Gastaldo R.A.,Colby College |
Kamo S.L.,University of Toronto |
Neveling J.,Council for Geosciences |
Geissman J.W.,University of Texas at Dallas |
And 2 more authors.
Geology | Year: 2015
The end-Permian extinction records the greatest ecological catastrophe in Earth history. The vertebrate fossil record in the Karoo Basin, South Africa, has been used for more than a century as the standard for understanding turnover in terrestrial ecosystems, recently claimed to be in synchrony with the marine crisis. Workers assumed that systematic turnover at the Dicynodon assemblage zone boundary, followed by the appearance of new taxa directly above the base of the Lystrosaurus assemblage zone, is the continental expression of the end-Permian event and recovery. To test this hypothesis, we present the first highprecision age on strata close to the inferred Permian-Triassic boundary. A U-Pb isotope dilution-thermal ionization mass spectrometry zircon age of 253.48 ± 0.15 Ma (early Changhsingian) is from a silicified ash layer ~60 m below the current vertebrate-defined boundary at Old Lootsberg Pass (southern South Africa). This section yields newly discovered plants and vertebrates, and is dominated by a normal polarity signature. Our collective data suggest that the Dicynodon-Lystrosaurus assemblage zone boundary is stratigraphically higher than currently reported, and older than the marine extinction event. Therefore, the turnover in vertebrate taxa at this biozone boundary probably does not represent the biological expression of the terrestrial end-Permian mass extinction. The actual Permian-Triassic boundary in the Karoo Basin is either higher in the Katberg Formation or is not preserved. The currently accepted model of the terrestrial ecosystem response to the crisis, both in this basin and its extension globally, requires reevaluation. © 2015 Geological Society of America.
Senger K.,University of Bergen |
Senger K.,University Center in Svalbard |
Buckley S.J.,University of Bergen |
Chevallier L.,Council for Geosciences |
And 9 more authors.
Journal of African Earth Sciences | Year: 2015
Igneous intrusions act as both carriers and barriers to subsurface fluid flow and are therefore expected to significantly influence the distribution and migration of groundwater and hydrocarbons in volcanic basins. Given the low matrix permeability of igneous rocks, the effective permeability in- and around intrusions is intimately linked to the characteristics of their associated fracture networks. Natural fracturing is caused by numerous processes including magma cooling, thermal contraction, magma emplacement and mechanical disturbance of the host rock. Fracturing may be locally enhanced along intrusion-host rock interfaces, at dyke-sill junctions, or at the base of curving sills, thereby potentially enhancing permeability associated with these features. In order to improve our understanding of fractures associated with intrusive bodies emplaced in sedimentary host rocks, we have investigated a series of outcrops from the Karoo Basin of the Eastern Cape province of South Africa, where the siliciclastic Burgersdorp Formation has been intruded by various intrusions (thin dykes, mid-sized sheet intrusions and thick sills) belonging to the Karoo dolerite. We present a quantified analysis of fracturing in- and around these igneous intrusions based on five outcrops at three individual study sites, utilizing a combination of field data, high-resolution lidar virtual outcrop models and image processing. Our results show a significant difference between the three sites in terms of fracture orientation. The observed differences can be attributed to contrasting intrusion geometries, outcrop geometry (for lidar data) and tectonic setting. Two main fracture sets were identified in the dolerite at two of the sites, oriented parallel and perpendicular to the contact respectively. Fracture spacing was consistent between the three sites, and exhibits a higher degree of variation in the dolerites compared to the host rock. At one of the study sites, fracture frequency in the surrounding host rock increases slightly toward the intrusion at approximately 3. m from the contact. We conclude by presenting a conceptual fluid flow model, showing permeability enhancement and a high potential for fluid flow-channeling along the intrusion-host rock interfaces. © 2014 Elsevier Ltd.
