies deposited on the inside of each bend. The Nankabunyana Canyon overlies the salt-withdrawal mini-basin at Beltana diapir. Unconformity-based channel breccias were deposited adjacent to and on downthrown side of growth faults. An angular unconformity, formed by slumping of transgressive shale in the canyon axis, is overlain by channel breccia with onlap of an overall fining-upward succes­sion of thin-bedded turbidites.

Correlation of a mega-slump unit in each canyon indicates sedimentation could not keep pace with subsidence, with overlying unconformity-based channel breccia marking next fin-ing-upward cycle. Overall fining-upward canyon fills are characterized by low net to gross. Channel complexes are encased within background sedimentation comprising fine-grained turbidites that provide excellent seal.