Kalahari Group Sediments Research Articles

The Kalahari sediments and hominins in southern Africa

The temporal coupling of the structural evolution of the Kalahari Basin and the accumulation of the Kalahari Group sediments has been an accepted paradigm leading to the assumption that the Kalahari group sediments have been accumulating gradually since the mid-Cretaceous. Here we review the first actual ages for the Kalahari Group based on cosmogenic ages from six geological localities. These results demonstrate that Kalahari Basin infill was a more dynamic process than previously thought and that the Kalahari Group sediments are mostly Plio-Pleistocene in age (∼4 Ma to 1–2 Ma). The hiatus between the initial structural subsidence of the basin, during the Cretaceous, and the general young age of the investigated sediments, implies a dynamic landscape in which significant phases of erosion occurred during the Mesozoic and Cenozoic. The magnitude of erosion is manifested by the fact that in many locations Kalahari Neogene to Quaternary sediments overlie Precambrian basement.The age of the present infill of the Kalahari thus falls within the temporal range of the genus Homo. In light of this new understanding, we provide a review of the archaeological evidence from the Kalahari Basin and along its southern fringe. Initial hominin presence is found at Wonderwerk Cave during the Olduvai Event and there is subsequent high-density occupation along the southern fringe of the Kalahari Basin during the Acheulean and the Fauresmith. Middle Stone Age occupation is limited to localities of limited size and small artifact counts and it appears that the focus of human occupation, particularly in the later stages of the Middle Stone Age, shifts southward, including along the coastal regions.

Eolian indicator mineral dispersion haloes from the Orapa kimberlite cluster, Botswana

This paper presents the results of an investigation into the structure of eolian kimberlite indicator minerals (KIMs) haloes present within Quaternary Kalahari Group sediments (up to 20 m thick) overlying the Late Cretaceous kimberlites in the Orapa field in North-East Botswana. A database of more than 8000 samples shows that kimberlites create a general mineralogical blanket of KIMs of various distances of transportation from primary sources in the Orapa area. Models of the reflection and dispersion patterns of KIMs derived from kimberlite pipes including AK10/ AK22/AK23 have been revealed based on 200 selected heavy mineral samples collected during diamond prospecting activities in Botswana from 2014 to 2017. Short distance eolian haloes situated close to kimberlite bodies cover gentle slopes within plains up to 500 × 1000 m in size. They have regularly have oval or conical shapes and are characterized by the presence mainly of unabraded or only slightly abraded KIMs. A sharp reduction of their concentration from hundreds and thousands of grains / 20 l immediately above kimberlites toto 10 grains/20 l at a distance of only 100–200 m from the pipes is a standard feature of these haloes. The variation of concentration, morphology and abrasion of specific KIMs with increasing distance from the primary sources has been investigated and presented herein. Sample volumes recommended for pipes present within a similar setting as those studied, with different depth of sedimentary cover are as follows: up to 10–20 m cover at 20–50 l, 20–30 m cover at 50–100 l and 30–80 m cover at 250 l. It is important to appreciate that the discovery of even single grains of unabraded or slightly abraded KIMs in eolian haloes are of high prospecting significance in this area. The results of the research can be applied to in diamond prospecting programs in various regions with similar environments.

Provenance and depositional environments of Quaternary sediments in the southern Kalahari Basin

The vast Kalahari Basin is characterized by uplifted margins, terrestrial sedimentation within semi endorheic sub-basins, subdued morphology and tectonic quiescence. This intracratonic basin has been subjected to a prolonged period of subsidence affecting its sedimentary fill by plate motion and climatic cycles. Provenance studies of Kalahari Group sediments mainly focused on the surficial deposits that represent only the last phase of sedimentation, leaving unresolved questions related to the rest of the strata.The southern Kalahari Group succession exposed along the walls of the Mamatwan Mine, Northern Cape, South Africa, reveals three main depositional environments: a bottom low-energy water-body, a middle fluvial, high-energy environment and an upper aeolian sandy unit. The entire section, which was deposited during the early to middle Quaternary, records significant environmental and depositional changes suggesting a highly dynamic landscape.The fully exposed section (55m) of the Kalahari Group at Mamatwan Mine was analysed for its mineralogy, elemental composition, Sr, Nd and Pb isotopic ratios and iron species. Isotopic fingerprints were used to determine the source rocks of the sediments. The basal part of the section was derived from the distant Angolan highlands supporting previous suggestions for the existence of a paleo, trans-Kalahari drainage system. Weathering sensitive indices show that these sediments underwent considerable chemical weathering typical to humid areas. Mineralogical assemblage and iron phases imply that a brackish and alkaline shallow water-body existed during the early-middle Pleistocene contemporaneous with relative dense hominin occupation of the area.During the Early-Middle Pleistocene transition, the lacustrine environment was rapidly filled with volcano-sedimentary Archean clasts that were derived mainly from the surrounding hills and experienced limited degree of source rock chemical weathering, but underwent subsequent groundwater alteration by iron, silica, and calcium-rich solutions and precipitation of calcrete and silcrete duricrusts. These changes in provenance and depositional environments may be related to tectonic adjustments along the southern margins of the Kalahari basin. Ultimately, a shift into an aeolian-transport dominated landscape occurred in the region. This was accompanied by reduced surface water availability and a supply of sediments from Paleoproterozoic western and north-western source areas following the establishment of the modern wind regime.

