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Structure, stratigraphy and sedimentology of the paleoproterozoic Nsuta manganese deposit, Ghana

  • Authors: Van Bart, Adrian
  • Date: 2008-07-18T13:42:03Z
  • Subjects: Manganese ores , Stratigraphic geology , Structural geology , Ghana
  • Type: Thesis
  • Identifier: uj:7367 , http://hdl.handle.net/10210/812
  • Description: The Nsuta manganese deposit is located in the Western Region of Ghana, approximately five kilometers south of Tarkwa Goldfields. The deposit has been an important source of manganese ore since mining began in 1916. The purpose of this project was to produce a concise model of the stratigraphy, sedimentology and structural evolution of the deposit in support of future exploration projects. The manganese ores occur as an up to 45m thick carbonate bed in a thick turbidite-greenstone succession that is part of the ~2.2 Ga Birimian Supergroup. Calc-alkaline volcanics, volcaniclastics, turbidites, argillites and phyllites are thought to have been deposited in a backarc basin environment. The entire sedimentary succession, including the manganese orebody, is a thick turbidite package hosted between an upper and lower greenstone unit consisting predominantly of volcaniclastic material. The entire lithological succession at Nsuta is interpreted to have been deposited within the middle to lower reaches of a submarine fan environment. Field evidence suggests a simple stratigraphy, commencing with a lower greenstone unit composed largely of volcaniclastic material. This is followed by an upward-fining lower turbidite unit deposited in response to a marked transgression and sea level rise. Maximum rate of sea level rise provided ideal conditions for manganese precipitation and concentration, as detrital influx ceased. The central portion of the carbonate orebody that formed hosts the manganese orebody. An upward-coarsening turbidite unit follows above the carbonate unit. This upward-coarsening succession reflects a regression and a highstand systems tract in terms of sequence stratigraphic principles. It is capped by an unconformity that formed during a period of rapid relative sea level fall. It is overlain by a second upward-fining turbidite succession. This succession is not fully preserved as there is a sheared contact between it and the overlying upper greenstone unit. Post-depositional deformation and metamorphic alteration are largely attributed to the Paleoproterozoic Eburnean Orogeny. A first phase of compression was directed along a NW-SE axis and produced a series of isoclinal anticlines and synclines (F1) with NE-SW striking axial planes. This was followed by thrusting between the anticlines and synclines. The age of this deformation and closely associated greenschist metamorphism can be accurately constrained between 2.09 Ga and 2.07 Ga. E-W oriented oblique listric faulting has a prominent effect on the appearance of the Nsuta manganese deposit, as it produced a series of imbricate fault blocks dipping to the north. Associated with this period of deformation is small-scale cross folding with axes plunging to the east (F2). The faults post-date the Eburnean Orogeny and must be associated with a second major tectonic event. Finally, a NNE-SSW striking normal fault, locally known as the German Line, caused further block rotation, notably in the northern parts of the mining concession. Late Mesozoic deep lateritic weathering and incision of the lateritic peneplane by modern rivers have resulted in the complex dissected appearance of the Nsuta orebody. However, based on the detailed structural analysis provided in this study, a feasible target for future exploration of manganese ore buried beneath Late Mesozoic and Cenozoic sediments and soils, has been identified. This target is located to the west of Hills A and B. , Dr. J.M. Huizenga Prof. Nic Beukes Prof. J. Gutzmer
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Neoarchaean clastic rocks on the Kaapvaal Craton : provenance analyses and geotectonic implications

  • Authors: Schneiderhan, Eva Anita
  • Date: 2008-08-13T12:19:27Z
  • Subjects: Geology South Africa , Stratigraphic geology , Cratons , North-West (South Africa)
  • Type: Thesis
  • Identifier: http://ujcontent.uj.ac.za8080/10210/377453 , uj:7672 , http://hdl.handle.net/10210/853
  • Description: The provenance of the Neoarchaean Ventersdorp Supergroup and several age-related supracrustal successions was analysed to gain insight into the geotectonic evolution of the Kaapvaal Craton during the transition from the Archaean to Proterozoic Eras. The studied successions include, besides the siliciclastic formations of the Ventersdorp Supergroup, the upper Wolkberg and Buffelsfontein Groups, the Godwan Formation and the Schmidtsdrift Subgroup of the basal Transvaal Supergroup in Griqualand West. Petrographic, whole rock geochemical and Sm-Nd isotopic analyses were combined with SHRIMP U-Pb age dating of detrital zircons. Furthermore, Rb-Sr isotopic studies were carried out on carefully selected suites of samples from surface exposure or, wherever