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The evolution of the Brosterlea Volcanic Complex, Eastern Cape, South AfricaSurtees, Grant Bradley January 2000 (has links)
Detailed field mapping (Map, Appendix B) has been conducted in and around the boundaries of a 14x18km, volcanic complex 35km northeast of Molteno in the Eastern Cape Province, South Africa. The structure is interpreted as a subsidence structure, and is filled with two volcaniclastic breccias, numerous lava flows, a number of sedimentary facies, and lies on a base of Clarens Formation overlying Elliot Formation rocks. This is an important study because 'widespread, voluminous fields of basaltic breccias are very rare (see Hanson and Elliot, 1996) and this is the first time that this type of volcanic complex and its deposits have been described. Detailed analyses of the two volcaniclastic breccias revealed changes in colour, clast types, clast sizes, and degree of alteration over relatively short distances both vertically and laterally within a single breccia unit. The variation in clast sizes implies a lack of sorting of the breccias. The lower of the two volcaniclastic breccias fills the subsidence structure, and outcrops between the Stormberg sedimentary sequence and the overlying Drakensberg basalts and was produced from phreatomagmatic eruptions signalling the start of the break-up of Gondwanaland in the mid-Jurassic. The upper volcaniclastic breccia is interbedded with the flood basalts and is separated from the lower breccia by up to 100m of lava flows in places, it is finer-grained than the lower volcaniclastic breccia, and it extends over 10km south, and over 100km north from the volcanic complex. The upper breccia is inferred to have been transported from outside the study area, from a source presumably similar to the subsidence structure in the volcanic complex. The pyroclastic material forming the upper breccia was transported to the subsidence structure as a laharic debris flow, based on its poorly sorted, unwelded and matrix-supported appearance. However, both breccias are unlikely to have been derived from epiclastic reworking of lava flows as they contain glass shards which are atypical of those derived from the autoclastic component of lava flows. The breccias are therefore not "secondary" lahars. There is also no evidence of any palaeotopographic highs from which the breccias could have been derived as gravity-driven flows. Based on the occurrence of three, 1m thick lacustrine deposits, localised peperite, fluvial reworking of sandstone and breccia in an outcrop to the south of the subsidence structure, and channel-lags encountered only in the upper units of the Clarens Formation and only within the subsidence structure, the palaeoenvironment inferred for the subsidence structure is one of wet sediment, possibly a shallow lake, in a topographic depression fed by small streams. Magmatic intrusions below the subsidence structure heated the water-laden, partly consolidated Clarens Formation sandstones, causing the circulation of pore fluid which resulted in the precipitation of minerals forming pisoliths in the sandstones. Intruding magma mixed, nonexplosively, with the wet, unconsolidated sediments near the base of the Clarens Formation (at approximately 100m below the surface), forming fluidal peperite by a process of sediment fluidisation where magma replaces wet sediment and cools slowly enough to prevent the magma fracturing brittly. Formation of fluidal peperite may have been a precursor to the development of FCIs (Fuel Coolant Interactions) (Busby-Spera and White, 1987). The breccias may represent the products of FCIs and may be the erupted equivalents of the peperites, suggesting a possible genetic link between the two. The peperites may have given way to FCI eruptions due to a number of factors including the drying out of the sediments and/or an increase in the volume of intruded magma below the subsidence structure which may have resulted in a more explosive interaction between sediment and magma. Phreatic activity fragmented and erupted the Clarens Formation sandstone, and stream flows reworked the angular sandstone fragments, pisoliths and sand grains into channelised deposits. With an increase in magmatic activity below the subsidence structure, phreatic activity became phreatomagmatic. The wet, partly consolidated Clarens Formation, and underlying, fully consolidated Elliot Formation sediments were erupted and fragmented. Clasts and individual grains of these sediments were redeposited with juvenile and non-juvenile basaltic material probably by a combination of back fall, where clasts erupted into the air fell directly back into the structure, and backflow where material was erupted out of the structure, but immediately flowed back in as lahars. This material formed the lower volcaniclastic breccia. A fault plane is identified along the southwestern margin of the subsidence structure, and is believed to continue up the western margin to the northwestern corner. A large dolerite body has intruded along the inferred fault plane on the western margin of the structure, and may be related to the formation of the lower volcaniclastic breccia, either directly through fluidisation of wet sediment during its intrusion, or as a dyke extending upwards from a network of sill-like intrusions below the subsidence structure. Geochemical analysis of the Drakensberg basalt lava flows by Mitchell (1980) and Masokwane (1997) revealed four distinct basalt types; the Moshesh's Ford, the Tafelkop, the Roodehoek, and the Vaalkop basalts. Basalt clasts sampled from the lower volcaniclastic breccia were shown to belong to the Moshesh's Ford basalt type which does not outcrop in situ within the subsidence structure. This implies that the Moshesh's Ford basalts were emplaced prior to the formation of the lower volcaniclastic breccia, and may have acted as a "cap-rock" over the system, allowing pressure from the vaporised fluids, heated by intruding basalt, to build up. The Moshesh's Ford basalt type was erupted prior to the resultant phreatomagmatic events forming the lower volcaniclastic breccia.
