Stable isotope study of the Archaean rocks of the Vredefort impact structure,central Kaapvaal Craton,South Africa

The Vredefort dome in the Kaapvaal Craton was formed as a result of the impact of a large meteorite at 2.02 Ga. The central core of Archaean granitic basement rocks is surrounded by a collar of uplifted and overturned strata of the Witwatersrand Supergroup, exposing a substantial depth section of the Archaean crust. Orthogneisses of the core show little variation in whole-rock δ ¹⁸O value, with the majority being between 8 and 10‰, with a mean of 9.2‰ (n = 35). Quartz and feldspar have per mil differences that are consistent with O-isotope equilibrium at high temperatures, suggesting minimal interaction with fluids during subsequent cooling. These data refute previous suggestions that the Outer Granite Gneiss (OGG) and Inlandsee Leucogranofels (ILG) of the core represent middle and lower crust, respectively. Granulite-facies greenstone remnants from the ILG have δ ¹⁸O values that are on average 1.5‰ higher than the ILG host rocks and are unlikely, therefore, to represent the residuum from the partial melting event that formed the host rock. Witwatersrand Supergroup sedimentary rocks of the collar, which were metamorphosed at greenschist-to amphibolite-facies conditions, generally have lower δ ¹⁸O values than the core rocks with a mean value for metapelites of 7.7‰ (n = 45). Overall, through an ∼20 km thick section of crust, there is a general increase in whole-rock δ ¹⁸O value with increasing depth. This is the reverse of what is normal in the crust, largely because the collar rocks have δ ¹⁸O values that are unusually low in comparison with metamorphosed sedimentary rocks worldwide. The collar rocks have δD values ranging from −35 to −115‰ (average −62‰, n = 29), which are consistent with interaction with water of meteoric origin, having a δD of about −25 to −45‰. We suggest that fluid movement through the collar rocks was enhanced by impact-induced secondary permeability in the dome structure. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Structural studies and their bearing on the Early Precambrian history of the Dharwar tectonic province,southern India

In the Dharwar tectonic province, the Peninsular Gneiss was considered to mark an event separating the deposition of the older supracrustal Sargur Group and the younger supracrustal Dharwar Supergroup. Compelling evidence for the evolution of the Peninsular Gneiss, a polyphase migmatite, spanning over almost a billion years from 3500 Ma to 2500 Ma negates a stratigraphic status for this complex, so that the decisive argument for separating the older and younger supracrustal groups loses its basis. Correlatable sequence of superposed folding in all the supracrustal rocks, the Peninsular Gneiss and the banded granulites, indicate that the gneiss ‘basement’ deformed in a ductile manner along with the cover rocks. An angular unconformity between the Sargur Group and the Dharwar Super-group, suggested from some areas in recent years, has been shown to be untenable on the basis of detailed studies, A number of small enclaves distributed throughout the gneissic terrane, with an earlier deformational, metamorphic and migmatitic history, provide the only clue to the oldest component which has now been extensively reworked.     

Mudstones of the Tanqua Basin, South Africa: an analysis of lateral and stratigraphic variations within mudstones, and a comparison of mudstones within and between turbidite fans

Permian deep‐water mudstones in the Tanqua Basin, South Africa, have been studied using geochemical and spectral gamma ray techniques. The mudstones occur as thick sequences between sand‐rich submarine fans, but also occur as thinner mud‐rich units within each fan. The interfan mudstones are interpreted to have accumulated during transgression and the consequent period of relatively high sea‐level, while the submarine fans and their intrafan mudstones were deposited during regression and relatively low sea‐level. Geochemical analyses revealed systematic differences between interfan and intrafan mudstones because the two types of mudstones have slightly different source lithologies. Differences between the two types of mudstone suggest that changes in relative sea‐level played a role in controlling exposure of sediment source areas. There are geochemical signals that display systematic stratigraphic trends within both interfan and intrafan mudstones. These are best explained by gradual denudation, exposure and weathering of different lithologies within a single sediment source area. Both interfan and intrafan mudstones have uniform geochemical signals along the flow direction except for the relative amount of uranium. It is most likely that the basinward increase in uranium in the mudstones is the result of reduced clastic dilution of uranium‐bearing pelagic fallout.     