possible, on deep diamond drill core. The Ventersdorp Supergroup is an up to 5 km thick undeformed, only slightly metamorphosed volcano-sedimentary succession deposited on the Kaapvaal Craton between 2714 Ma and 2665 Ma. A lack of major time hiati to the underlying Mesoarchaean Witwatersrand Supergroup and covering Neoarchaean to Palaeoproterozoic Transvaal Supergroup render the Ventersdorp Supergroup very well suited for the investigation of the geotectonic evolution of the Kaapvaal Craton near the Archaean-Proterozoic boundary. This is supported by its excellent preservation, which also allowed detailed studies of sedimentological structures, such as seismites indicating Neoarchaean earthquakes. The provenance analyses carried out on the clastic formations of the Ventersdorp Supergroup point to a gradual change in tectonic evolution from typically Archaean to post-Archaean processes rather than a drastic, unique transition in the case of the Kaapvaal Craton. Texturally immature wackes of the Kameeldoorns Formation, representing the oldest clastic units of the Ventersdorp Supergroup, are derived mainly from Mesoarchaean source rocks, whereas the stratigraphically younger Bothaville Formation displays geochemical signatures comparable with Archaean trondhjemite-tonalite granodiorite-suites (TTGs), thus suggesting crustal addition in the so-called ‘Archaean-style’. The extension of provenance analyses to supracrustal successions that are tentatively correlated with the Bothaville Formation, revealed contributions from granitoid V sources that formed under post-Archaean and Archaean conditions. Furthermore, the geochemical data for all analysed formations support a passive margin setting. Arc settings, as indicated in some samples, are due to the input of less fractionated volcanic material that provides evidence of distal volcanism. Analyses of Nd-isotopic systematics and U-Pb ages of detrital zircons revealed a Mesoarchaean age for the source rocks of the formations. U-Pb age dating of detrital zircons of the Godwan Formation suggests that this formation is of Mesoarchaean age, and therefore not a correlative of the other Neoarchaean successions. Hence, the results suggest that the continental crust of the Kaapvaal Craton was thick enough since the Mesoarchaean (2.8 - 3.1 Ga) to allow long-term crustal recycling, and therefore modern plate tectonic processes could have operated earlier than on other well-studied cratonic blocks. During the Neoarchaean, however, crustal thickening of the Kaapvaal Craton took place by accretion of Archaean-style TTGs along the margins of the craton. Thus, Archaean and post-Archaean tectono-magmatic processes co-existed. Furthermore, the Neoarchaean supracrustal successions represent the first sedimentation events on an entirely stabilised and tectonically quiescent Kaapvaal Craton. Input from distal volcanic sources marks the last sign of volcanic activity prior to the craton-wide deposition of carbonate rocks of the Transvaal Supergroup. Geochronological data also imply a connection of the Neoarchaean Kaapvaal Craton to further cratonic blocks that may hold source rocks for the studied formations, as for some small age populations of older detrital zircons (ca. 3.1 - 3.4 Ga), no suitable source area could be identified on the Kaapvaal Craton itself. However, it seems unlikely that the Zimbabwe Craton was one of these cratonic blocks, because the Rb-Sr whole rock ages of all studied formations yield a model age of 2092 ± 55 Ma, which is thought to correspond to a craton-wide influence of the 2.05 Ga old Bushveld Igneous Complex on the Rb-Sr isotope systematics of all analysed clastic successions. This influence is apparently missing in the Southern and Central Marginal Zones of the Limpopo Belt, suggesting that the collision between the Kaapvaal and Zimbabwe Cratons only took place after the emplacement of the Bushveld Igneous Complex, i.e. after 2.05 Ga. , Dr. U. Zimmermann Prof. J. Gutzmer
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Paleomagnetism of selected neoarchean-paleoproterozoic cover sequences on the Kaapvaal Craton and implications for Vaalbara

  • Authors: De Kock, Michiel Olivier
  • Date: 2008-08-25T06:27:36Z
  • Subjects: Paleomagnetism , Stratigraphic geology , Cratons , Paleogeography
  • Type: Thesis
  • Identifier: uj:3676 , http://hdl.handle.net/10210/905
