NW Botswana Research Articles

The Elusive Congo Craton Margin During Gondwana Breakup: Insights from Lithospheric Mantle Structure and Heat Flow beneath the Xaudum Kimberlite Province, NW Botswana

Abstract The continental lithospheric mantle (CLM) beneath the southern margin of the Congo craton has remained elusive, mainly because of the thick Phanerozoic sedimentary cover concealing possible kimberlite and lamproite diatremes. In this study, we explore this lithospheric mantle section using major and trace element compositions of mantle-derived clinopyroxene and garnet xenocrysts from kimberlites of the ca. 84 Ma Nxau Nxau cluster in northwest (NW) Botswana, which is part of the poorly known Xaudum kimberlite province extending into northern Namibia. We utilize these data to better understand the thermal and compositional evolution of the lithospheric mantle at the southern margin of the Congo craton. The clinopyroxene population (83 individual grains) comprises Cr-rich and Cr-poor diopsides with variable major (Al2O3, Na2O, Mg#) and incompatible trace element (U, Th, Zr, Hf, Nb, Ta, REEs) compositions. The large garnet population studied (496 individual grains) is dominated by lherzolitic G9 (38%) and ‘megacrystic’ G1 (41%) compositions, with minor contributions from Ti-metasomatized G11 (7%) and eclogitic G3 (6%) cratonic mantle sources. Harzburgitic G10 garnet is very rare (two grains only), consistent with a lherzolite-dominated CLM section in a craton margin position. The eclogitic garnet population has compositions similar to garnet from high-Mg cratonic mantle eclogite xenoliths, and such compositions have recently been interpreted as metasomatic in origin within the mantle xenoliths literature. Pressure–temperature calculations using the single-grain clinopyroxene technique reveal a relatively cold cratonic geotherm of 37–38 mW/m2 for the study region during the Late Mesozoic. For peridotitic garnets, projections of calculated Ni-in-garnet temperatures onto the independently constrained regional conductive geotherm suggest that lherzolite dominates at <145 km depth, whereas high-Ti lherzolitic G11 garnets and ‘megacrystic’ G1 garnets originate mostly from greater depths, reaching down to the lithosphere base at 150 to 210 km depth. The apparent confinement of ‘megacrystic’ G1 garnet to the bottom of the lithosphere suggests formation from infiltrating asthenosphere-derived proto-kimberlite liquids during melt–rock interactions. In general, the data suggest that the CLM beneath NW Botswana is depleted to about 145 km depth, and between 145 and 210 km depth, a thick metasomatized layer is identified, representing the transition into the underlying asthenosphere. A relatively thin lithosphere beneath NW Botswana is consistent with the proposed craton margin setting, especially when compared with the thicker cratonic roots beneath the central regions of the Congo and Kalahari cratons in Angola and South Africa, respectively, reaching down to 250 km depth and possibly even deeper. The compositional dissimilarity between the deepest-derived garnets from kimberlites in NW Botswana (i.e. from the diamond stability field) and garnets that occur as inclusions in diamond from cratons worldwide suggests extensive overprinting of the lowermost cratonic lithosphere by oxidative melt-related metasomatism. This finding, together with the very low diamond grades of the Xaudum kimberlites, points to a diminished diamond potential of the large and mostly unexposed ‘cratonic’ region (e.g. covered by thick desert sand) located between the major diamond mining districts of the Congo craton to the north (e.g. Catoca) and the Kalahari craton to the south (e.g. Orapa and Jwaneng).

Late Quaternary palaeoenvironmental reconstruction of a Chobe Enclave landform (NW Botswana) based on pedological and geochemical indices

Late Quaternary palaeoenvironmental reconstruction of a Chobe Enclave landform (NW Botswana) based on pedological and geochemical indices

Spatial and temporal controls on the solute behavior of rivers in arid watersheds: The Okavango River, NW Botswana

