Cryogenian glaciomarine megaclasts of the MacDonald Corridor, Bimbowrie Conservation Park, Olary Region, South Australia

Ten glacially related megaclasts, unique perhaps from a global perspective, have been identified in the MacDonald Corridor. Included are perhaps the largest known glacially-related basement-derived megaclasts, the Bimbowrie Megaclast measuring 1.25 km in exposed width. The MacDonald Corridor is the most outward of four half-grabens to the east of the main Adelaide Geosyncline rift complex. All four contain Neoproterozoic low-grade metasedimentary rocks situated between inliers of Paleoproterozoic-Mesoproterozoic basement at the southwestern margin of the Curnamona Province. These corridors originated as extensional structures during the last of three major episodes of rifting that led to the eventual break-up of the supercontinent Rodinia. This rift phase was synchronous with the older (Sturtian-age) of the two major Cryogenian glaciations that are recorded in the rift and sag-basin complex of the Adelaide Geosyncline in South Australia. Those megaclasts selected for description vary in size from 0.1 to 1.25 km in exposed width, and were derived from the eastern rift flank, the Kalabity Inlier They therefore comprise either Paleoproterozoic metamorphic or Mesoproterozoic rocks or a composite of both. The megaclasts fall into two groups, those contained within the sedimentary fill of the rift basin, the Bimbowrie and Adlaweena megaclasts being the largest, and those resting as erratics on the flank. Of these, the 450 m wide Old Boolcoomata Megaclast resides within the remains of a steeply dipping U-shaped glacial valley. Tectonic activity and ice action are considered to have been responsible for creating the high-relief topography of the Kalabity Inlier. Ice was also instrumental in separating the megaclasts from their source and transporting them into the tectonically active MacDonald Corridor half-graben. Additionally, the variation of floating ice-cover controlled iron deposition within the glacigene Benda Siltstone. The present morphology of the MacDonald Corridor is largely due to the Cambrian Delamerian Orogeny, which while causing some structural complexity, exhumed and tilted the infill of the corridor to view.