Ramotoroko C.D.,University of Botswana |
Ranganai R.T.,University of Botswana |
Nyabeze P.,Council for Geosciences
Journal of African Earth Sciences | Year: 2016
The main goal of this study is to use suitably processed potential field data between longitude 24°E to 26°E and latitude 25°S to 26.5°S to gain a better geological and structural understanding of the extension of the Madibe-Kraaipan granite-greenstone terrane in southeast Botswana. Specifically, 150 new gravity measurements at 2–4 km intervals are reduced and later merged with existing gravity data in Botswana and South Africa for an integrated crustal interpretation with regional aeromagnetic data. Gravity and aeromagnetic anomalies of the region present partly coincident medium to high amplitude regions alternating with low zones. Analysis of the data revealed the existence of relatively narrow N[sbnd]S trending rocks of dense and high magnetic intensity extending to the village of Mmathethe, corresponding to the northern extent of the Kraaipan greenstone belt. Much of the north-central area forms a broad magnetic low and gravity high implying Kraaipan metavolcanic rocks are more extensively developed in this area than previously recognized, under a blanket of Kalahari sediments that are ∼55 m thick as indicated by borehole data. The whole area lies within an ENE-trending Pre-Transvaal dyke swarm visible on the regional aeromagnetic data and much clearer on (proprietary) high resolution aeromagnetic data. The derivative and analytic signal techniques applied for both gravity and magnetic data spatially map the greenstone belt and multiple granite plutons very well. Depth estimates obtained by the 3D Euler method in combination with two-dimensional power spectrum technique locate the high magnetic intensity horizon at around 4.0 km. The depths were further confirmed by gravity model results along two profiles across the granite-greenstone terrane in Botswana and South Africa in a W-E direction. The models show generally steep-sided bodies of comparable width with a maximum depth extent of 4.7 km for the greenstones and 4.4 km for the younger plutons. The distribution and configuration of the greenstones and plutons suggest assembly of this Kaapvaal craton western domain by plate tectonic processes. Episodic intrusion of granitoids into Kraaipan greenstone successions led to deformation of the belt with significance to Au and Au-PGE mineralization. The mapped structures could be important for groundwater exploration in this dry farming terrane. © 2016 Elsevier Ltd
Abiye T.A.,University of Witwatersrand |
Leshomo J.T.,Council for Geosciences
Environmental Earth Sciences | Year: 2013
The Namaqualand area is located in the Northern Cape Province of South Africa which is characterised by arid climate where groundwater is the only source of water supply for local communities. Extensive groundwater sampling was carried out in the area and the physico-chemical parameters, inorganic constituents, stable isotopes and trace metals were measured. The hydrochemistry of the area indicates dissimilar groundwater composition due to complex geochemical processes where groundwater flow takes place from catchment F30A to the catchment D82B, and the chemistry is controlled primarily by Redox reaction, dissolution and mixing processes. The Br-/Cl- ratio revealed that the salinity in the area is derived from seawater mixing, halite dissolution and atmospheric deposition/sea aerosol spray. Under excessive evaporative condition due to climatic aridity, groundwater salinity shows increasing trend. Isotope results show the presence of shallow-weathered zone and deep-fracture controlled circulating groundwater in the crystalline basement aquifer of the area. In comparison to the South African water quality standard, the results obtained for gross alpha activity which could be derived from uranium isotopes show that 41 % of the reported data in the area fall above the drinking water limit. © 2012 Springer-Verlag Berlin Heidelberg.
Abiye T.,University of Witwatersrand |
Leshomo J.,Council for Geosciences
Environmental Earth Sciences | Year: 2014
This paper presents the enrichment tendency and spatial distribution of metals in the groundwater which is pumped out from the granitic aquifers in South Africa. Groundwater is the sole source of water supply for the local community in the study area (Namaqualand), and hence, it was necessary to understand the controlling geochemical processes and interrelationship of metals in the groundwater. The geochemical association of metals has been assessed based on the geostatistical methods. The results show that geochemical processes such as oxidation, leaching, and evaporation besides water–rock interaction are very important in controlling metal enrichment in the groundwater from highly mineralized rocks. The metal enrichment index for selected toxic metals in groundwater increases in the order of Cd > U > Cr > Pb. The observed enrichment trend could be considered as a result of mineralization of basement rocks which is facilitated by active geochemical process in the arid environment. The lack of aquifer flushing due to negligible recharge helps the metals to concentrate at shallow groundwater zones supported by severe evaporation process. © 2014, Springer-Verlag Berlin Heidelberg.
Temme A.J.A.M.,Wageningen University |
Schaap J.D.,Aequator Groen en Ruimte B.V. |
Sonneveld M.P.W.,Wageningen University |
Botha G.A.,Council for Geosciences
Quaternary International | Year: 2012
Palaeosols have long been studied as valuable records of past climate and landscape changes. The influence of palaeosols on the functioning of present-day landscapes is receiving closer attention due to the relevance of palaeosols on long-term hydrological processes and the future hydrological and erosive response of catchments. This study describes a sequence of interbedded colluvial sedimentary deposits and buried palaeosols exposed by gully erosion in the Drakensberg escarpment foothills in KwaZulu-Natal province, South Africa. Model simulations are presented that explore present-day hydrological effects of pedogenetic and textural differentiation under conditions of groundwater lowering caused by gully erosion at 14 sites. The results suggest that the colluvial deposits and palaeosols cause a significant increase in total annual drainage from most of the 14 studied sites compared with similar sites without texturalor structural differentiation. In the simulations, sediment deposition, not palaeosol formation, has the most profound influence on the present-day hydrological functioning of the catchment: a true deposystem service. Effects of the limited palaeosol development on soil water holding capacity and actual soil water storage appear to be negligible. © 2011 Elsevier Ltd and INQUA.