Solute cycling models for rivers in arid watersheds must identify solute sources and their spatial and temporal connectivity to rivers to assess the processes that control the solute behavior in these rivers. In this study, we collected hourly time series of total dissolved ions (TDI) concentrations, water levels, water temperatures and air temperatures at select stations distributed longitudinally in a river in an arid watershed. Our objectives were to: (1) document the spatial and temporal variations in solute concentrations and (2) determine the processes controlling the spatial and temporal variations in solute concentrations. We conducted this investigation over a two-year period at four stations along the Okavango River flowing through the Okavango Delta (Delta) in the middle Kalahari Desert, Botswana. At the spatial scale, we observed progressive downriver enrichment in TDI concentrations associated with evapotranspiration of river water because of the 5 months river transit across the hot Delta. At a temporal scale, the TDI concentrations increased seasonally from evapoconcentration and sub-seasonally from solute transfer from the watershed to the river. We analyzed sub-seasonal TDI concentration perturbations using the slope (β) of the relationship between TDI concentrations and normalized water level (C-NWL) relationships. We find that at low discharge during flow recession and at the beginning of rising discharge from flooding, the solute enrichment anomalies with β > 0 are related to hydrologic connectivity between the river and solute stores in the river floodplains, hundreds of thousands of salt islands and isolated evapoconcentrated wetland pools. Our findings indicate that hydrologically-driven river connectivity to solute stores in the local watershed and evapotranspiration jointly control the solute behavior at variable spatial and temporal scales in rivers in arid watersheds. We anticipate that our findings will inform solute transport and solute cycling models for rivers in arid watersheds.

Understanding High-grade Mineralization in the Khoemacau Zone 5 Cu-Ag Deposit, Kalahari Copperbelt, NW Botswana, using μXRF Techniques

Understanding High-grade Mineralization in the Khoemacau Zone 5 Cu-Ag Deposit, Kalahari Copperbelt, NW Botswana, using μXRF Techniques

The Elusive Southern Margin of the Congo Craton During Gondwana Breakup: Insights from Lithospheric Mantle Structure and Heat-Flow Beneath the 85 Ma Xaudum Kimberlite Province in Nw Botswana

The Elusive Southern Margin of the Congo Craton During Gondwana Breakup: Insights from Lithospheric Mantle Structure and Heat-Flow Beneath the 85 Ma Xaudum Kimberlite Province in Nw Botswana

Geochemistry of soils of a deep pedon in the Okavango Delta, NW Botswana: Implications for pedogenesis in semi-arid regions

Pedogenic parameters and data on geochemical compositions were analysed to improve our understanding of pedogenesis and soil morphology in an ecologically and geologically unique and relatively quiescent tectonic setting of the Okavango Delta, northern Botswana. A complex interaction of pedogenic and gradational processes in the area resulted in the formation of a deep (~4 m) pedon with three distinct morphologies: a weakly structured surface horizon, modern pedogenic carbonate enriched B horizons at stage III development and slightly pedogenically altered sediment C horizons. The pH ranged from 8.1 to 10.0 implying alkaline soil environment, while electrical conductivity ranged from 2 to 55 dS/cm. Gleying, an evidence hydromorphism, was present in the wetting front (C horizon). Results of the total elemental composition show the dominance of silica followed by CaO, Al2O3, MgO and Fe2O3 as the major oxides. Trace elements had lower HFSE (e.g. Zr and Nb) than LILE (e.g. Ba, Sr, Rb, Cr and V) contents. Calcite, quartz and opal-CT are the minerals present in the fine silt and clay fractions of the soils. Geochemical indicators points to incipient weathering and pedogenesis. Evolutionary accumulation of pedogenic carbonate in the diagnostic horizons bestows Calcisols as the Reference Soil Group and not Arenosols as previously known. Other pedogenic processes including leaching losses, redox processes and salt accumulation were strongly influenced by the saline groundwater processes. By comparing these site-specific findings with examples from other settings, this study has demonstrated the applicability of morphological, geochemical and mineralogical properties of soils in assessing pedogenic processes and classification in deep calcic soils associated with alluvial and aeolian depositions.

