Complex development of a 300-million-year old subglacial unconformity in southern Namibia

The expansion of ice masses across southern Africa during the Late Palaeozoic Ice Age (LPIA) at about 300 Ma has been recognised in the literature for over a century, including the distribution of upland areas in controlling the configuration of ice masses (Davis, 1908). In Namibia, increasing attention has focussed on long and deep palaeovalley networks in the north, but comparatively little work has been attempted in the topographically subdued plains of the south. The desert terrain of the Mariental area exposes diamictites of the Dwyka Formation discontinuously over about 300 km, extending further south to the Noordoewer 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 some preliminary results from fieldwork in austral winter 2019, in which we describe a highly varying basal contact that records the processes of growth, flow and expansion of ice masses across this part of Gondwana. Subglacially-produced unconformities may exhibit classic glacially-striated pavements at basin margins, which substitute for soft-sediment striated surfaces in comparatively more “basinal” areas. Where these features are absent, additional criteria may be sought. In Mariental, spectacular soft-sediment shear zones exhibit a combination of brittle and ductile end products are recognised, overprinted by shear bands. 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” formed in which in single clast-thick boulder-dominated intervals 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, which is suspected to have been dominated by Piedmont glaciers.