Sourcing river rock and Middle Stone Age artifacts discovered along the Cunene River, Angola-Namibia border [Prospectando rochas fluviais e artefatos do Paleolítico Médio Africano descobertos ao longo do Rio Cunene, fronteira entre Angola e Namíbia]

Abstract

Although many important prehistoric sites are known from South Africa, few comparable contexts have been discovered and documented in the northern Namibia and southern Angola borderlands. During a geomorphic assessment of riparian corridors in Namibia’s Kaokoveld region, Middle Stone Age (MSA in Africa; broadly correlative with European Paleolithic) lithic artifacts were found preserved in unstratified plein air sites located atop a terrace adjacent to the perennial Cunene River. These remains attest to hominin activities along the northern edge of the modern hyperarid Namib Desert, which receives less than 100 mm of rainfall in a year. The location of the archaeological site is quite remote, and is along the eastern perimeter of the hyperarid Cunene erg (sandsea), and downstream of the Marienfluss–Hartmann Valley near Serra Cafema, At the Cafema site (as it is known), more than 30 lithic artifacts are preserved in the Cunene River valley, in context of a former river terrace. The artifacts discovered by a walking survey include quartzite flakes, cores, and points with some edge abrasion and varnish, including the first Levallois-Mousterian points found in this region of Africa. Since the archaeology of this area is poorly known, these cultural assemblages enable initial correlations across the continent, and provide a basis for reconstructing provenience, procurement and tool manufacture during the Middle Pleistocene, the time frame marked by the first appearance and the dispersal of the modern human species Homo sapiens.

To assess the potential source areas for lithic raw materials in this region, pebble counting methods and compositional assessment were conducted on the artifacts and Quaternary alluvium (Qal) of the relict river terrace at Cafema. Based on the composition of the artifacts found on the surface, siliceous rock was an important resource for lithic manufacture during the MSA; this is reflected in the observation that quartzite was overwhelmingly the dominant material used in the stone tool manufacture. Quartzite is generally known to be a preferred material for making tools of consistent size, ease of knapping, quality of form-shape, and persistence of edge retention.

To define regional procurement areas where hominin may have acquired quartzite raw materials at local (0-5 km), regional (6-20 km) and supra-regional (21-100 km) scales, we examined the geological outcrops in the region to identify potential sources. Due to its remoteness, the geology of this region of southern Africa is not well known, and the available maps are only available at coarse scales of resolution. Geologic outcrops along the Cunene River include some of the world’s oldest rocks dating to the Vaalian ~1760 Ma, and comprise part of a Large Igneous Province (LIP) that stretches across the African continent. The local bedrock outcrops near Cafema include a medium-to-high grade metamorphic complex, granitoids, and surrounding country rock -- these rocks comprise the rugged mountainous terrain incised by the Cunene River.

Since quartzite is the dominant clast type occurring as raw material in the river terrace where the MSA artifacts themselves were found, we can conclude that river terrace materials (Qal) themselves were among the likely raw material sources exploited during antiquity. Rounded quartzite boulders and cobbles are present as surface lag, along with the artifacts in the Qal within the relict river terrace at the plein air site. The river terrace itself is the closest source to the observed artifacts.  If the raw material source was alluvial (i.e., within the Qal unit), quartzite river cobbles may have been derived from outcrops located further upstream the Cunene system, which is a large river network that drains a diversity of geologic units. Although it is not possible to identify the precise formation and procurement area of origin, we offer some relative assessments about likely source areas within the region, based on the geology. The specific geologic units that may have contributed quartzite clasts to the Qal river terrace include the (1) Damara Sequence (Nda); and (2) lithologies within the undifferentiated Mokolian unit, which are not well mapped in detail. The closest potential primary sources of quartzite raw materials in Nda rock outcrops (i.e., not alluvium within the Qal terrace at the site) are located within 2 km of the Cafema site. However, confirmed source locales in the past could not be specifically identified in the field.

The hypothesis offered is that the Qal alluvial components within the relict terrace of the Cunene River was a preferred source for quartzite lithic raw materials used by mobile hunter-gatherers to make tools during the MSA, sometime after ~225 kya. Cafema is the first MSA site in northern Namibia that is in direct stratigraphic context with a securely dated unit. A replicate OSL-SAR date ~220 kyr has provided initial age constraints on a sandy unit preserved within the cobble-boulder Qal terrace fill, and constrains the maximum age for the overlying archaeological assemblage. These findings advance the reconstruction of this cultural landscape through a geoarchaeological lens, and form a basis for understanding the relict Pleistocene landscape and environment, its plant resources, and proximity to raw material sources within the riparian corridor of the perennial Cunene River.