Lithostratigraphic classification of the Tsodilo Hills Group: a Palaeo- to Mesoproterozoic metasedimentary succession in NW Botswana

The Tsodilo Hills Group strata exposed in the Tsodilo Hills are an association of meta-arenites, meta-conglomerates, quartz-mica schists, sandstone, red siltstone and sedimentary breccia deposited on an open siliciclastic marine shelf between the Late Palaeoproterozic and Late Mesoproterozoic, and outcropping in NW Botswana. The succession is dominated by three micaceous quartzite units interlayered with subordinate lenses and wedges of other rock types. Facies gradients from S to N are expressed by decreasing content of muscovite at all levels of metasediment organisation from thin wedge-shaped units to thick quartzite complexes, as well as a decrease in pebble content and increase in the arenaceous matrix in some metal conglomerate beds, matching regional palaeotransport direction. Well-rounded pebbles of rocks are flat, suggesting redeposition from a beach environment. Lenticular conglomeratic bodies with erosional lower boundaries represent infills of local incisions in the sandy bottom sediments. The abundance of laterally discontinuous lithological units reflects shelf palaeotopography controlled and modified by deposition and migration of large bed forms, ranging from megaripple marks (or submarine dunes) to sand waves. Deposition was influenced by tides and two regressive events. The older regression resulted in a marker unit of tidal mudflat-related red-bed facies: mudstone, siltstone, channel-fill sandstone and sedimentary breccia. The second regression is indicated by a tabular conglomerate marker reflect-ing increased input of coarse terrigenous material.

Biogenic corrosion caused by bats in Drotsky's Cave (the Gcwihaba Hills, NW Botswana)

Located in north-west Botswana, Drotsky's Cave is a vast cavity hosted within dolomite known for the discovery of archaeological strata dating back to the Late Stone Age. It is home to a colony of bats comprising several ten thousands of individuals who are responsible for the accumulation of a thick deposit of guano, on which we have conducted research in some of the deeper parts of the cavity.A detailed study of the morphologies of the dolomite walls and roofs demonstrates that part of the cave's speleogenesis is probably due to biogenic corrosion. This type of weathering and corrosion process is still poorly described in underground environments. However, it is now accepted that the role played by bats and their droppings on the condition of walls, their weathering, and the enlargement of galleries and chambers by condensation-corrosion processes, should be taken into account when considering the way deep networks evolve. In this article, we discuss the mechanisms of this biogenic corrosion in Drotsky's Cave, as well as their morphogenic impacts (the dissolution and recession of walls, the formation of lateral notches, arches, bell holes, and ceiling spherical cupolas), and the consequences for the wall retreat of conduits. Finally, we propose a new typological classification.The consequences of this biogenic corrosion are important for archaeological questions related to the taphonomy of archaeological remains in the buried deposits and the conservation of cave art. Because of the important impacts of guano on buried biological archaeological remains, such a population would have encouraged accelerated weathering, corrosion and dissolution of the walls, potentially contributing to the total, or partial, disappearance of any trace of human activity on the surrounding rocks.

OSL dating of a carbonate island in the Chobe Enclave, NW Botswana

Abstract Carbonate platform islands are important landscape features in the northeastern part of the Okavango Delta region of Botswana, the Chobe Enclave. However, the formation processes and the timing of these “Chobe Islands” remain unclear. They are assumed to be the result of late Quaternary hydrological changes. Records of such changes are poorly preserved, though the occurrence of beach ridges in the Middle Kalahari Basin attests to the existence of large paleo-lakes in the past. Carbonate rocks from the Chobe Island appear to be relics of palustrine environments, but their relationship with the other hydrological archives is still unclear. Here, we report optically stimulated luminescence dating of key sedimentary beds in and around a single Chobe Island. It was necessary to model dose rate evolution for each sample individually, taking into account post-depositional changes in the sediment chemistry and its burial depth. The resulting ages suggest that the dated units were deposited between MIS6 to MIS1. The carbonate platform itself appears to have been deposited in two phases, separated by either an unexpectedly long (∼40 ka) depositional hiatus or an episode of erosion. This study demonstrates the potential of using luminescence dating in such settings, and offers the possibility of linking sedimentary processes within the Chobe Enclave to regional paleo-hydrological records.

Effect of evapotranspiration on dissolved inorganic carbon and stable carbon isotopic evolution in rivers in semi-arid climates: The Okavango Delta in North West Botswana

