Preservation of late Paleozoic glacial rock surfaces by burial prior to Cenozoic exhumation, Fleurieu Peninsula, Southeastern Australia

Abstract

The antiquity of the Australian landscape has long been the subject of debate, with some studies inferring extraordinary longevity (>108 myr) for some subaerial landforms dating back to the early Paleozoic. A number of early Permian glacial erosion surfaces in the Fleurieu Peninsula, southeastern Australia, provide an opportunity to test the notion of long-term subaerial emergence, and thus tectonic and geomorphic stability, of parts of the Australian continent. Here we present results of apatite fission track analysis (AFTA) applied to a suite of samples collected from localities where glacial erosion features of early Permian age are developed. Our synthesis of AFTA results with geological data reveals four cooling episodes (C1-4), which are interpreted to represent distinct stages of exhumation. These episodes occurred during the Ediacaran to Ordovician (C1), mid-Carboniferous (C2), Permian to mid-Triassic (C3) and Eocene to Oligocene (C4).The interpretation of AFTA results indicates that the Neoproterozoic−Lower Paleozoic metasedimentary rocks and granitic intrusions upon which the glacial rock surfaces generally occur were exhumed to the surface by the latest Carboniferous−earliest Permian during episodes C2 and/or C3, possibly as a far-field response to the intraplate Alice Springs Orogeny. The resulting landscapes were sculpted by glacial erosive processes. Our interpretation of AFTA results suggests that the erosion surfaces and overlying Permian sedimentary rocks were subsequently heated to between c. 60 and 80°C, which we interpret as recording burial by a sedimentary cover comprising Permian and younger strata, roughly 1 km in thickness. This interpretation is consistent with existing thermochronological datasets from this region, and also with palynological and geochronological datasets from sediments in offshore Mesozoic−Cenozoic-age basins along the southern Australian margin that indicate substantial recycling of Permian−Cretaceous sediments. We propose that the exhumation which led to the contemporary exposure of the glacial erosion features began during the Eocene to Oligocene (episode C4), during the initial stages of intraplate deformation that has shaped the Mt Lofty and Flinders Ranges in South Australia. Our findings are consistent with several recent studies, which suggest that burial and exhumation have played a key role in the preservation and contemporary re-exposure of Gondwanan geomorphic features in the Australian landscape.