Proterozoic canyons of the Flinders Ranges (South Australia): submarine canyons or drowned river valleys?

Abstract

A number of kilometer-sized erosional incisions (canyons) occur within a specific stratigraphic interval in the late Proterozoic succession of the Adelaide Geosyncline, Flinders Ranges, South Australia. This paper describes in detail four adjacent east-west orientated incisions in the northern Flinders Ranges which are interpreted to be part of an ancient sinuously meandering canyon system. At least 600 m of siltstones and fine sandstones of the Bunyeroo and the Brachina Formations were removed during the erosional event that produced the canyon. Infilling sediments, which exhibit a similar succession in each incision, were deposited during emplacement of the Wonoka Formation. Although an identical sedimentary succession occurs within each incision, palaeocurrents determined from flute casts at the bases of sandstone beds alternate by close to 180° from one erosional feature to the next. Basal infilling sediments commonly contain lenticular breccias, interpreted as slumps derived from the Brachina Formation, which forms the incision walls. It is significant that clasts in these breccias preserve evidence of lithification predating the erosional event that produced the incisions. Channel-filling debris-flow units which occur mainly above the basal slumps, along with associated sandstones, were most likely emplaced episodically. Asymmetric ripple cross-laminae in the sandstone beds above and within debris-flow units commonly are wave modified. Isolated occurrences of hummocky cross stratification are observed in most units of the incision-fill. A thin ( < 100 cm) but extensive limestone unit, developed above the erosional unconformity east of the canyon and on southern canyon walls, has a non-marine isotopic signature which suggests subaerial exposure of at least upper portions of canyon walls. The final position of what is interpreted as a laterally migrating thalweg channel is located adjacent to the north wall of each incision. Current debate centers on whether the incisions represent the meandering portion of an ancient submarine canyon system, or whether they were cut subaerially and were subsequently drowned and filled by coastal onlap. Both models are discussed in this paper. Sinuous meanders can form on a variety of scales, both in subaerial as well as in submarine environments. The abundance of wave-generated and wave-modified sedimentary structures within the canyon fill and the presence of what appears to be a non-marine limestone precipitated on the canyon walls favour the subaerial erosion model.