  • Description: The Kaapvaal craton of southern Africa and the Pilbara craton of Western Australia, two of the best-preserved Archean cratons in the world, are covered by remarkably similar early Precambrian cover sequences. This has led to the proposal of the so-called Vaalbara hypothesis, which promotes the existence of the two cratons as a single crustal entity, and possibly, Earth’s oldest assembled continent in Neoarchean-early Paleoproterozoic times. Previous studies have failed to prove the existence of Vaalbara conclusively, principally due to a lack of reliable ages or because of uncertainty and gaps in the paleomagnetic record from the Kaapvaal craton. During the present study paleomagnetic samples were collected from selected Neoarchean- Paleoproterozoic cover sequences of the Kaapvaal craton for the establishment of well-defined paleomagnetic poles. In addition, the Hartswater Group of the Ventersdorp Supergroup was sampled for zircon SHRIMP analyses in order to constrain the ages of poles defined from that succession. The paleopoles established here, together with existing paleopoles from the Kaapvaal craton, are used to evaluate the apparent polar wander path of the craton during the Neoarchean-Paleoproterozoic and are compared with poles of similar age from the Pilbara craton as a test of the Vaalbara hypothesis. Regarding the age of the Hartswater Group, zircon SHRIMP ages of 2735 ± 3 Ma and 2724 ± 6 Ma cast doubt on younger ages from the Klipriviersberg Formation, which comprise the base of the Ventersdorp Supergroup. Traditional (younger) age constraints from the Ventersdorp Supergroup do not support the original Vaalbara correlation. A new correlation is suggested here, taking the new ages into account, showing that the Ventersdorp Supergroup overlaps in time with the Fortescue Group of the Pilbara craton. Most importantly, the new ages also provide constraints on the magnetization within the Platberg Group and the Allanridge Formation. Six new paleopoles, of various quality, are added to the existing database from that craton. These poles from the ~2.73 Ga Platberg Group and ~2.7Ga Allanridge Formation of the Ventersdorp Supergroup, the ~2.5Ga lower Transvaal Supergroup, the lower two unconformitybounded sequences of the Waterberg Group (2.05 Ga and ~1.99 Ga) and the upper Soutpansberg Group (~1.76 Ga) have, together with existing poles from the Kaapvaal craton, led to the definition of an APWP for that craton for a period ~2.78 to ~1.76 Ga. Particularly the poles from the Waterberg and Soutpansberg Groups provided the information to identify complexities (looping) in the APWP that have gone unrecognized in the past. The paleomagnetic data gathered and the newly defined APWP could be used in conjunction with geological evidence from the Kaapvaal and Pilbara cratons to evaluate, and validate, the Vaalbara hypothesis. A good match between the APWP’s of the two cratons for the period ~2.78 to ~2.70 Ga and the geological features (lithology and structure) of the two cratons provide the best evidence that Vaalbara existed as a cratonic unit in the late Archean. Paleomagnetic data constrain the position of the Pilbara craton in immediate proximity to the northwest of the Kaapvaal craton (in a Kaapvaal reference frame). The position of the Zimbabwe craton relative to the Pilbara and Kaapvaal cratons is still unresolved, but indications are that it was most likely in a proximal position to the Kaapvaal craton at 2.7 Ga in a configuration not much different from its present day configuration. This would imply that Vaalbara was most probably the Earth’s oldest assembled continent as proposed by earlier workers. The new paleomagnetic data further suggest that Vaalbara did not exist anymore at ~2.0Ga. When evaluated in conjunction with geological evidence a strong argument can be made for the existence of the Vaalbaran continent up until ~2.22 Ga and that the Pilbara and Kaapvaal cratons became separate entities from about ~2.05 Ga. , Prof. NJ Beukes Prof. DAD Evans
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Provenance ages and timing of sedimentation of selected Neoarchean and Paleoproterozoic successions on the Kaapvaal Craton

Selected magnetostratigraphic studies in the main Karoo Basin (South Africa): implications for mass extinction events and the supercontinent of Pangea

  • Authors: De Kock, Michiel Olivier
  • Date: 2009-01-27T07:18:31Z
  • Subjects: Stratigraphic geology , Paleomagnetism , Paleoclimatology , Pangaea (Geology) , Karoo Basin (South Africa)
  • Type: Thesis
  • Identifier: uj:14829 , http://hdl.handle.net/10210/1953
  • Description: M.Sc. , The Late Carboniferous to early Jurassic Karoo Supergroup of South Africa witnessed two of the “big five” Phanerozoic mass extinction events, and the formation and subsequent break-up of the supercontinent Pangea. The closure of the Permian Period witnessed the greatest biotic crisis in the history of life. What is known about the Permian-Triassic boundary (hereafter referred to as the PTB) comes almost exclusively from marine successions in Europe and Asia. Although a major extinction event has been recognized in terrestrial successions, surprisingly little is known about its effects and timing. The exact placement of the PTB in the Karoo basin is not well constrained due to shortcomings of stratigraphic methods employed to date. This has made it extremely difficult to correlate the mass extinction events in the marine and non-marine environments; however, paleomagnetic studies could provide answers to both problems of absolute placement and correlation of the PTB in non-marine and marine successions. The PTB appears to lie within an interval of reversed polarity in many marine successions. A detailed magnetostratigraphic survey across the presumed PTB in the Karoo succession