The Late Palaeozoic Ice Age unconformity in southern Namibia viewed as a patchwork mosaic

David A. Osleger,Daniel P. Le Heron,Neil P. Griffis,Isabel P. Montañez,Christoph Kettler,Axel Hofmann,Guilhem Douillet,Roland Mundil,Pierre Dietrich

doi:10.1002/dep2.163

Abstract

AbstractThe 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’ recognised 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.

Highlights

The Dwyka Group of southern Africa preserves a world-class archive of the Late Palaeozoic Ice Age (LPIA), whose glacial record has enjoyed a rich history of research for more than a century (Sutherland, 1870; Lomas et al, 1905)
The motivation of this study is to reveal the processes at work at the icebed interface to resolve the style and dynamics of LPIA in this region of Gondwana
Reappraisal of the basal Dwyka surface at several locations at the margins of the Aranos and Karasberg basins in Namibia reveal that a complex suite of processes was responsible for the generation of the basal unconformity

Summary

Introduction

In spite of the rich tradition of investigation, the size of the main Karoo Basin of South Africa and neighbouring basins to the north and across southern and central Africa (Catuneanu et al, 2005), means that large regions still remain under-investigated, and many new insights remain possible through ongoing field investigations. In the Aranos Basin, the Gibeon Formation, representing the basal Dwyka Group and the lowermost deglacial sequence, dips gently westward In this region, several localities expose the basal contact with the Dwyka, including the so-called Airport Canyon south of Mariental, a set of buttes, sections at Gibeon, outcrops along the Fish River and north of Keetmanshoop (Figure 1B). Basal facies exposed at the buttes locality is interpreted to record an unconformity cut through fluvial incision This interpretation is supported by (i) trough cross-bedded sandstones (Figure 4C) which form the bulk of the Dwyka Group interpreted as fluvial deposits, (ii) multi-metre scale downcutting and a geometric relationship between the morphology of the incision and overlying trough cross-bedded strata, together with (iii) the organisation of individual sandstone buttes into a sinuous morphology at the landscape scale. Thereafter, given the thickness of the succession above in the Owl Gorge region 1 km to the north, it is likely that the succession records a transition to deeper water (Griffis et al, in press) during an initial deglaciation cycle

DISCUSSION

Conclusions