Windhoek, Namibia
Windhoek, Namibia

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Hofmann M.,Private Bag X607 | Linnemann U.,Senckenberg Naturhistorische Sammlungen Dresden | Hoffmann K.-H.,Geological Survey of Namibia | Germs G.,Private Bag X607 | And 6 more authors.
Precambrian Research | Year: 2015

The transition from supercontinent Rodinia to Gondwana took place in the Neoproterozoic. The western margin of the Kalahari Craton in southern Namibia underwent rifting at c. 750. Ma, caused by the break-up of Rodinia, followed by drift-events and ongoing sedimentation throughout the Cryogenian (at least from 750 to 630. Ma) in Namibia. These sediments comprise at least three different deposits of glacio-marine diamictites (Kaigas at c. 750-720. Ma, Sturtian at c. 716. Ma and Marinoan at c. 635. Ma). The Ediacaran is characterised by collision during the assembly of Gondwana and includes a fourth glacial deposit (post-Gaskiers Vingerbreek glaciation at c. 547. Ma). This study presents more than 1050 single zircon grain U-Pb analyses of different diamictite horizons from southern Namibia and discusses their correlation. For all samples from sediments related to the Kaigas, Sturtian and Marinoan glacial events, the youngest obtained zircon ages were at c. 1.0. Ga, making differentiation by the maximum age of sedimentation impossible. But a correlation was still possible by using the complete detrital zircon U-Pb age patterns, with a significant change in the relative abundance of concordant Mesoproterozoic to Paleoproterozoic zircons. This P/M ratio seems to be a good tool to distinguish the Cryogenian diamictites (Marinoan: P/M. <. 0.4, Sturtian: 0.4. <. P/M. <. 10, Kaigas: P/M. >. 10). Although all the observed ages from the detrital zircons can be explained by derivation of local material, none of our samples correspond to the Cryogenian rifting events in southern Namibia. Therefore the source area cannot be local and more probably is located in the east of the studied areas. The constancy of the main U-Pb ages suggests a constant sediment supply direction throughout the Cryogenian. The same age populations occurring in the Ediacaran Aar Member indicate the same sediment transport direction from the east, but with an increased proportion of zircon grains older than 2.2. Ga. This marks a transition to the unconformably overlying Vingerbreek (post-Gaskiers) diamictite horizons, which show a significant change in the age spectra. Probably due to mixed input from the east (Kalahari Craton) and from the west (Gariep Belt), the Vingerbreek diamictites show a wider range of zircon ages with youngest ages at c. 590. Ma. This time is characterised by collision events and the Gondwana formation. The Hf isotope record shows that the only input of juvenile material in our samples occurred in the Mesoproterozoic during the Namaqua Natal Orogeny (formation of the Namaqua Belt). In total, four Archaean to Proterozoic crustal growth events are recognised in the western part of the Kalahari Craton: (1) Meso- to Paleoarchean (c. 3.42-2.8. Ga), (2) lower Paleoproterozoic to Neoarchaean (c. 2.8-2.27. Ga), (3) lower to upper Paleoproterozoic (c. 2.27-1.7. Ga) and (3) Mesoproterozoic (c. 1.6-1.0. Ga). © 2014 Elsevier B.V.


Van Schijndel V.,Gothenburg University | Cornell D.H.,Gothenburg University | Hoffmann K.-H.,Geological Survey of Namibia | Frei D.,Stellenbosch University
Geological Society Special Publication | Year: 2011

The African continental crust was assembled by a series of orogenies over a period of billions of years mainly in Precambrian times. Tracing the build-up history of this stable crust is not always straightforward due to multiphase deformation and regions with poor outcrop. Episodes of metamorphism and magmatism associated with multiple Wilson cycles are recorded in zircons, which found their way into sediments derived from the hinterland. Dating of zircon populations in detrital rocks can hence provide age spectra which reflect the metamorphic and magmatic events of the region. Microbeam dating of detrital zircon is used to characterize the crustal development history of the Rehoboth Province of southern Africa. We investigated a quartzite of the Late Palaeo-Early Mesoproterozoic Billstein Formation, formed in a continental basin, and a quartz-feldspar arenite layer of the late Mesoproterozoic Langberg Formation conglomerates, immature sediments formed within a felsic volcanic system (both close to Rehoboth Town). The combined data indicate three episodes of crustal evolution in the Rehoboth Province. The oldest phase is only documented in the Billstein quartzite by three 2.98-2.7 Ga Archaean zircons. A Palaeoproterozoic phase between 2.2 and 1.9 Ga is older than any known exposures of the Rehoboth Province. The Billstein quartzite shows a main peak at 1.87 Ga, corresponding to the 1863±10 Ma Elim Formation. The Langberg sample reflects magmatism related to the entire Namaqua-Natal Wilson cycle between c. 1.32 and 1.05 Ga. The absence of zircons of that age range in the Billstein quartzite indicates a pre-Namaqua age for the Billstein Formation. Our data shows that there were at least three episodes of crustal development at 2.98-2.7 Ga, 2.05-1.75 and 1.32-1.1 Ga. We have documented the existence of a previously unrecognized 2.98-2.7 Ga Archaean crustal component, which was probably exposed in the Rehoboth Province during the Palaeoproterozoic and thus indicates a much longer geological history for the Rehoboth Province than previously known. © The Geological Society of London 2011.