at localities in the north and south of the main Karoo Bain reveal two magnetic chrons, reversed followed by normal (with the boundary close to the reversal), which extends to slightly younger results from a previous study that identified an N/R pattern, thereby identifying a R/N/R pattern. The normal chron might correlate with the long basal Triassic normal polarity interval and the reversed polarity zones above and below it known from marine successions in the Alps, Russia, Pakistan and China. The PTB is thought to be situated coincident with the LAD of Dicynodon and the event bed of Ward et al. (2000), apparently above but not necessarily diachronous with a lithology change from predominantly green- to predominantly red mudstone. This placement falls within a normal polarity interval, but could conceivably have taken place at a time of reverse polarity due to delayed acquisition of magnetic remanence. The idea of an extraterrestrial impact as the cause of the end-Permian mass extinctions is strongly enhanced by a synchronous relationship between them. The configuration of the supercontinent Pangea during this time of earth history has been the matter of debate for the last three decades, with numerous alternative reconstructions to the classic Pangea A1 having been proposed for the time preceding the Jurassic. Paleomagnetic data from the Karoo allow for the definition of a new paleopole for West Gondwanaland, which prove a valuable tool for evaluating these various reconstructions. It is neither consistent with a Pangea B-type not C reconstruction for Pangea during this time interval, because of geological ambiguities. The most likely solution to the problem is that of a persistent non-dipole field contribution to the geomagnetic field during this time. Approximately 50 million years later Pangea was unambiguously in a classic Pangea A1 configuration, and life on earth suffered yet another set back. The end-Triassic mass extinction, which marks the sequence boundary between the Triassic and the Jurassic, has not received as much attention as the other four big Phanerozoic biotic disasters. In the Karoo a pronounced turnover in faunal assemblages from typical Triassic fauna to Jurassic Fauna (dinosaurs) is seen in the Elliot Formation. Magnetostratigraphic study of localities in the north and south of the Karoo Basin provided a magnetic zonation pattern for the Elliot Formation, a tool that has led to the constraining of the sequence boundary to the transition from the lower Elliot Formation to the middle Elliot and added to the hypothesis that the faunal turnover is globally synchronous. The determination of a paleolatitude for the Elliot Formation in combination with characteristically arid lithologies (eolian sandstones) provided the base for the evaluation of the paleoclimate that characterized Pangea during the Late Triassic to Early Jurassic. Key words: Karoo Basin, Magnetostratigraphy, Mass Extinction, Paleoclimate, Paleogeography, Paleomagnetism, Pangea, Permian-Triassic, Triassic-Jurassic
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Genetic stratigraphy of the paleoproterozoic Pretoria Group in the Western Transvaal

  • Authors: Coetzee, Louis Lodewyk
  • Date: 2009-01-27T07:18:41Z
  • Subjects: Stratigraphic geology , Pretoria Group (South Africa)
  • Type: Thesis
  • Identifier: uj:14830 , http://hdl.handle.net/10210/1954
  • Description: M.Sc. , The sedimentary succession of the Paleoproterozoic Pretoria Group is very important for understanding Earth’s ancient history. It represents a time of extreme environmental changes on Earth, from global ice-ages to hot-houses. However, the genetic stratigraphy of the succession is poorly understood so that the stratigraphic relationships between the events remain uncertain. This dissertation provides a genetic stratigraphic model of the succession by utilising an integrated sedimentological and geochemical approach which culminates in a new sequence stratigraphic subdivision of the Pretoria Group. The study focuses on the Potchefstroom area in the western part of the Transvaal depository. The Pretoria Group commences with the Rooihoogte Formation which overlies the Chuniespoort Group with erosional contact. New stratigraphic data indicates that the Rooihoogte Formation is a correlative of the Duitschland Formation in the eastern Transvaal. The succession was deposited in a foreland basin. An important new finding is that a diamictite at the base of the formation contains striated and bull-nosed pebbles and is of glacial origin. The discordantly overlying Timeball Hill Formation is composed of a coarsening upward carbonaceous shale – hematite oolite-bearing quartzite unit overlain by a second carbonaceous shale, capped by a second glacial diamictite (the well known Rietfonteindam diamictite). The oolitic ironstones in the quartzites suggest that they formed in a warm oxidizing environment. The shales display mature chemical indices of alteration which supports this theory. ä13Corganic values increase from –35‰ to –24‰ from the bottom to the top of the Timeball Hill Formation