New chronology for the southern Kalahari Group sediments with implications for sediment-cycle dynamics and early hominin occupation

Kalahari Group sediments accumulated in the Kalahari basin, which started forming during the breakup of Gondwana in the early Cretaceous. These sediments cover an extensive part of southern Africa and form a low-relief landscape. Current models assume that the Kalahari Group accumulated throughout the entire Cenozoic. However, chronology has been restricted to early–middle Cenozoic biostratigraphic correlations and to OSL dating of only the past ~300ka. We present a new chronological framework that reveals a dynamic nature of sedimentation in the southern Kalahari. Cosmogenic burial ages obtained from a 55m section of Kalahari Group sediments from the Mamatwan Mine, southern Kalahari, indicate that the majority of deposition at this location occurred rapidly at 1–1.2Ma. This Pleistocene sequence overlies the Archaean basement, forming a significant hiatus that permits the possibility of many Phanerozoic cycles of deposition and erosion no longer preserved in the sedimentary record. Our data also establish the existence of a shallow early–middle Pleistocene water body that persisted for >450ka prior to this rapid period of deposition. Evidence from neighboring archeological excavations in southern Africa suggests an association of high-density hominin occupation with this water body.

Origins and palaeo-environments of Kalahari duricrusts in the Moshaweng dry valleys (Botswana) as detected by major and trace element composition

Geo-chemical analyses of duricrusts from the margins of the Moshaweng dry valleys (Kalahari, central Botswana) was undertaken mainly to provide a diagnostic basis for palaeo-environmental conditions and to help stimulate the development of a worldwide duricrusts database. The Moshaweng duricrusts fall into three main petrological groups: calcrete, intergrade cal-silcrete and quartz pebble conglomerate and two distinct geochemical groups for elements involved in non-pedogenic calcrete and cal-silcrete formation, comprising mainly SiO 2, CaO, CO 2 and Sr. For most other elements, calcretes and silcretes define a single compositional variation trend indicating their genetic link. Results of this work show that the different behaviours of Rb and Ba (no anomaly) and Na 2O and CaO (important anomalies) have resulted from in situ chemical weathering likely at the aquifer scale. The source solutions for calcretes and calc-silcrete formation are also derived from the aquifer host clastic sedimentary rocks, namely the Mmamabula Formation (Karoo) and the derived Kalahari Group Sediments. Hence these sediments represent the main source of CaO and SiO 2 solutes that formed the Moshaweng duricrusts. Changes in geochemical conditions within the aquifer led to changes in pH such that recharge through infiltration led to Ce oxidation generating Ce-negative anomalies in percolating water implying initially low pH values (<7.5). During down drainage flow, groundwater alkalinity increased. In the confined part of the aquifer this was marked by pH > 8.5 during the dry season such that the groundwater was supersaturated in carbonates leading to the precipitation of calcretes. The resulting highly alkaline and saline water may have promoted silica dissolution engendering calc-silcrete formation during the subsequent wet season when a decrease of pH to ∼7–8 was triggered by the inflow of fresh water. On the basis of the hydrochemical constraints discussed in this paper, we show that calcretes precipitated at pH > 8.5 and cal-silcretes at pH < 8 likely under alternating wetter and drier environments. Wetter conditions have previously been dated as occurring at least 200,000 years ago and probably re-curred since then as indicated by the multiple calcrete–calc-silcrete sequences in the Moshaweng dry valleys. Drier conditions are implied by stable isotope results, which suggest that calcrete formation occurred when C4 plants were predominant.