Temnospondyls from the Beaufort Group (Karoo Basin) of South Africa and Their Biostratigraphy

The Permo-Triassic Beaufort Group (Karoo Basin) of South Africa is biostratigraphically subdivided into eight, temporally successive assemblage zones based on therapsids (‘mammal-like reptiles’). The Temnospondyli, fossil tetrapods usually regarded as extinct amphibians, are second only to therapsids in terms of diversity and abundance in these strata, with nine higher-level taxa (‘families’) known. Temnospondyls are also playing an increasingly important role in biostratigraphy and correlation of the Beaufort strata. The lower Beaufort Group (Late Permian) contains six of the eight biozones, but only one temnospondyl ‘family’, the Rhinesuchidae, whose record in the Karoo is the richest in the world. However, rhinesuchid taxonomy remains in flux and the group is thus of limited biostratigraphic utility. The Early Triassic Lystrosaurus Assemblage Zone (middle Beaufort Group) contains the Rhinesuchidae, Amphibamidae, Lydekkerinidae, Tupilakosauridae, Rhytidosteidae, Mastodonsauridae and Trematosauridae, although the biostratigraphy of temnospondyls within this biozone is poorly constrained. The uppermost reaches of the Lystrosaurus biozone contain a paucity of fossils but includes ‘Kestrosaurus’ (Mastodonsauridae) and ?Trematosuchus (Trematosauridae), taxa previously thought to pertain to the lower part of the overlying Cynognathus biozone. The late Early to Middle Triassic Cynognathus Assemblage Zone (upper Beaufort Group) hosts the Mastodonsauridae, Trematosauridae, Brachyopidae, Laidleriidae and, possibly, the Rhytidosteidae. Based largely on the spatial and temporal distribution of mastodonsaurids, this biozone has been biostratigraphically subdivided into a lower A, middle B and upper C subzones, characterised by differing ages and faunas.     

The Okavango giant mafic dyke swarm (NE Botswana): its structural significance within the Karoo Large Igneous Province

The structural organization of a giant mafic dyke swarm, the Okavango complex, in the northern Karoo Large Igneous Province (LIP) of NE Botswana is detailed. This N110°E-oriented dyke swarm extends for 1500 km with a maximum width of 100 km through Archaean basement terranes and Permo-Jurassic sedimentary sequences. The cornerstone of the study is the quantitative analysis of N>170 (exposed) and N>420 (detected by ground magnetics) dykes evidenced on a ca. 80-km-long section lying in crystalline host-rocks, at high-angle to the densest zone of the swarm (Shashe area). Individual dykes are generally sub-vertical and parallel to the entire swarm. Statistical analysis of width data indicates anomalous dyke frequency (few data <5.0 m) and mean dyke thickness (high value of 17 m) with respect to values classically obtained from other giant swarms. Variations of mean dyke thicknesses from 17 (N110°E swarm) to 27 m (adjoining and coeval N70°E giant swarm) are assigned to the conditions hosting fracture networks dilated as either shear or pure extensional structures, respectively, in response to an inferred NNW–SSE extension. Both fracture patterns are regarded as inherited brittle basement fabrics associated with a previous (Proterozoic) dyking event. The Okavango N110°E dyke swarm is thus a polyphase intrusive system in which total dilation caused by Karoo dykes (estimated frequency of 87%) is 12.2% (6315 m of cumulative dyke width) throughout the 52-km-long projected Shashe section. Assuming that Karoo mafic dyke swarms in NE Botswana follow inherited Proterozoic fractures, as similarly applied for most of the nearly synchronous giant dyke complexes converging towards the Nuanetsi area, leads us to consider that the resulting triple junction-like dyke/fracture pattern is not a definitive proof for a deep mantle plume in the Karoo LIP.     