indicating net carbon burial, which translate to a decrease in atmospheric CO2 and colder climates as deposition evolved. In turn this can be linked to the presence of the glacial Rietfonteindam diamictite in the upper part of the Timeball Hill Formation. The Rietfonteindam diamictite is overlain by conglomerate, quartzite and shale of the Boshoek Formation, which were deposited as an upwards fining transgressive sedimentary unit following on post-glacial eustatic sea-level rise. It is in turn overlain by the 2.22Ga. Hekpoort basalt. This basalt is metasomatically altered, but has remained virtually unaffected by regional metamorphism, as shown by detailed SEM petrographic analyses. Excellent examples of zeolite- filled amygdales are preserved in the lavas. The Hekpoort lavas are overlain by fluvial red beds of the Dwaalheuwel Formation. A lateritic paleosol (Hekpoort paleosol) is developed below the red bed succession. The red beds are overlain with sharp gradational contact by the carbonaceous shelf mudstone of the Strubenskop Formation which grades up into the shallow marine Daspoort quartzite. The Silverton Formation, mainly composed of carbonaceous shale, overlies the Daspoort Formation with sharp gradational contact and grades upwards into shallow marine Magaliesberg quartzite. ä13Corganic values decrease from –25‰ to –29‰, from middle to top of the Silverton Formation, most probably indicating carbon input into the atmosphere and therefore rising atmospheric temperature. The Machadodorp lava, which was previously thought to be restricted to the eastern part of the Transvaal basin, was found to be present in the Potchefstroom area as well. Five unconformity-bounded sequences are present in the succession. Estimates are that they were deposited in time intervals of 60m.y. each.
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The Pongola Supergroup in Swaziland

  • Authors: Nhleko, Noah
  • Date: 2009-01-28T09:42:41Z
  • Subjects: Geology , Stratigraphic geology , Geochemistry , Paleomagnetism
  • Type: Thesis
  • Identifier: uj:14843 , http://hdl.handle.net/10210/1966
  • Description: D.Phil. , The Mesoarchean Pongola Supergroup, cropping out in the southeastern region of the Kaapvaal craton, is one of the oldest known supracrustal successions in the world. It represents an erosional remnant of a once extensive cratonic cover sequence. The succession is subdivided into the lower Nsuze Group, a volcano sedimentary succession, and the Mozaan Group composed mainly of siliciclastics and minor volcanics. The Mozaan Group is host to the world’s oldest known glacial deposit. The Pongola Supergroup is well preserved both geochemically and structurally. Outcrops extend from the northern part of KwaZulu-Natal and Mpumalanga Province in South Africa into Swaziland. This study presents results of an integrated stratigraphic, sedimentological, geochemical, geochronological, and paleomagnetic investigation of the Pongola Supergroup in Swaziland and drill cores from the Nongoma Graben in KwaZulu-Natal. The Nsuze Group displays marked cyclicity between volcanic and siliciclastic rocks that were probably deposited in an intracratonic ‘sag and dome’ basin. The volcanicsedimentary cycles are thought to represent periodic heat loss from a hot regional mantle beneath the Kaapvaal craton. In the sag basins the rate of subsidence was gradual and sedimentation marked by near shore deposition. Volcanism in the Agatha Formation was episodic and displays a cyclicity of 2-14Myr in duration. Magmatic eruption was marked by the development of a low crustal level magma chamber. Crustal contamination trough assimilation and fractional crystallization at these low crustal level magma storages is recorded by compositional bimodal volcanism of basaltic andesite-rhyodacite and andesite-rhyolite association. After cessation of the volcanism of the Nsuze Group, subsequent development of the Pongola basin was marked by thermal subsidence and marine transgression in an epicratonic basin, at the time of deposition of the Mozaan Group. The Mozaan Group overlies the Nsuze Group with an erosive unconformity developed over an in situ weathering profile, i.e. a paleosol. The Mozaan Group consists of alternating quartzite, shale, conglomerate, iron-formation. Three units of contemporaneous flood basalt, namely the Tobolsk, Gabela and Ntanyana formations are interbedded with the siliciclastic deposits in the upper part of the succession. Unimodal paleocurrent directions based on fluvial quartzites indicate initial provenances to the south and north. This indicates that the basin morphology was in a form of a trough. Eventually paleocurrent distribution patterns turn southeast, and marine flooding extended further towards the west, to form the Greater Witwatersrand Basin, which was then modified by development of a foreland basin towards the west and northwest. This suggests that the present outcrops of the Mozaan Group represent mere remnants of an extensive basin. Siliciclastic deposits of the Mozaan succession harbours a wealth of information on the crust-forming events