The Mesozoic–Cenozoic interior sag basins of Central Africa: The Late-Cretaceous–Cenozoic Kalahari and Okavango basins

The Kalahari basin formed as a response to down-warp of the interior of the southern Africa, probably in the Late Cretaceous. The down-warp, along with possible uplift along epeirogenic axes, back-tilted rivers into the newly formed Kalahari basin and deposition of the Kalahari Group sediments began. Initial deposition of basal gravels occurred in the channels of the Cretaceous rivers, with other unsorted gravel beds deposited at the base of scree slopes along the edges of valleys and fault-bounded structures. The accumulation of gravels continued as the down-warp of the basin progressed with interbedding of the gravel layers with sand and finer sediment carried by the rivers. Thick clay beds accumulated in the lakes that formed as a result of the back-tilting of rivers, with sandstone being deposited in braided streams interfingering with the clays and covering them in some areas as the shallow lakes filled up with sediment. A period of relative tectonic stability during the mid-Miocene saw the silcretisation and calcretisation of older Kalahari Group lithologies, and this was followed in the Late Miocene by relatively minor uplift of the eastern side of southern Africa and along certain epeirogenic axes in the interior. More significant uplift that followed in the Pliocene along epeirogenic axes may have elevated Karoo Supergroup and basal Kalahari Group sedimentary rocks above the basin floor where they were exposed to erosion. The eroded sand was washed into the basin where it was reworked and redeposited by aeolian processes during drier periods, resulting in the extensive dune fields that are preserved today. Propagation of the East African Rift System into southern Africa has led to the formation of local depressions and the disruption of drainages, notably of the Kwando and Okavango Rivers. Lakes periodically occupied these depressions in the past. Large alluvial fans have developed in the depressions as a result of loss of flow confinement, and they currently support extensive wetlands (Okavango and Linyanti swamps).

Petrology, geochemistry and environmental significance of silcrete‐calcrete intergrade duricrusts at Kang Pan and Tswaane, central Kalahari, Botswana

AbstractCalcrete–silcrete intergrade duricrusts are an important component of the Kalahari Group sediments of central southern Africa and yet have neither been analysed systematically nor in any detail. In this study, the petrological and geochemical characteristics of suites of calcrete, silcrete and intergrade duricrusts from two fresh, relatively deep exposures at Kang Pan and Tswaane (adjacent to the Okwa Valley) in the central Kalahari, Botswana, are described. The duricrust profile at Kang Pan consists of a highly indurated crystalline non‐pedogenic calcrete which has been extensively silicified by chalcedony, and, rarely, cryptocrystalline silica or microquartz. Silicification is most extensive in lower parts of the profile, where replacement is related to groundwater fluctuations, and in upper sections due to periodic flooding by ephemeral surface water. The exposure at Tswaane consists of a sequence of pale green glauconitic non‐pedogenic silcrete and cal‐silcrete overlain by non‐pedogenic calcrete, all of which have formed within sediments situated upon granitoid‐gneiss bedrock. The siliceous duricrusts are dominated by cryptocrystalline silica cements and appear to have developed through the replacement of a pre‐existing non‐pedogenic calcrete. These siliceous duricrusts have also been calcified at a later date during the formation of the overlying calcrete to produce a complex range of silica–carbonate cements. At both sites, the style and type of silicification present appears to be determined by the duration of wetting and the permeability of the precursor calcrete. Geochemical evidence indicates a lack of chemical weathering profile development within the granitoid–gneiss bedrock and considerable differences between the chemical signature of the bedrock and combined duricrusts from Kang and Tswaane. This suggests that bedrock made a minimal contribution in terms of silica and carbonate species to duricrust formation, and that the majority of cementing agents were non‐local. It would therefore appear likely that the geomorphological context of each site had a major influence upon the development of calcrete, silcrete and intergrade duricrust cements. Copyright © 2004 John Wiley &amp; Sons, Ltd.

A multi-element baseline geochemical database from the western extension of the Central Africa Copperbelt in northwestern Zambia

A stream sediment geochemical survey was undertaken in the extreme north-west of Zambia with three primary objectives: (i) to provide a baseline environmental database for an area uncontaminated by heavy industry, mining and intensive farming; (ii) to detect metal anomalies that may indicate potentially economic mineral deposits; and (iii) to provide a tightly controlled geochemical database that can be used as a standard for future mineral exploration programmes throughout central Africa. Three major geological domains underlie the studied area. These are crystalline Neoarchaean to Palaeoproterozoic basement, an overlying sequence of Neoproterozoic Katanga strata within the western part of the Lufilian Arc (a Pan African orogenic belt that hosts major copper–cobalt deposits in the Central African Copperbelt of northern Zambia and the Democratic Republic of Congo [DRC]), and extensive Caenozoic Kalahari Group sediments. Stream sediments collected from each of the three geological domains have distinctive geochemical signatures. Relatively high values for a range of elements including uranium, thorium and yttrium were found in stream sediments within the pre-Katanga basement domain. Enhanced values of a larger number of elements are present in the stream sediments underlain by Katanga strata including significant, previously unknown, copper anomalies that warrant detailed follow-up work. The high metal values in sediments derived from Katanga strata contrasts sharply with sediment samples collected from drainage channels in areas of thick Kalahari sand, which mostly have very low values of all analysed elements. Our results suggest that it will be difficult to detect contamination of streams in the Copperbelt areas of Zambia and the Democratic Republic of Congo due to their naturally high metal concentrations. By contrast, it will be relatively easy to monitor pollution of areas underlain by Kalahari sands as these have naturally very low values of all elements.