Prevec R.,Rhodes University |
Gastaldo R.A.,Colby College |
Neveling J.,Council for Geosciences |
Reid S.B.,Colby College |
Looy C.V.,University of California at Berkeley
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2010
The discovery of a plant-fossil assemblage, situated ~70 m below the vertebrate-defined Permian-Triassic boundary, allows for the characterization of part of the Late Permian landscape in the southern Karoo Basin. The megafloral and microfloral assemblages are preserved in association with an O-horizon above a weakly developed palaeosol, exposed in both New and Old Wapadsberg Pass, Eastern Cape Province, South Africa. Stable-isotope geochemical evidence indicates that pedogenesis occurred under conditions of a high regional water table, and the palaeosol was entombed by overbank or avulsion deposits in an aggradational landscape that includes tuffite. The low-diversity macroflora contains gymnospermous (Glossopteris) and sphenopsid (Phyllotheca, Trizygia) elements that include vegetative (axial, leaf) and reproductive (strobilar) remains. Glossopteris leaves are microphyllous and may reflect the taxon's physiological response to purported climate perturbation or wetland edaphic conditions, and exhibit a range of plant-animal interactions. Palynological residues contain sphenophyllalean cuticles, and the microfloral assemblage is consistent with a low-diversity glossoperid-dominated woodland with a sphenopsid understory. Extrabasinal elements include representatives of peltasperms (Alisporites), corystosperms (Falcisporites), and conifers (Lueckisporites). Pollen conforming to Lunatisporites pellucidus is present in the assemblage and is considered an indicator taxon for the latest Permian to Early Triassic. When compared to palynological assemblage zones in Australia, the Wapadsberg Pass flora is assigned a Late Changhsingian age and, therefore, represents the youngest autochthonous, Glossopteris-dominated flora to be reported from Gondwana. These results extend a palynostratigraphic framework across the Southern Hemisphere that complements other zonations presently applied to the Karoo Basin succession. © 2010 Elsevier B.V. All rights reserved.
Gastaldo R.A.,Colby College |
Knight C.L.,Colby College |
Neveling J.,Council for Geosciences |
Tabor N.J.,Southern Methodist University
Bulletin of the Geological Society of America | Year: 2014
Terrestrial settings preceding the end-Permian crisis are reported to trend toward increasingly dry and arid conditions, resulting in landscape change and a shift in fluvial architectures and regimes. Much of the latest Permian (Changhsingian) stratigraphic record in the Karoo Basin, South Africa, consists of paleosols, which record the physical conditions across time and space. Preboundary sequences at Wapadsberg Pass, Eastern Cape Province, provide insight into the climate regime that infl uenced paleosol formation at that time. A high-resolution sedimentological and geochemical study of two, stacked aggradational paleosols, in conjunction with stable isotope geochemical characterization of paleosol carbonate-cemented concretions over a 90 m section at this locality, demonstrates that these landscapes were predominantly wetland terrains without a demonstrable trend in increasing drying up to the Permian-Triassic boundary, as defi ned by vertebrates in the area. Two paleosols examined 70 m below the Permian-Triassic boundary are identifi ed as Protosols, and the former soil-air interface of each is marked by an autochthonous forestfl oor litter in which canopy leaves of Glossopteris and groundcover plants of Trizygia are preserved. Molecular weathering ratios (e.g., base loss, clayeyness, chemical index of alteration minus potassium [CIA-K], etc.) determined from these horizons are indicative of immature soil development under watersaturated conditions. Assuming that paleosol-matrix concentrations of trace elements are indicative of Permian soil-solution chemistries, high concentrations of Ni, Cu, Ba, and Cr may have been growth-stress factors that may account for the small glossopterid leaf size in the megafl oras, in contrast to current models that implicate stress in response to climate change. Stable isotope δ18O and δ13C values are presented for micritic and microspar (<20 μm) calcite cements from carbonate nodules collected at 15 horizons through a 90 m stratigraphic interval up to, and including, the Permian-Triassic boundary. These isotopic ratios exhibit dissimilar trends. No clear trend exists in δ18O (Peedee belemnite [PDB] values range from -14.7‰ to -21.8‰). In contrast, a trend exists in δ13C values, where carbonate cements almost certainly precipitated under well-drained conditions in an interval that is 60 m below the Permian-Triassic boundary (-5.3‰), while d13C values as low as -16.9‰, indicative of water-logged conditions, begin 90 m below and continue up to the Permian-Triassic boundary. Hence, no evidence is found for a preboundary trend toward increasing aridity at this locality. The fi rst estimates of the latest Permian atmospheric pCO2 from paleosols, based on coexisting calcite and organic matter δ13C values from paleosols that developed under well-drained conditions, provide a range of values from 900 to 1900, and 500 to 1300 ppmV, respectively, which are signifi cantly lower than the latest Early Permian, when terrestrial biome replacement is documented to have occurred. © 2014 Geological Society of America.