that affected the Kaapvaal craton. Detrital zircons from quartzites and diamictite samples yield ages that record magmatic events that extend from the early (ca. 3.6Ga) to middle (2.89Ga) Archean. Late magmatic events were coeval with the development of the Mozaan basin. The Mozaan succession also is host to the oldest known glacial deposits, namely the Klipwal and the Mpatheni Member diamictites. The absence of deformation, erosional contacts and the presence of incorporated underlying material suggest that grounded glaciers were not the mode of deposition of these diamictites, but that the glacial deposits can be explained as gravity flows from collapse of oversteepened slopes of low relief glaciomarine setting that degenerated in more distal part of the basin into clast poor turbidity flows. Analysis of shale and matrix composition in diamictites show that mechanical erosion processes dominated the source area. CIA values range between 70 and 81, suggesting negligible chemical weathering in the sediment source area. CIA values increase only in the upper part of the stratigraphy, i.e. Ntanyana Formation. Incompatible to compatible trace element ratios are low and suggest that mafic and ultramafic rock dominated the source area. This, perhaps, demonstrates that the greenstone belts were a major source of detritus to the Mozaan basin. Well-constrained paleomagnetic data sets acquired from the Klipwal Member diamictite and Tobolsk lava give a very good estimate of the paleogeographic setting of the Kaapvaal craton during the deposition of the Mozaan Group. The Klipwal diamictite was apparently deposited at high-paleolatitude setting of 48°. The craton then moved slightly to the north to latitude ~43° at the time of eruption of the Tobolsk lavas. From the results it appears that available geochemical classification schemes based on the composition of Phanerozoic volcanic rocks are not suitable to classify the lavas of the Agatha Formation unequivocally. To arrive at any tectonic model for these igneous rocks it is necessary to consider stratigraphic relationships, physical volcanology and geochemical characteristics of the rock succession. On the other hand, the wellconstrained paleomagnetic data indicate that the global climate system in the Mesoarchean was similar to modern day earth where glacial deposits are constrained largely to Polar Regions.
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Geological controls on no. 4 seam roof conditions at New Denmark Colliery, Highveld Coal Field, Karoo Basin, South Africa

  • Authors: Stanimirovic, Jasmina
  • Date: 2009-01-28T09:43:30Z
  • Subjects: Facies (Geology) , Coal , Stratigraphic geology , Sedimentology , Mine roof control , Karoo Supergroup , Mpumalanga (South Africa)
  • Type: Thesis
  • Identifier: uj:14849 , http://hdl.handle.net/10210/1971
  • Description: M.Sc. , The coal-bearing Permian Vryheid Formation of the Ecca Group (Karoo Supergroup) was investigated at New Denmark Colliery, situated in the north east section of the Karoo Basin, South Africa. The lithostratigraphy of the sequence is defined in terms of conventional lithostratigraphic terminology but also by applying detailed genetic stratigraphic schemes that have previously been proposed for the adjacent coalfields. The succession is divided up into depositional sequences named after the underlying and overlying coal seams, the No. 2, 3, 4 and 5 seam sequences. The sedimentary succession was divided up into five facies, namely: conglomerate facies, sandstone facies, interlaminated sandstone-siltstone facies, siltstone facies and coal facies. These were interpreted hydrodynamically. Facies assemblages were then interpreted palaeoenvironmentally. Glacial, fluvial, deltaic and transgressive marine sequences were responsible for forming this sedimentary succession. Attention was then focussed on the main economic No. 4 seam, which is mined underground at the colliery. Detailed subsurface geological cross-sections, core sequences and isopach maps of the No. 4 seam coal and the lithologies above, were used to determine specific aspects of the depositional environment that could contribute to unstable roof conditions above No. 4 seam. Coarsening-upward deltaic cycles, fining-upward bedload fluvial cycles, glauconite sandstone marine transgressions and crevasse-splay deposits are recognized in the overlying strata. Poor roof conditions occur parallel to palaeochannel margins because the interbedded channel sandstone and adjacent flood plain argillites cause collapsing along bedding plane surfaces. Rider coals overlying thin crevasse-splay sequences in close proximity to the No. 4 seam, create one of the most serious roof conditions; complete collapse occurs along the rider coal contact with the underlying splay deposits. Differential compaction of mudrock/shale/siltstone over more competent sandstone causes slickensided surfaces that weaken the roof lithologies. Correct identification of these sedimentological features will enable the prediction of potential poor roof conditions during mining operations and mine planning.