Abstract Study region Okavango Delta, Middle Kalahari, NW Botswana. Study focus We investigated the effect of evapotranspiration on the evolution of dissolved inorganic carbon (DIC) and stable carbon isotopes of DIC (δ 13 C DIC ) in the Okavango River. We measured the DIC concentrations and the δ 13 C DIC for samples collected over a 400 km reach of the river in the Okavango Delta during flood conditions and non-flood conditions. In addition, we incubated river samples collected from the proximal portion (Mohembo) and the distal portion (Maun) of the Delta and subsequently evaporated the samples by ∼90% under ambient conditions. New hydrological insights We found a 379% and 500% increase in the DIC concentrations and a δ 13 C DIC increase of 3.9‰ and 6.1‰ in the river during the flood non-flood conditions, respectively. The DIC concentrations of evaporated river samples increased by 535% for the Mohembo and by 850% for the Maun samples. The increase in the δ 13 C DIC of the evaporated river samples resulted from CO 2(g) loss during chemical equilibrium with atmospheric CO 2(g) followed by carbon exchange between DIC and atmospheric CO 2(g) . Although the δ 13 C DIC increased spatially for the Okavango River, it never reached the value of ∼0‰ expected for equilibration of river DIC with atmospheric CO 2(g) . The results of the evaporated river samples suggest that isotopic enrichment from equilibration in Okavango River was balanced by respiration and photo-oxidation of carbon-depleted dissolved organic matter.

DC resistivity and seismic refraction survey across the SE margin of Lake Ngami, NW Botswana

Seismic refraction survey and DC resistivity measurements were made across the margin of the Lake Ngami. The structure and stratigraphy at the lake were determined. High resolution aeromagnetic data showed a prominent anomaly coinciding with the Kunyere Fault. Estimated depths to magnetic sources are increasing towards the lake. Two velocity layers were mapped. The top layer (500 m/s) is thin outside the lake and thicker inside the lake. The underlying layer (3125 m/s) has undeterminable thickness. Resistivity sounding results inside the lake showed that the low velocity layer has four sub-units: dry hard clays; diatomaceous earth; soft clays interlayered with silts; and wet whitish clays interlayered with silts. Normal faults were mapped along the profile with a total displacement up to 50 m. The results of the study indicate that the formation of the Lake Ngami basin was structurally controlled and probably initiated by the tectonics of the Okavango Rift Zone.

Pleistocene to Recent rejuvenation of the Hebron Fault, SW Namibia

Abstract The Hebron Fault in SW Namibia is associated with a <1 m to 9.6 m high scarp displacing Proterozoic basement and Middle to Late Pliocene crystalline conglomerates. The young age of strata exposed in the fault scarp together with evidence for displacement of aeolian dunes, post-dating the Middle Stone Age, suggests that latest fault displacements occurred during the Late Pleistocene to recent. Recorded historical seismic events show that the fault zone is still active. Latest movements of the fault are recorded by: down-to-the-SW offset of calcrete-cemented conglomerate; fluvially modified, asymmetric hanging wall, graben-like structures; at least two left-stepping jogs in the fault trace and structural data from basement rocks in which late-stage crush zones overprint earlier cataclasite. These features provide consistent evidence that the present scarp formed predominantly by normal dip-slip displacement on a NW-striking, steeply SW-dipping master fault with only a minor dextral strike-slip component. Strongly veined cataclastic fault rocks adjacent to the scarp in basement most probably originated at depths of 4–10 km. The conclusion is therefore that recent fault activity has reactivated a pre-existing, much older fault. Aerial photographic lineaments and similar fault scarps identified NW and SE of the present study area are interpreted as extensions of the same fault structure. Hence the total length of the Hebron Fault is at least 300 km subparallel to the Atlantic margin of southern Africa. Our observations confirm that the Hebron Fault is a neotectonic feature of regional significance that may relate to late Cenozoic and particularly Quaternary neotectonic activity in NE Namibia and NW Botswana.

Deltamethrin in sediment samples of the Okavango Delta, Botswana

Deltamethrin concentrations were determined in 35 sediment samples collected from three different habitats: channel, lagoon and pool sites from Xakanaxa in the Okavango Delta, NW Botswana. The samples were Soxhlet-extracted in acetone to extract deltamethrin residues and subsequently cleaned-up with silica gel 60. The final determination was carried out with a gas chromatograph equipped with an electron capture detector (GC-ECD). The sample work-up and determination gave deltamethrin recoveries of 54 to 97%, and detection limits of 0.004 mg/kg dw. The concentration of deltamethrin residues in the sediment samples collected from the three sprayed areas in the Okavango delta ranged between 0.013 and 0.291 mg/kg dw, with the highest concentrations observed in samples obtained from the pool sites. Analysis of samples for organic matter content showed percentage total organic carbon (% TOC) ranging between 0.19% and 8.21%, with samples collected from the pool having the highest total organic carbon. The concentrations of deltamethrin residues and the % TOC in sediment samples showed a similar trend with the highest levels recorded in the pool samples. These data confirmed that a simple method based on GC-ECD, after Soxhlet extraction, was robust enough to enable quantification of deltamethrin in the sediments, because comparable results were obtained with a more sophisticated system consisting of a GC coupled to a mass spectrometer with a time of flight (TOF) analyser. Water SA Vol.32 (4) 2006: pp.483-488

