A large‐scale meandering submarine canyon: outcrop example from the late Proterozoic Adelaide Geosyncline, South Australia

Abstract

ABSTRACTThe late Proterozoic Adelaide Geosyncline, along with overlying Cambrian strata, comprises a thick sequence of sediments and sparse volcanics which accumulated in a major rift and passive margin setting. During late syn‐rift or early post‐rift phases, large volumes of terrigenous and carbonate sediments of the late Proterozoic Umberatana and Wilpena Groups and Cambrian Hawker Group filled the rift. Submarine canyon development was related to at least four of these depositional cycles, the most notable of which resulted in incision and subsequent filling of the major (several kilometres in width and up to 1.5 km deep) submarine canyons by the Wonoka Formation.The Wonoka Formation canyons are not obviously fault controlled. They are interpreted to have been eroded by turbidity currents during a relative low‐stand of sea‐level. They were subsequently filled by a fining‐upwards suite of sediments which reflects subsequent relative rise of sea‐level and carbonate platform development. Ultimately the canyon complex was buried by north‐westerly progradation of overlying fluvial and slope sequences (Billy Springs Beds and possibly correlative upper Pound Subgroup). It is considered likely that more distal elements of this prograding clastic wedge provided the necessary material for canyon erosion, prior to canyon filling and ultimate burial by what may have been elements of the same depositional cycle.It is considered possible that the series of isolated outcrops of canyon cross‐sections within the Wonoka Formation are sections of a single canyon thalweg developed within a considerably broader zone of slope degradation. If this interpretation is correct, then the gorge‐like Patsy Springs Canyon lies in more proximal regions of the basin‐slope, whereas 40 km to the north‐east the lower slope is cut by the Fortress Hill Canyon Complex. Palaeocurrent analyses of channel‐fill turbidites within the canyons imply that the Fortress Hill Complex is in fact the outcropping western edge of a sinuous, incised canyon thalweg.The Wonoka Formation canyons, containing basal sedimentary breccias but only minor conglomerates, are considered typical of passive margin canyon development. They are contrasted with the generally highly conglomeratic channel‐fills observed in outcropping Tertiary and Cretaceous examples of active margin canyons and upper fan valleys.