ON THE EVOLUTION OF THE UPPER-MOLOPO DRAINAGE

ABSTRACT The evolution of the Molopo drainage can be explained both in terms of tectonics and climatic variations. The upper-Molopo expanded from the more westerly situated ancient proto-Molopo in a topographic low that developed concurrently with the deepening of the Kalahari Basin, in-between incipient forms of two intra-continental axes of uplift. A subsequent contraction of this maximum phase of drainage expansion is linked with the subsidence of the adjacent Bushveld Complex, reduced surface runoff through the exposure of the belt of Malmani Dolomites, and lowering of the regional groundwater table. There are also strong indications that the upper-Molopo drainage has responded to climatic variations. A steadily declining competence can be linked to a generally aridifying trend since the humid late Cretaceous. Superimposed on this trend the effects of pulses of more humid conditions can be recognised. A strong rejuvenation phase that resulted in a 20–30 m incision of the Molopo drainage into the Kalahari group sediments exemplifies such a major humid pulse. The present Molopo has no record of any permanent flow during living memory and has acquired all the characteristics of a fossil drainage.

Duricrust development and valley evolution: Process–landform links in the kalahari

AbstractDuricrusts are an important landscape component of the Kalahari region of central southern Africa. Their exposures within the dry valleys (mekgacha) of the Kalahari provide some of the most widespread surface outcrops of the terrestrial Jurassic to Holocene Kalahari Group sediments. Exposures have been extensively used in the construction of lithostrati‐graphic sequences, on the assumption that valley systems have incised their courses through a pre‐existing duricrust sequence. Recent work, however, has identified the role of groundwater erosion processes in valley development, which may have influenced duricrust formation.Studies of duricrusts from boreholes drilled within two mekgacha show that duricrust type is intrinsically related to the presence of a valley. Analyses of calcretes and silcretes in a series of profiles and thin sections from the Letlhakeng area of Botswana also indicate extensive alteration and diagenesis in association with former higher water tables. Sedimentary sequences within duricrust host materials can be identified but there is no evidence for correlation of duricrust cements between exposures. Profile studies from the Auob Valley in Namibia, however, suggest that this valley has incised through a sequence of duricrusts. Caution is advised in future attempts to correlate duricrust types on the basis of valley exposures, with the recommendation that where such exposures are used in a lithostratigraphic context, only duricrust host material characteristics and not cementing materials should be considered.

The evolution and characteristics of the Kalahari, southern Africa

The Kalahari is an ancient depositional environment with a complex history. The Mega Kalahari sand sea, at 2·5 million km 2 the largest on earth, is more extensive than the ill-defined Kalahari Desert and can best be considered as the surface expression of the Cretaceous to recent sediments in the Kalahari-Congo structural basin. These Kalahari Group sediments have accumulated and evolved through fluvio-deltaic, aeolian and groundwater processes, with characteristics due to primary deposition and subsequent modification being difficult to distinguish. Low fossil content, limited exposures and poor differentiation within the dominant and ubiquitous Kalahari Sand, together with the failure of scientists to comprehend the nature of the extensive duricrust suite, have delayed understanding the sedimentological and environmental history of the basin. However, despite the absence of a satisfactory stratigraphical framework, the post-Cretaceous tectonic history and drainage development provides a basis for subsequent evaluation of the palaeoenvironmental significance of the depositional landform suites found in the surface sediments of the Mega Kalahari.

The deposition and development of the Kalahari Group sediments, Central Southern Africa

Kalahari Group sediments cover 2.5 × 106 km2 of central southern Africa. This paper analyses their deposition and development by integrating geological and geomorphological evidence. The post-Gondwanaland tectonic evolution of the sub-continent, drainage development, climatic change and post depositional modification are all identified as important for the development of the Kalahari Group. It is concluded that local, regional and sub-continent wide influences have all played important roles. Consequently, caution should be implemented when attempting to correlate sedimentary units in different areas.