Early structural development of the Okavango rift zone, NW Botswana

Aeromagnetic and gravity data collected across the Okavango rift zone, northwest Botswana are used to map the distribution of faults, provide insights into the two-dimensional shallow subsurface geometry of the rift, and evaluate models for basin formation as well as the role of pre-existing basement fabric on the development of this nascent continental rift. The structural fabric (fold axes and foliation) of the Proterozoic basement terrane is clearly imaged on both gravity and magnetic maps. The strike of rift-related faults (030–050° in the north and 060–070° in the south) parallels fold axes and the prominent foliation directions of the basement rocks. These pre-existing fabrics and structures represent a significant strength anisotropy that controlled the orientation of younger brittle faults within the stress regime present during initiation of this rift. Northwest dipping faults consistently exhibit greater displacements than southeast dipping faults, suggesting a developing half-graben geometry for this rift zone. However, the absence of fully developed half-grabens along this rift zone suggests that the border fault system is not fully developed consistent with the infancy of rifting. Three en-echelon northeast trending depocenters coincide with structural grabens that define the Okavango rift zone. Along the southeastern boundary of the rift, developing border faults define a 50 km wide zone of subsidence within a larger 150 km wide zone of extension forming a rift-in-rift structure. We infer from this observation that the localization of strain resulting from extension is occurring mostly along the southeastern boundary where the border fault system is being initiated, underscoring the important role of border faults in accommodating strain even during this early stage of rift development. We conclude that incipient rift zones may provide critical insights into the development of rift basins during the earliest stages of continental rifting.

Shallow seismic refraction and magnetic studies at Lake Ngami, The Okavango Delta, Northwest Botswana

A seismic refraction analysis was carried out on data acquired on an 8.5 km profile at Lake Ngami, NW Botswana, to determine the structure and stratigraphy underlying the lake. The seismic spread comprised a 48-channel seismograph with a 9.5 m receiver and source spacing. The seismic source comprised 10 hammer blows on an aluminum plate, from which a vertical stack was recorded. The data were processed with WinSism™ version 10 program using the intercept time method. A total field magnetic survey was also carried out along the same line. The seismic refraction profile shows a low velocity layer (400–1600 m/s) extending from the surface at the southern end of the line to a maximum depth of 30 m in the middle of the profile, returning to a depth of 20 m from 4.5 km to the northern end of the line. The low velocity layer consists of the unconsolidated sediments and is underlain by more compact and saturated material with seismic velocities of 1600–3600 m/s. This material is interpreted to comprise semi- to fully consolidated sandstone. The base of these sandstones, however, could only be mapped near the southern end of the line out to 2 km. High seismic velocity rocks (3600–5000 m/s), interpreted to be weathered to fresh basement, were mapped below the sandstones at the southern end of the line. The magnetic profile shows a high at the beginning of the line which progressively declines to a low at 3 km. The field strength increases rapidly over the next 1.5 km to reach a plateau level at 4.5 km. The magnetic high at the beginning of the line corresponds to shallow basement which is mapped in the refraction profile. The magnetic low at 3 km is indicative of a thickened sediment section at this location. The southern end of the line with high seismic velocities comprises basement rocks located in the footwall block of the Kunyere Border Fault of the Okavango Rift. The step-wise downthrown hanging wall block consists of the low to intermediate velocity basin deposits in the northern part of the profile.