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Provenance of ordovician to silurian clastic rocks of the Argentinean precordillera and its geotectonic implications

  • Authors: Abre, Paulina
  • Date: 2009-03-31T09:20:36Z
  • Subjects: Structural geology , Stratigraphic geology , Historical geology , Argentina
  • Type: Thesis
  • Identifier: uj:8232 , http://hdl.handle.net/10210/2344
  • Description: D. Phil. , A Mesoproterozoic basement and a Cambrian-Ordovician carbonate platform characterize the Precordillera terrane. These characteristics and its distinct geologic history mark a difference between this suspected exotic-to-Gondwana terrane and the Gondwanan autochthonous, leading to speculation that the Precordillera was derived from Laurentia. The surprising similarities of the carbonate sequences between the Precordillera and certain parts of southeast Laurentia suggest a common geological history. However, other models interpret the origin of the Precordillera terrane as being para-autochthonous with respect to Gondwana. All these models are still controversial. A combination of several methodologies including petrography and heavy minerals analysis, geochemistry, Sm-Nd and Pb-Pb isotopes and zircon dating were applied to several Ordovician and Ordovician to Silurian units of the Precordillera terrane. Geochemistry and petrography indicates that all the Formations studied have similar characteristics, with at least two sources providing detritus to the basin. The dominant source has an unrecycled upper continental crust composition whereas the other component is more depleted. The study of detrital chromian spinels suggests that mid-ocean ridge basalts, continental intraplate flood basalts and ocean island basaltrelated rocks were among the sources for the detrital record of the Precordillera terrane. Nevertheless, the mafic sources and their ages remain unknown. Nd isotopes account for negative εNd values and TDM ages in a range of variation found elsewhere within Gondwana and basement rocks of the Precordillera. The Sm/Nd ratios of certain samples indicate fractionation of LREE. Pb isotopes indicate that a source with high 207Pb/204Pb was important, and point to Gondwanan sources. Detrital zircon dating constrain the sources as being dominantly of Mesoproterozoic age (but with a main peak in the range 1.0 to 1.3 Ga), with less abundant populations of Neoproterozoic (with a main peak in the range 0.9 to 1.0 Ga), Palaeoproterozoic, Cambrian and Ordovician ages in order of abundance. i The uniformity shown by the provenance proxies indicate that there were no important changes in the provenance from the Lower Ordovician until the Early Silurian. Several areas are evaluated as sources for the Precordillera terrane. The rocks that fit best all the provenance constraints are found within the basement of the Precordillera terrane and the Western Pampeanas Ranges. Basement rocks from the Arequipa-Antofalla area (Central Andes) also match the isotopic characteristics, but a northern source is less probable, except for the Western tectofacies. On the other hand, areas such as Antarctica, Falklands/Malvinas Microplate, the Natal-Namaqua Metamorphic belt and the Grenville Province of Laurentia can be neglected as sources. The proposal of these areas as sources is in agreement with palaeocurrents and facies analyses and suggests proximity between them and the Precordillera since at least the Late Arenig to Early Llanvirn. This has important implications for the proposed models regarding the geotectonic evolution of the Precordillera terrane. The models would need to be adjusted to the here proposed youngest timing of collision.
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Lithostratigraphy, depositional environments and sedimentology of the Permian Vryheid Formation (Karoo Supergroup), Arnot North, Witbank Coalfield, South Africa

  • Authors: Uys, Joanne
  • Date: 2009-04-30T09:28:39Z
  • Subjects: Stratigraphic geology , Sedimentology , Facies (Geology) , Lithofacies , Karoo Supergroup , Mpumalanga (South Africa)
  • Type: Thesis
  • Identifier: uj:8338 , http://hdl.handle.net/10210/2464
  • Description: M.Sc. , This work documents the lithostratigraphy and interpreted depositional environments of the Permian Vryheid Formation in the most northern proximal setting yet studied in the Witbank Coalfield. Data from 924 boreholes from two mining companies (Anglo Operations Ltd. and Xstrata Coal Ltd.) drilled over 50 years, covering an area of 910km2 revealed a 35m sequence of terrigenous clastic sedimentary rocks containing two coal seams. These seams are numbered No. 1 at the base and No. 2 at the top. Delineation of facies type, facies assemblages, lateral facies distributions and computer-based three-dimensional modeling facilitated the interpretation of the palaeodepositional environments. Eleven lithofacies are defined and interpreted hydrodynamically. Facies classification is based primarily on grain size and sedimentary structures. The modeling of the borehole information uses the finite element method to interpolate the thickness, roof and floor surfaces and trend of each seam and inter-seam parting between boreholes. The spatial position of the boreholes is defined using a digital terrain model that represents the current surface topography. Lateral distributions were correlated by repositioning the boreholes using the base of the No. 2 seam as a datum. Glaciofluvial, glaciolacustrine, bed-load (braided) fluvial and constructive progradational deltaic environments are interpreted in the study area. Fluvial channel sequences are dominant and cause the thinning of the coal seams below channel axes as well as splitting of both the No. 1 and No. 2 seams. Glaciofluvial influences also affect the lower portion of the No. 1 seam. Basement palaeotopography restricts the distribution of the lower splits of the No. 1 seam. The coals either ‘pinch-out’ or are absent above basement highs but blanket the adjacent low-lying areas. In contrast to the greater Witbank Coalfield, but concurrent with other studies in the more northern proximal regions, fluvial systems dominate over deltaic systems in the study area. Glaciodeltaic, fluviodeltaic and anastomosed channel fluvial systems recognized in the remainder of the Karoo Basin were fed by the braided fluvial systems in the study area. The close proximity of the study area to the northern edge of the basin accounts for the subtle differences in lithostratigraphy and interpreted depositional environments when compared with more distal sites to the south. For example, glaciofluvial clastic sediment input in the lower portions of the No. 1 seam and post-Karoo erosion that has removed the overlying seams; the deltaic progradational sequence, above the No. 2 seam, occurs twice in succession and the bioturbation, that has become characteristic of sedimentary sequence of the Vryheid Formation above the No. 2 seam in the central and southern parts of the Karoo Basin, is not as identifiable. These differences are explained by the extreme proximal location of the study area on the northern basin margin relative to the remainder of the Karoo Basin.