Significance of transition between Talchir Formation and Karharbari Formation in Lower Gondwana basin evolution — A study in West Bokaro Coal basin, Jharkhand, India

Basal part of the Gondwana Supergroup represented by Talchir and Karharbari Formations (Permo-Carboniferous) records an abrupt change-over from glacio-marine to terrestrial fluviolacustrine depositional environment. The contact between the two is an unconformity. Facies analysis of the glacio-marine Talchir Formation reveals that basal glaciogenic and reworked glaciogenic sediments are buried under storm influenced inner and outer shelf sediments. Facies associations of the Karharbari Formation suggest deposition as fluvio-lacustrine deposits in fault-controlled troughs. An attempt has been made in this paper to explain the sedimentation pattern in Talchir and Karharbari basins, and the abrupt change-over from glacio-marine to terrestrial fluviolacustrine depositional environment in terms of glacio-isostacy.     

Shelf‐margin clinothem progradation,degradation and readjustment: Tanqua depocentre,Karoo Basin (South Africa)

Degradation of basin‐margin clinothems around the shelf‐edge rollover zone may lead to the generation of conduits through which gravity flows transport sediment downslope. Many studies from seismic‐reflection data sets show these features, but they lack small‐scale (centimetre to metre) sedimentary and stratigraphic observations on process interactions. Exhumed basin‐margin clinothems in the Tanqua depocentre (Karoo Basin) provide seismic‐reflection‐scale geometries and internal details of architecture with depositional dip and strike control. At the Geelhoek locality, clinothem parasequences comprise siltstone‐rich offshore deposits overlain by heterolithic prodelta facies and sandstone‐dominated deformed mouth bars. Three of these parasequences are truncated by a steep (6 to 22°), 100 m deep and 1·5 km wide asymmetrical composite erosion surface that delineates a shelf‐incised canyon. The fill, from base to top comprises: (i) thick‐bedded sandstone with intrabasinal clasts and multiple erosion surfaces; (ii) scour‐based interbedded sandstone and siltstone with tractional structures; and (iii) inverse‐graded to normal‐graded siltstone beds. An overlying 55 m thick coarsening‐upward parasequence fills the upper section of the canyon and extends across its interfluves. Younger parasequences display progressively shallower gradients during progradation and healing of the local accommodation. The incision surface resulted from initial oversteepening and high sediment supply triggering deformation and collapse at the shelf edge, enhanced by a relative sea‐level fall that did not result in subaerial exposure of the shelf edge. Previous work identified an underlying highly incised, sandstone‐rich shelf‐edge rollover zone across‐margin strike, suggesting that there was migration in the zone of shelf edge to upper‐slope incision over time. This study provides an unusual example of clinothem degradation and readjustment with three‐dimensional control in an exhumed basin‐margin succession. The work demonstrates that large‐scale erosion surfaces can develop and migrate due to a combination of factors at the shelf‐edge rollover zone and proposes additional criteria to predict clinothem incision and differential sediment bypass in consistently progradational systems.     

