Axel Hofmann’s research while affiliated with University of Johannesburg and other places

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.

The expansion of ice masses across southern Africa during the Late Palaeozoic Ice Age has been known for 150 years, including the distribution of upland areas in controlling the configuration of glaciation. In Namibia, increasing attention has focussed on long and deep palaeovalley networks in the Kaokoland region in the north, but comparatively little work has been attempted in the topographically subdued plains of the south, in the Aranos and Karasburg basins. The desert terrain of the Aranos area exposes diamictites of the Dwyka Group discontinuously over about 300 km, extending further south to the Karasburg area at the Namibian-South African border along the Orange River. Whilst examined at a stratigraphic level, the nature of the contact between the Dwyka glacial rocks and underlying lithologies has not been systematically investigated. This paper presents the results from fieldwork in austral winter 2019, in which a highly varying basal contact is described that records the processes of growth, flow and expansion of ice masses across this part of Gondwana. At the basin margins, subglacially-produced unconformities exhibit classic glacially striated pavements on indurated bedrock. In comparison, the basal subglacial unconformity in the more basinward regions is characterised by soft-sediment striated surfaces and deformation. In the Aranos Basin, soft-sediment shear zones originated in the subglacial environment. This type of subglacial unconformity developed over well differentiated, unconsolidated, siliciclastic materials. Where ice advanced over more poorly sorted material or cannibalised pre-existing diamictites, “boulder-pavements” recognized as single clast-thick boulder-dominated intervals formed. Importantly, these boulder-pavements are enriched in clasts, which were facetted and striated in-situ by overriding ice. By integrating measurements of striation orientations, fold vergence and palaeocurrent information, former ice flow pathways can potentially be reconstructed over a wide area.

In the eastern part of the Karoo Basin of South Africa, the sedimentary record of the Late Palaeozoic Ice Age, the Dwyka Group, consists of an up to 200 m thick accumulation of massive to crudely‐stratified diamictite occasionally interstratified with siltstone, sandstone and conglomerate horizons. Three distinct sedimentary units, separated by intervening glacial erosion surfaces, are viewed as ice‐margin fluctuation sequences. The lowermost one, resting on highly uneven, glacially‐abraded Archaean basement, has been interpreted as a grounding zone wedge deposited after the retreat and stabilization of the ice margin after the inundation of the Karoo Basin. The grounding zone wedge interpretation is based on its thickness (up to 100 m), the dominance of diamictite, and its facies assemblage and inferred depositional processes (rain‐out of debris, dropstone dumping, mass and debris flow, till). Overlying the grounding zone wedge deposits are sedimentary units interpreted as glaciofluvial or ice‐contact delta and grounding zone wedges, respectively. By analogy with Quaternary sedimentary sequences, deposition of the Dwyka Group in the study area might have been very rapid (tens to hundreds of thousand years) and may hence correlate with the ultimate deglacial sequence of the Western Karoo Basin, as both successions are covered by the postglacial Ecca Group. Although commonly observed and imaged on modern, high‐latitude continental shelves, grounding zone wedges have never been interpreted in the ancient geological record. This paper therefore outlines a model defining criteria necessary for identifying grounding zone wedges. This article is protected by copyright. All rights reserved.

The Gondwana-wide Permo-carboniferous glaciogenic deposits, known as the Dwyka Group in South Africa, form the basal succession of the classic Karoo Supergroup. In southeastern South Africa, these deposits are commonly viewed as bearing the record of a single deglacial event. Conversely, its western equivalent (Namibia and western South Africa) is assumed to record four stages of growth and decay of ice masses separated by intermittent interstadials. The present work reveals, however, that at least three icemargin fluctuation sequences, formed by both glacial and non-glacial facies, form the infill of 100 m deep, 1-3 km wide U-shaped cross-shelf paleo-valleys carved into the bedrock. These valleys possibly represent paleo-fjords as observed elsewhere in Southern Gondwana but not recognized as such in this part of the Karoo Basin so far. In the studied area (KwaZulu-Natal province), three superimposed units displaying both glacial and non-glacial signatures compose the Dwyka Group. The up to 70 m thick lower unit consists of massive diamictite bearing abundant up to boulder-sized lonestones. This lower unit wedges out and onlaps on valley flanks while its top is characterized by glacial striae and grooves. Above, the 50 m thick coarsening-upward sequence forms the second unit which also consists in massive lonestone-bearing diamictite interstratified by normally-graded sandstone beds devoid of lonestones. These facies grade upward into a 10 m thick trough and sigmoidal cross bedded conglomeratic sandstones that is locally highly disturbed by subglacial deformation. The third unit formed by a massive diamictite bearing carbonaceous concretions and rare pebblesized lonestones is topped by discontinuous patches of highly glacially-deformed conglomeratic sandstones. Black shales of the Ecca Group, usually interpreted as postglacial deposits, directly rest on this ultimate glacial surface. Although diamictite intervals likely result from deposition in a glaciomarine environment by important rain-out beneath or immediately in front of a floating ice shelf, cross bedded and normally-graded sandstone horizons, respectively interpreted as fluvioglacial and subaqueous sediment density flows deposits, indicate local ice-free conditions. The vertical superimposition of these facies are then strong indicator of ice margin fluctuations throughout the study area. In such a context, the glacial maximum is likely marked by the basal erosion surface that carved valleys into the bedrock while the glacial surface recorded on top of the first unit is suspected to represent a temporary stillstand marked into an ice margin retreat. Deformed sandstones on top of both the second and third units thus represent fluvioglacial deposits emplaced during ice free conditions in a context of relative sea level fall forced by the glacio-isostatic rebound and deformed by subsequent ice margin advance. Conversely, marine and glaciomarine deposits resting on top of these deformed beds are interpreted as being deposited by rapidly retreating ice margin in glacio-isostatic depressions.