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1.
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. 相似文献
2.
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. 相似文献
3.
The Okavango giant mafic dyke swarm (NE Botswana): its structural significance within the Karoo Large Igneous Province 总被引：1，自引：0，他引：1
Bernard Le Gall Gomotsang Tshoso Jrme Dyment Ali Basira Kampunzu Fred Jourdan Gilbert Fraud Herv Bertrand Charly Aubourg William Vtel 《Journal of Structural Geology》2005,27(12):783
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. 相似文献
4.
《Sedimentology》2018,65(3):809-841
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. 相似文献
5.
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. 相似文献
6.
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. 相似文献
7.
The coal deposits of southern Africa (Botswana, Malawi, Mozambique, Namibia, South Africa, Swaziland, Tanzania, Zambia and Zimbabwe) are reviewed. The coal seams formed during two periods, the Early Permian (Artinskian–Kungurian) and the Late Permian (Ufimian–Kazanian). The coals are associated with non-marine terrestrial clastic sedimentary sequences, most commonly mudrock and sandstones, assigned to the Karoo Supergroup. The Early Permian coals are most commonly sandstone-hosted while the younger coals typically occur interbedded with mudstones. The sediments were deposited in varying tectono-sedimentary basins such as foreland, intracratonic rifts and intercratonic grabens and half-grabens. The depositional environments that produced the coal-bearing successions were primarily deltaic and fluvial, with some minor shoreline and lacustrine settings. Coals vary in rank from high-volatile bituminous to anthracite and characteristically have a relatively high inertinite component, and medium- to high-ash content. In countries where coal is mined, it is used for power generation, coking coal, synfuel generation, gasification and for (local) domestic household consumption. 相似文献
8.
Over the past decade researchers working on the rocks of the Beaufort Group in the main Karoo Basin of South Africa have vastly increased our understanding of this important Permo-Triassic sequence. Many new fossil forms have been discovered, allowing for breakthroughs into the biodiversity, biogeography and biostratigraphy of the group. Taxonomic and phylogenetic advances are many and varied, and cover most of the vertebrate taxa, but with emphasis placed on the temnospondyl amphibians, archosauriforms and non-mammalian synapsids, in particular the anomodontia. Biostratigraphic breakthroughs have centered on the Middle Permian Eodicynodon and Tapinocephalus assemblage zones, the Late Permian Dicynodon Assemblage Zone, and the Triassic Lystrosaurus and Cynognathus assemblage zones. Correlation of these biozones with better dated sequences in Europe, Russia and China has allowed for many chronostratigraphic refinements, which are in turn vital for sequence stratigraphical analysis of the basin fill. Based on fossil data, both the lower (Ecca–Beaufort) and upper (Beaufort–Molteno) contacts of the group have been proved to be highly diachronous. The refined chronostratigraphic framework has also allowed for a better analysis of the basin evolution through time, particularly in terms of the correlation of external stimuli that affect basin sedimentation patterns. 相似文献
9.
南非陆相二叠系-三叠系界线研究进展 总被引：2，自引：2，他引：2
从岩石地层研究着手,介绍了南非卡鲁盆地内综合地层研究的相关成果.在生物地层方面,脊椎动物和植物化石的研究在该区具有传统优势,孢粉化石的研究则是在最近几年才取得较大进展,在陆相界线地层研究中菌孢化石在二叠系-三叠系界线处约1 m厚的地层内的富集事件属首次报道.在地球化学地层方面,南非有机碳同位素在二叠系-三叠系界线附近的变化趋势与海相二叠系-三叠系界线地层存在相似的对应关系,可以作为海、陆相界线地层对比的辅助标志.在磁性地层方面,南非卡鲁盆地内的磁极性存在从反向到正向然后再反转(R-N-R)的变化,二叠系-三叠系界线则位于第一反向极性带的顶部.同时,结合目前全球陆相二叠系-三叠系界线的研究现状及特点,分析了陆相界线地层研究中存在的基本问题,指出华南地区可能为突破传统观念的研究地区,陆地事件地层研究意义重大. 相似文献
10.
Recent mineral separate ages obtained on the Karoo large igneous province (southern Africa) suggest that the province was built by several distinct magmatic pulses over a rather long period on the order of 5–6 Ma concerning the main erupted volume [Jourdan, F., Féraud, G., Bertrand, H., Kampunzu, A.B., Tshoso, G., Watkeys, M.K., Le Gall., B., 2005. The Karoo large igneous province: Brevity, origin, and relation with mass extinction questioned by new 40Ar/39Ar age data, Geology 33, 745–748]. Although this apparently atypical province is dated in more detail compared to many other large igneous provinces, volumetrically important areas still lack sufficient high-quality data. The timing of the Karoo province is crucial as this event is correlated with the breakup activity of the Gondwana supercontinent. The Lesotho basalts represent a major lava sequence of the province, but have not yet been precisely dated by systematic analysis of mineral separates. We analyzed plagioclase separates from five lava flows encompassing the complete 1.4-km-thick Lesotho sequence from top to bottom using the 40Ar/39Ar method. We obtained five plateau and mini-plateau ages statistically indistinguishable and ranging from 182.3 ± 1.6 to 181.0 ± 2.0 Ma (2σ). We derived an apparent maximum duration for this event of 0.8 Ma by neglecting correlated errors embedded in the age uncertainties.
A critical review of previous ages obtained on the Lesotho sequence [Duncan R.A., Hooper, P.R., Rehacek, J., Marsh, J.S., Duncan, A.R., 1997. The timing and duration of the Karoo igneous event, southern Gondwana. Journal of Geophysical Research 102, 18127–18138] shows that groundmass analyses are unreliable for high-resolution geochronology, due to alteration and 39Ar recoil effects. Discrepancy between our ages and a previous plagioclase age at 184 Ma obtained by the later workers is tentatively attributed to the heterogeneity of the monitor used and/or cryptic excess 40Ar. The current age database suggests that at least three temporally and spatially distinct brief major events (the Lesotho and southern Botswana lava piles and the Okavango dyke swarm) are so far recognized in the Karoo province. Identification of brief and volumetrically important Karoo magmatic events allows detecting the migration of the Karoo magmatism and potentially the stress regime that affected the southern African lithosphere at this time. A filtered compilation of 60 ages obtained with homogeneous intercalibrated standards suggests a shorter duration for the main pulses of the magmatism between 3 and 4.5 Ma, compared to a whole province duration of 10 Ma, between 182 and 172 Ma. 相似文献