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Diagenetic carbonates and biogeochemical cycling of organic matter in selected Archean-Paleoproterozoic sedimentary successions of the Kaapvaal Craton, South Africa

  • Authors: Cochrane, Justin Michael
  • Date: 2010-06-03T05:38:44Z
  • Subjects: Stratigraphic geology , Geochemistry , Petrology , Mineralogy , Sedimentation and deposition , Kaapvaal Craton (South Africa)
  • Type: Thesis
  • Identifier: uj:6855 , http://hdl.handle.net/10210/3288
  • Description: M.Sc. , The Kaapvaal craton is one of few regions on earth with an almost continuous record of wellpreserved supracrustal rocks ranging in age from ~3.5 Ga to the late Paleoproterozoic at ~1.75 Ga. In this study diagenetic carbonates from the Paleoarchean Buck Reef Chert and Joe’s Luck Formation of the Swaziland Supergroup, the Mesoarchean Thalu and Promise Formations of the Mozaan/Witwatersrand Supergroups and the Paleoproterozoic Timeball Hill and Silverton Formations of the Transvaal Supergroup were sampled and analyzed. The aim of the study was to determine possible variations in the composition of the carbonates through time and their significance especially with regards to microbial activity in diagenetic systems in early Earth history. Results indicate similar petrographic observations and geochemical signatures in diagenetic carbonates of iron formations in the Buck Reef Chert, Joe’s Luck and Griquatown Iron Formation. The carbonates all tend to be siderites with iron derived from hydrothermal input and all are depleted in 13C relative to Peedee Belemnite standard. It suggested that siderite formed as a result of microbial respiration. Microbes degrade organic matter and reduce iron in this process. This resulted in the depletion in 13C and in the precipitation of siderite. However in order for iron reduction to have occurred the reduced iron first had to be oxidized. This most probably occurred through iron oxidizing chemolithoautotrophs under microaerophilic conditions. Diagenetic carbonate concretions of the Thalu and Promise Formations are manganiferous and are highly depleted in 13C relative to PDB. There is also strong evidence for hydrothermal input of manganese and iron into the system because of positive europium anomalies. The carbonates from both of the formations strongly suggest the presence of some free oxygen. The reasoning behind this conclusion is as follows: The depletion of 13C in the carbonates points to microbial decomposition of organic matter and manganese respiration (the decomposition of organic matter by microbial MnO2 reduction) is shown to be the most reasonable process that led to the formation of the carbonate concretions. The implication is that MnO2 must first have been precipitated and that can only be achieved in the presence of free oxygen with the oxidation reaction often catalyzed by manganese oxidizing chemolithoautotrophs. The carbonates of the Timeball Hill and Silverton Formationsare calcites ad contain little no iron. There is also little or no evidence for hydrothermal input and the basin appears to be a clastic dominated. It is generally accepted that a major rise in oxygen in the oceans and the atmosphere occurred at about 2.32 Ga. This rise in oxygen levels is reflected in the diagenetic calcite concretions of the Silverton Formation. Both iron and manganese reduction where not very effective because of the depletion in the basin water of these two elements, organic carbon taken up in the calcite concretions, indicated by negative δ13CPDB carbonate values, was most probably derived from aerobic and/or nitrate respiration. The most important conclusion from this study is that sufficient free oxygen and hence oxygenic photosynthesis were present to oxidize both Fe and Mn at least as far back as the Paleo-Mesoarchean.
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