Prave A.R.,University of St. Andrews | Condon D.J.,British Geological Survey | Hoffmann K.H.,Geological Survey of Namibia | Tapster S.,British Geological Survey | Fallick A.E.,Scottish Universities Environmental Research Center
Geology | Year: 2016

The end-Cryogenian glaciation (Marinoan) is portrayed commonly as the archetype of snowball Earth, yet its duration and character remain uncertain. Here we report U-Pb zircon ages for two ash beds from widely separated localities of the Marinoan-equivalent Ghaub Formation in Namibia: 639.29 ± 0.26 Ma and 635.21 ± 0.59 Ma. These findings verify, for the first time, the key prediction of the snowball Earth hypothesis for the Marinoan glaciation, i.e., longevity, with a duration of ≥4 m.y. They also show that the nonglacial interlude of Cryogenian time spanned 20 m.y. or less and that glacigenic erosion and sedimentation, and at least intermittent open-water conditions, occurred 4 m.y. prior to termination of the Marinoan glaciation. © 2016 Geological Society of America.


Penny A.M.,University of Edinburgh | Wood R.,University of Edinburgh | Curtis A.,University of Edinburgh | Bowyer F.,University of Edinburgh | And 2 more authors.
Science | Year: 2014

Reef-building inmetazoans represents an important ecological innovationwhereby individuals collectively enhance feeding efficiency and gain protection from competitors and predation. The appearance of metazoan reefs in the fossil record therefore indicates an adaptive response to complex ecological pressures. In the Nama Group, Namibia, we found evidence of reef-building by the earliest known skeletal metazoan, the globally distributed Cloudina, ∼548 million years ago.These Cloudina reefs formed open frameworks without a microbial component but with mutual attachment and cementation between individuals. Orientated growth implies a passive suspension-feeding habit into nutrient-rich currents.The characteristics of Cloudina support the view that metazoan reef-building was promoted by the rise of substrate competitors and predators.


Kasemann S.A.,University of Bremen | Pogge von Strandmann P.A.E.,Birkbeck, University of London | Prave A.R.,University of St. Andrews | Fallick A.E.,Scottish Universities Environmental Research Center | And 2 more authors.
Earth and Planetary Science Letters | Year: 2014

A marked ocean acidification event and elevated atmospheric carbon dioxide concentrations following the extreme environmental conditions of the younger Cryogenian glaciation have been inferred from boron isotope measurements. Calcium and magnesium isotope analyses offer additional insights into the processes occurring during this time. Data from Neoproterozoic sections in Namibia indicate that following the end of glaciation the continental weathering flux transitioned from being of mixed carbonate and silicate character to a silicate-dominated one. Combined with the effects of primary dolomite formation in the cap dolostones, this caused the ocean to depart from a state of acidification and return to higher pH after climatic amelioration. Differences in the magnitude of stratigraphic isotopic changes across the continental margin of the southern Congo craton shelf point to local influences modifying and amplifying the global signal, which need to be considered in order to avoid overestimation of the worldwide chemical weathering flux. © 2014 Elsevier B.V.


Peters G.,Geoscience Australia | Street G.,Tin Htay International Pty Ltd | Kahimise I.,Geological Survey of Namibia | Hutchins D.,Geological Survey of Namibia
Exploration Geophysics | Year: 2015

A regional scale TEMPEST208 airborne electromagnetic survey was flown in north-east Namibia in 2011. With broad line spacing (4km) and a relatively low-powered, fixed-wing system, the approach was intended to provide a regional geo-electric map of the area, rather than direct detection of potential mineral deposits. A key component of the geo-electric profiling was to map the relative thickness of the Kalahari sediments, which is up to 200m thick and obscures most of the bedrock in the area. Knowledge of the thickness would allow explorers to better predict the costs of exploration under the Kalahari. An additional aim was to determine if bedrock conductors were detectable beneath the Kalahari cover. The system succeeded in measuring the Kalahari thickness where this cover was relatively thin and moderately conductive. Limitations in depth penetration mean that it is not possible to map the thickness in the centre of the survey area, and much of the northern half of the survey area. Additional problems arise due to the variable conductivity of the Kalahari cover. Where the conductivity of the Kalahari sediment is close to that of the basement, there is no discernable contrast to delineate the base of the Kalahari. Basement conductors are visible beneath the more thinly covered areas such as in the north-west and south of the survey area. The remainder of the survey area generally comprises deeper, more conductive cover and for the most part basement conductors cannot be detected. A qualitative comparison with VTEM data shows comparable results in terms of regional mapping, and suggests that even more powerful systems such as the VTEM may not detect discrete conductors beneath the thick conductive parts of the Kalahari cover. © ASEG 2015


Kasemann S.A.,University of Bremen | Prave A.R.,University of St. Andrews | Fallick A.E.,Scottish Enterprise | Hawkesworth C.J.,University of St. Andrews | Hoffmann K.-H.,Geological Survey of Namibia
Geology | Year: 2010