Characterization of arsenic occurrence in the water and sediments of the Okavango Delta, NW Botswana

Detailed chemical analyses were performed on surface water, groundwater and sediment samples collected from the Okavango Delta between February and November 2003 in order to examine the distribution and geochemistry of naturally occurring As in the area. Surface water in the Okavango Delta, which is neutral to slightly acidic and has high dissolved organic C (DOC), was found to be slightly enriched in As when compared to a global value for stream water. Of the 20 new borehole analyses from this project, six were found to have values exceeding 10 μg/L, the current World Health Organization provisional guideline value for As. The results from field speciation indicate that As(III) is slightly more predominant than As(V). There is a positive correlation between As and pH and between As and DOC in the groundwater samples. For the sediment samples, there is a positive correlation between As and Co, As and Fe, As and loss on ignition (LOI) and between As and the percent fines in the sample. Reductive dissolution of oxides and hydroxides in the sediments with organic C as an electron acceptor is the likely mechanism for the release of As from the sediments into the groundwater.

Use of the geochemical and biological sedimentary record in establishing palaeo-environments and climate change in the Lake Ngami basin, NW Botswana

Sediment samples from a continuous 4.6 m profile in the dry bed of Lake Ngami in NW Botswana were analysed for geochemistry and dated using both 14C and TL methods. Certain units in the profile were found to be diatom rich and these, with the geochemical results, were used as indicators of high and low lake levels within the basin. The Lake Ngami sediments contain a high proportion of SiO 2 (51–92.5 wt%, avg. 72.4 wt%) and variable levels of Al 2O 3 (2.04–17.2 wt%, avg. 8.88 wt%). Based on elevated Al 2O 3 and organic matter (LOI orgC ) results, lacustrine conditions occurred at ca. 42 ka until 40 ka and diatom results suggest that relatively deep but brackish conditions prevailed. At 40 ka, the lacustrine sedimentary record was terminated abruptly, possibly by tectonic activity. At ca. 19 ka, shallow, aerobic, turbulent conditions were prevalent, but lake levels were at this time increasing to deeper water conditions up until ca. 17 ka. This period coincides with the Late Glacial Maximum, a period of increased aridity in the central southern Africa region. Generally, increasing Sr/Ca ratios and decreasing LOI orgC and Al 2O 3, from ca. 16 to 5 ka, suggest decreasing inflow into the basin and declining lake levels. Based on the enrichment of LREE results, slightly alkaline conditions prevailed at ca. 12 ka. Diatom results also support shallow alkaline conditions around this time. These lake conditions were maintained primarily by local rainfall input as the region experienced a warmer, wetter phase between 16 and 11 ka. Lake levels rose rapidly by 4 ka, probably in response to enhanced rainfall in the Angolan catchment. These results indicate that lake levels in the Lake Ngami basin are responding to rainfall changes in the Angolan catchment area and local rainfall. The results confirm that the present-day anti-phase rainfall relationship between southern Africa and regions of equatorial Africa was extant during the late Quaternary over the Angolan highlands and NW Botswana.

Cryptic indicators of provenance from the geochemistry of the Okavango Delta sediments, Botswana

The siliciclastic sediments of the Okavango inland Delta of northwest Botswana have a modal composition of quartz arenites and result from a complex history, including transport by river and deposition in a nascent rift basin located in a desert environment with input of aeolian sands. The geochemical composition of sediments from the Okavango Delta was determined in order to constrain the role of weathering at the source and the composition of the source rocks. The chemical analyses and the interelement ratios show a broad compositional range usually encompassing the PAAS composition. The chemical index of alteration (CIA) values and the A–CN–K diagram define an evolution trend which can be interpreted using a mixing model involving a strongly weathered component which corresponds to the sedimentary fraction transported by the Okavango River and a relatively immature component which corresponds to the aeolian sand component of the Okavango sediments. Field geological data supported by geochemical ratios involving elements with affinity for mafic–ultramafic and felsic rocks such as Th/Cr, Th/Sc, La/Sc, La/Co and Eu/Eu* support a source area including mafic–ultramafic and felsic rocks, with or without intermediate rocks. The relationships between certain elements (Cr–Ni, Na 2O–Al 2O 3, K 2O–Al 2O 3) refine the interpretation by pointing to the existence of at least three source rock end-members, including a felsic rock source and pyroxene-rich and olivine-rich mafic–ultramafic source rocks. Proterozoic granitoid–gabbro and related volcanic and ortho-metamorphic rock complexes exposed in NW Botswana and adjacent Angola and Namibia are the source rocks of the sediment component which was mixed with aeolian sand and interacted with a variable proportion of diagenetic carbonates to produce the Okavango sediments.