Tectonic evolution of the Cape and Karoo basins of South Africa

The Cape and Karoo basins formed within the continental interior of Gondwana. Subsidence resulted from the vertical motion of rigid basement blocks and intervening crustal faults. Each basin episode records a three-stage evolution consisting of crustal uplift, fault-controlled subsidence, and long periods of regional subsidence largely unaccompanied by faulting or erosional truncation. The large-scale episodes of subsidence were probably the result of lithospheric deflection due to subduction-driven mantle flow. The early Paleozoic Cape basin records the combined effects of a north-dipping intra-crustal décollement (a late Neoproterozoic suture) and a right-stepping offset between thick Rio de la Plata craton and Namaqua basement. Following the Saldanian orogeny, a suite of small rift basins and their post-rift drape formed at this releasing stepover. Great thicknesses of quartz sandstone (Ordovician–Silurian) and mudstone (Devonian) accumulation are attributed to subsidence by rheological weakening and mantle flow. In contrast, the Karoo basin is a cratonic cover that mimics the underlying basement blocks. The Permian Ecca and lower Beaufort groups were deposited in a southward-deepening ramp syncline by extensional decoupling on the intra-crustal décollement. Reflection seismic and deep-burial diagenetic studies indicate that the Cape orogeny started in the Early Triassic. Deformation was partitioned into basement-involved strike-slip faults and thin-skinned thrusting. Uplift of the Namaqua basement resulted in erosion of the Beaufort cover. East of the Cape fold belt, contemporaneous subsidence and tilting of the Natal basement created a late Karoo transtensional foreland basin, the Stormberg depocentre. Early Jurassic tectonic resetting and continental flood basalts terminated the Karoo basin.     

Modelling the sustainability and productivity of pastoral systems in the communal areas of Namaqualand

We investigated the effects of rainfall and the number of animals on changes in vegetation and on the output of milk and meat from the communal areas of Namaqualand. Previously published short- and long-term models link processes that range from the levels of tissue (in, for example, the mammary gland), to the milk yields of individual animals, to the growth and survival of their young and to long-term changes in plant species populations at the ecosystem level. These models have been used to study how different factors and management strategies affect livestock productivity and vegetation composition on a 20,000 ha rangeland in Namaqualand. First, the inter-relations between rainfall, stocking rate and productivity were studied using the short-term model. This model shows that in addition to total rainfall and stocking rate, the timing of rainfall within a year also influences doe live weight and survival to the end of the year. When the long-term model is run, using recorded rainfall, predictions of small stock numbers agree closely with livestock data recorded over the same 30-year period. One thousand replicates of 100-year runs of the long-term model were then used to study the probable impact of different upper limits to stock numbers on animal performance. Off take (sales and slaughterings) are maximal when stock numbers are limited to 2000 adults. Animal numbers increase marginally as the limit is increased above this level, but the variability between years in numbers increases. Secondly, the long-term model was used to study the long-term effects of the stocking rate strategies on rangeland condition. The model predicts that although these effects are variable, when moderately degraded range is stocked with an upper limit at the recommended level it is unable to recover to less degraded states over 100 years. Thirdly, the model was used to examine the effects of reduction in stock numbers and slaughtering of kids in a drought year on goat numbers during the subsequent 5 years. Finally, the model predicts that a 10% reduction in mean annual rainfall will lead to a 35% reduction in animal numbers over 200 years.     

Seeds dispersed in dung of insectivores and herbivores in semi-arid southern Africa

We investigated the incidence of endozoochory in the semi-arid South African Karoo shrubland and Kalahari savanna by dissecting and germinating seed from dung samples from a wide range of wild mammals and domestic livestock. Intact seeds occurred at a mean density of 1575 seeds kg⁻¹in livestock air-dried dung and 3613 seeds kg⁻¹in air-dried dung of indigenous animals. Seedlings emerged from dung at an average density of 153 kg⁻¹air-dry dung. Seeds of Aizoaceae, Mesembryanthemaceae, Chenopodiaceae and Poaceae were abundant in the dung of wild and domestic herbivores as well as such ant- and termite-eating insectivores as aardvark (Orycteropus afer) and bat-eared fox (Otocyon megalotis). Seeds of fleshy-fruited shrubs occurred in dung of browsing herbivores and the bat-eared fox. Five non-indigenous weeds were found in the dung samples. Although the floras of the Kalahari and Karoo are considered to be largely wind and water-dispersed, endozoochory is a primary or secondary dispersal mechanism in many plant families and for many plant life-forms. Animals dispersed seeds of many species characteristic of fertile and disturbed habitats. In Mesembryanthemaceae, a family in which seeds are primarily dispersed very short distances by raindrops, endozoochory enables occasional long-distance dispersal.