The Neoproterozoic Earth underwent at least two severe glaciations, each extending to low paleomagnetic latitudes and punctuating warmer climates. The two widespread older and younger Cryogenian glacial deposits in Namibia are directly overlain by cap carbonates deposited under inferred periods of high atmospheric carbon dioxide concentrations. Oceanic uptake of carbon dioxide decreases ocean pH; here we present a record of Cryogenian interglacial ocean pH, based on boron (B) isotopes in marine carbonates. Our data suggest a largely constant ocean pH and no critically elevated pCO2 throughout the older postglacial and interglacial periods. In contrast, a marked ocean acidification event marks the younger deglaciation period and is compatible with elevated postglacial pCO2 concentration. Our data are consistent with the presence of two panglacial climate states in the Cryogenian, but indicate that each had its own distinct environmental conditions. © 2010 Geological Society of America.


Evans R.L.,Woods Hole Oceanographic Institution | Jones A.G.,Dublin Institute for Advanced Studies | Garcia X.,Barcelona Center for Subsurface Imaging | Muller M.,Dublin Institute for Advanced Studies | And 9 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2011

A regional-scale magnetotelluric (MT) experiment across the southern African Kaapvaal craton and surrounding terranes, called the Southern African Magnetotelluric Experiment (SAMTEX), has revealed complex structure in the lithospheric mantle. Large variations in maximum resistivity at depths to 200-250 km relate directly to age and tectonic provenance of surface structures. Within the central portions of the Kaapvaal craton are regions of resistive lithosphere about 230 km thick, in agreement with estimates from xenolith thermobarometry and seismic surface wave tomography, but thinner than inferred from seismic body wave tomography. The MT data are unable to discriminate between a completely dry or slightly "damp" (a few hundred parts per million of water) structure within the transitional region at the base of the lithosphere. However, the structure of the uppermost ∼150 km of lithosphere is consistent with enhanced, but still low, conductivities reported for hydrous olivine and orthopyroxene at levels of water reported for Kaapvaal xenoliths. The electrical lithosphere around the Kimberley and Premier diamond mines is thinner than the maximum craton thickness found between Kimberley and Johannesburg/Pretoria. The mantle beneath the Bushveld Complex is highly conducting at depths around 60 km. Possible explanations for these high conductivities include graphite or sulphide and/or iron metals associated with the Bushveld magmatic event. We suggest that one of these conductive phases (most likely melt-related sulphides) could electrically connect iron-rich garnets in a garnet-rich eclogitic composition associated with a relict subduction slab. Copyright 2011 by the American Geophysical Union.


Stollhofen H.,Friedrich - Alexander - University, Erlangen - Nuremberg | Stanistreet I.G.,University of Liverpool | Von Hagke C.,California Institute of Technology | Nguno A.,Geological Survey of Namibia
Sedimentary Geology | Year: 2014

Its location on a tectonically relatively stable passive margin and its degree of interaction with the sea make the Horingbaai fan-delta, NW Namibia, an exceptional record of coastal activity, providing insights into the response of ephemeral fluvial systems to changes in climate, sea level and continent-scale uplift. The fan comprises upper, middle and lower segments. The upper fan and middle fan are dominated by a braided river system; only in the upper fan are fluvial sediments interleaved with hyperconcentrated flow deposits. Plio-Pleistocene sea level highstands have left marine terraces on the fan surface which enable correlation with the offshore marine record and provide timelines to constrain fan growth. The bulk of fan-delta progradation took place at ~. 2.7-2.4. Ma, between the formation of a widespread erosional surface incising the middle Pliocene Karpfenkliff and Kamberg Formations, but prior to the emplacement of the warm-water fauna-bearing late Pliocene "Oyster Terrace" (~. 2.4-2.2. Ma), an equivalent of the +. 30 mP (marine terrace package) in coastal southwestern Africa. Major fan progradation is contemporaneous with widespread regional uplift (~. 12. ±. 5. m/Ma) and climate change in southwestern Africa, the latter associated with intensification of northern hemisphere glaciations. Younger fan growth phases are weaker and constrained by <. 10. m asl marine terrace bodies that yield mostly cold-water fauna, corresponding to the onset of strong glacial/interglacial climatic fluctuations superimposed on a general aridification trend and the introduction of colder sea-surface temperatures after 2.2. Ma. © 2014 Elsevier B.V.


Schneider G.,Geological Survey of Namibia
Special Paper of the Geological Society of America | Year: 2016

African geological surveys are playing an important role in local and global development. Core functions are mapping, mineral exploration, geophysics, geochemistry, research, and geoengineering. Regulatory functions, geohazards, training, environment, water resources, and land-use planning are also addressed by some. Most African geological surveys were founded to assist in the search for mineral resources, but they have had to adapt to a much wider fi eld of responsibilities in more recent times. Examples from the Geological Survey of Namibia are given. Faced with challenges of funding and skills shortage, African geological surveys have united under the auspices of the Organization of African Geological Surveys in order to jointly resolve the diffi culties experienced. © 2016 The Geological Society of America. All rights reserved.

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