The glacial paleolandscapes of Southern Africa: the legacy of the Late Paleozoic Ice Age

The modern relief of Southern Africa is characterized 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 breakup of Gondwana, i.e., since the mid-Mesozoic. We show in this contribution that the modern topography over large parts of Southern Africa bears glacial relief inherited from the Late Paleozoic Ice Age (LPIA) that occurred between 370 and 280 Myr ago and 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, particularly glacial landforms. In this contribution, we review published field and remote data and provide a new large-scale interpretation of the geomorphology of these paleohighlands of Southern Africa. Our foremost finding is that over Southern Africa vast surfaces are exhumed glacial landscapes tied to the LPIA. These glacial landscapes manifest in the form of centimeter-scale striated pavements; meter-scale fields of roches moutonnées, whalebacks, and crag and tails, narrow gorges cut into mountain ranges; and kilometer-scale glacial erosion surfaces and large U-shaped valleys, overdeepenings, fjords, and troughs up to 200 km in length. These forms are frequently found covered or filled with coarse-grained, glaciogenic sediments (frontal and lateral moraines, grounding zone wedges, IRD-bearing muds, etc.), whose distribution largely follows the pattern of glacial forms. Importantly, these glacial forms still today control many modern aspects of the surficial processes, such as glacial valleys that funnel the modern drainage network of some transects of the main rivers of Southern Africa. To explain how the glacial landscape has survived for such an extended period, we argue that its preservation and modern exposure may be attributed to burial under substantial layers of Karoo sediments and lavas for approximately 120 to 170 million years, followed by its exhumation since the middle Mesozoic, linked 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 re-exposed. This remarkable preservation allows 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. Moreover, we propose that in many instances, glacial erosion processes have superimposed an older, nonglacial land system whose original form is still expressed in the modern geomorphology of Southern Africa. Notably, some escarpments that delineate high-standing plateaus from coastal plains could be surficial expressions of crustal-scale faults whose offset likely operated before the LPIA and on which glacial processes are marked in the form of striae. Additionally, some hill or mountain ranges may have already existed during LPIA times, potentially reflecting remnants of Pan-African orogenic belts. Whether these features were later reactivated or persisted unchanged since that time is uncertain, but they were shaped by glacial erosion. We further propose that a network of pre-existing alluvial valleys could have existed before the LPIA, possibly formed during an extended period of exhumation and erosion in Southern Africa. These valleys may have later facilitated ice flow from highlands to lowlands, although the extent and configuration of such features remain speculative. The exhumed pre-LPIA landforms may in some cases be taken for pediments, pediplains, and pedivalleys and interpreted as recording the topographic evolution of Southern Africa after the dislocation of Gondwana during the Mesozoic. Some glacial valleys are also taken for rift structures. We therefore emphasize the need of considering the legacy of LPIA geomorphology when assessing the topographic evolution of Southern African and its resulting modern aspect, as well as inferences about climate changes and tectonic processes.