Forcing factors influencing deposition of the Wonoka Formation, Flinders Ranges, South Australia, and triggers for canyon development.

Abstract

The latest Neoproterozoic Wilpena Group of the Flinders Ranges, South Australia, has been the subject of many studies, particularly concerning the Wonoka Formation, as the origin of canyons that cut down from this formation into lower formations has puzzled workers for decades. Two models for the ancient Wonoka canyons have been proposed by various authors, the subaerial model, and the submarine model. A tectonic mechanism with regional uplift resulting in a fall of sea-level has been proposed for the subaerial model with previously little evidence to support this in areas away from canyon incision. This study uses measured stratigraphic sections to show deepening and then shallowing of sea-level for older parts of the Wonoka Formation away from areas of canyon incision. Studied evidence includes the appearance and disappearance of turbidite beds, slumping within unit 2 and the formation of an irregular erosional surface associated with minor brecciolas low in unit 3. The down-cut surface is presumed to represent a significant sequence boundary coeval with known deep erosional canyons to the south and in the northern Flinders Ranges. Stable carbon isotope work was carried out and the results indicate that a stratified sea is possible for lower Wonoka time, coinciding with canyon incision and down-draw or fall of sea-level. The data collected is consistent with a tectonic event that caused the relative fall of sea-level by regional uplift and prompted canyon incision low in unit three of the Wonoka Formation. Several models for fall of sea-level including lowering due to glaciation (land-ice), salt diapirism, tectonic influences and a Messinian-style down-draw are considered.