Late Proterozoic Patsy Springs Canyon, Adelaide Geosyncline: submarine or subaerial origin?

Abstract

ABSTRACTA significant aspect of Late Proterozoic sedimentation in the Adelaide Geosyncline, South Australia, is the presence of kilometre‐deep erosional incisions which have been termed canyons. These structures were formerly described to be of submarine origin, cut and filled in an inferred basin‐slope setting by subaqueous processes. Subsequent detailed research, particularly on a specific incision known as Patsy Springs Canyon, indicates that sedimentary structures within some of the canyon‐filling sediments are indicative of deposition above fair weather wave base. In addition, an unusual carbonate unit, which is observed to veneer upper portions of canyon shoulders and to contribute to carbonate breccias interbedded with canyon‐fill, has a stable isotope signature which may imply a non‐marine origin. The presence of the carbonate veneer, where it is in situ, suggests that at least upper portions of the canyons could have been emergent during the canyon‐filling phase. Considering these observations, and combining them with regional stratigraphical relationships, an alternative model for canyon genesis is proposed involving subaerial erosion and subsequent filling by coastal onlap. Such a model requires base‐level changes of the order of 1 km, in order to account for observed canyon cutting and filling. Vertical movements associated with halokinesis, or thermally‐induced uplift of the order of 1 km, could have resulted in the observed erosional events. Alternatively, a Messinian‐style evaporitic lowering of base‐level is currently receiving serious attention. With present knowledge this mechanism most satisfactorily explains all observations.