The Glacial Paleolandscapes of Southern Africa: the Legacy of the Late Paleozoic Ice Age

Abstract

Abstract. The modern relief of Southern Africa is characterised by stepped plateaus bordered by escarpments. This morphology is thought to result from stepwise uplift and ensuing continental-scale erosion of the region as it rode over Africa’s mantle ‘superplume’ following the break-up of Gondwana, i.e. since the mid-Mesozoic. We demonstrate in this contribution that this modern morphology of Southern Africa is in fact largely inherited from glacial erosion associated to the Late Paleozoic Ice Age (LPIA) that occurred between 370 and 260 Myr ago, during which Gondwana – which included Southern Africa – was covered in thick ice masses. Southern Africa hosts vast (up to 10⁶ km²) and thick (up to 5 km) sedimentary basins ranging from the Carboniferous, represented by glaciogenic sediments tied to the LPIA, to the Jurassic-Cretaceous. These basins are separated by intervening regions largely underlain by Archean to Paleoproterozoic cratonic areas that correspond to paleohighlands that preserve much of the morphology that existed when sedimentary basins formed, and particularly glacial landforms. In this contribution, we review published field and remote data and provide new large-scale interpretation of the geomorphology of these paleohighlands of Southern Africa. Our foremost finding is that over Southern Africa, vast surfaces, tens to hundreds of thousands km² (71.000–360.000 km²) are exhumed glacial landscapes tied to the LPIA. These glacial landscapes manifest in the form of cm-scale striated pavements, m-scale fields of roches moutonnées, whalebacks and crag-and-tails, narrow gorges cut into high-standing mountain ranges, and km-scale planation surfaces and large U-shaped valleys, overdeepenings, fjords and troughs up to 200 km in length. Many modern savannahs and desertic landscapes of Southern Africa are therefore relict glacial landscapes and relief ca. 300 Myr old. These exhumed glacial relief moreover exerts a strong control on the modern-day aspect of the geomorphology of Southern Africa as (1) some escarpments that delineate high-standing plateaux from valleys and coastal plains are inherited glacial relief in which glacial valleys are carved, (2) some hill or mountain ranges already existed by LPIA times and were likely modelled by glacial erosion, and (3) the drainage network of many of the main rivers of Southern Africa is funnelled through ancient glacial valleys. This remarkable preservation allowed us to reconstruct the paleogeography of Southern Africa in the aftermath of the LPIA, consisting of highlands over which ice masses nucleated and from which they flowed through the escarpments and toward lowlands that now correspond to sedimentary basins. Our findings therefore indicate that glacial landforms and relief of continental-scale can survive over tens to hundreds of million years. This preservation and modern exposure of the glacial paleolandscapes were achieved through burial under piles of Karoo sediments and lavas over ca. 120 to 170 million years and a subsequent exhumation since the middle Mesozoic owing to the uplift of Southern Africa. Owing to strong erodibility contrasts between resistant Precambrian bedrock and softer sedimentary infill, the glacial landscapes have been exhumed and rejuvenated. We therefore emphasise the need of considering the legacy of glacial erosion processes and the resulting presence of glacial landscapes when assessing the post-Gondwana-breakup evolution of Southern African topography and its resulting modern-day aspect, as well as inferences about climate changes and tectonic processes. Finally, we explore the potential pre-LPIA origin for some of the landscapes. In the Kaoko region of northern Namibia, the escarpments into which glacial valleys are carved may correspond to a reminiscence of the Kaoko Pan-African Belt, whose crustal structures were either reactivated or where relief persisted since then. In South Africa, the escarpment bordering the paleohighland corresponds to crustal-scale faults that might have been reactivated during LPIA by subsidence processes. These inherited morphological or crustal features may have been re-exploited and enhanced by glacial erosion during the LPIA, as it is the case for some Quaternary glacial morphology.