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Carbonate alteration of serpentinite in the Murchison Greenstone Belt, Kaapvaal craton : implications for gold mineralization.Madisha, Moropa Ebenezer 15 August 2012 (has links)
M.Sc. / Please refer to full text to view abstract
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The tectonic evolution of the rocks comprising the Venetia Klippe, Limpopo Belt, South Africa, with emphasis on carbonate and calc-silicate rocks and pegmatiteTwiggs, C. 16 August 2012 (has links)
M.Sc. / This thesis involves a study ofthe geology surrounding the —530 Ma to —519 Ma Venetia kimberlite pipes situated between AIldays and Messina (now renamed Musina) in the Beit Bridge Terrane of the Limpopo Belt, South Africa. The Limpopo Belt is an eastnortheast trending high grade metamorphic terrane thought until recently to be the result ofa collisional event between the Kaapv_aal and Zimbabwe Cratons between 2.7 and 2.65 Ga. However, recent studies have challenged this concept and suggest that the assembly was more complex and took place over an extended period of time ending at —2.04 Ga. This study involved surface mapping of the Farms Rugen (south) and Ostrolenca, providing additional information to help with mine planning, grade control and exploration. It forms a portion of a project initiated between Venetia Mine, the Venetia- Limpopo Nature Reserve and Professor Jay Barton of RAU to geologically map in detail the area around the pipes (scale < 1:10 000) and to study various aspects of the regional geology. The rock types into which the Venetia kimberlite pipes intruded belong to the Venetia klippe, an east-west trending synclinal structure with the axial plane dipping steeply northwards. Lithologically, the Venetia klippe comprises four layered units in which interlayered granitic or arkosic quartzofeldspathic gneisses, with and without biotite and garnet, and para and ortho-amphibolite, quartzite and meta-carbonate rocks (marble and limestone), banded iron formation and calc-silicate rock occur. Geochemical analysis (SEM and electron microprobe) of the meta-carbonates (re-crystallised magnesian limestone, coarse-grained marble and fine-grained foliated marble), indicate the precursors to be magnesian limestone, dolomite and limestone. Several events have been identified during the structural evolution of the area. They include: formation of gneissic metamorphic layering, tectonic suturing between different lithologies, formation of a syncline and east-west strike-slip faulting, north-south trending folds and northeast-southwest dextral strike-slip faulting, tourmaline bearing pegmatite emplacement, dolerite intrusion, tourmaline absent pegmatite emplacement, kimberlite emplacement and reactivation of pre-existing structures. Depositional structures only in the fine-grained foliated marble are preserved, e.g. graded bedding, cross-bedding, rip-up clasts and channels. These structures suggest deposition of the carbonates in two main depositional environments; peritidal (channels and rip-up clasts) and subtidal shelf (graded bedding and cross-bedding). A study of pegmatites in the area shows two main pegmatite types: tourmaline bearing and tormaline absent, each locally displaying a zonation. Mineralogically, the pegmaties are rich in quartz and albite and lack K-feldspar so they are classified as sodic-rich or plagio-pegmatites. Step heating 40Ar/39Ar analyses of muscovite from undeformed pegmatite yields an age of —2.0 Ga, which is interpreted to represent the time of pegmatite emplacement into the Venetia klippe rocks. Structurally, the pegmatites are sheet-like bodies cross-cutting compositional layering, joints, faults, folds and the dolerite, except for the older tourmaline bearing pegmatite that has intruded along east-west faults, but not northeast-southwest trending faults. By applying the principles of a dike propagation model, the source of the Venetia pegmatites should be greater than 5X5X5 km in volume and a maximum of 10km away. An appropriate granitic source has been recognized on the farm Gotha to the south of the mine by Martina Barnett. Leucocratic granodiorite, tonalite and granite with minor xenoliths of amphibolite, quartzite and magnetite quartzite define the Gotha Granitic Complex and pegmatite decreases in abundance away from it to the north and east. Deposition of Unit 3 lithologies into a rifted basin and an ancient epeiric sea is possible. However, there is more evidence (peritidal and shelf environments of the metacarbonates) and clean mature quartzites to suggest deposition into a passive continental margin or epeiric sea similar to the Malmani dolomites of the Transvaal Supergroup.
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Structure and evolution of basin and petroleum systems within a transformrelated passive margin setting : data-based insights from crust-scale 3D modelling of the Western Bredasdorp Basin, offshore South AfricaSonibare, Wasiu Adedayo 04 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: This study investigates the crustal structure, and assesses the qualitative and quantitative impacts of crust-mantle dynamics on subsidence pattern, past and present-day thermal field and petroleum
system evolution at the southern South African continental margin through the application of a
multi-disciplinary and multi-scale geo-modelling procedure involving both conceptual and
numerical approaches. The modelling procedure becomes particularly important as this margin
documents a complex interaction of extension and strike-slip tectonics during its Mesozoic
continental rifting processes. Located on the southern shelf of South Africa, the Western
Bredasdorp Basin (WBB) constitutes the focus of this study and represents the western section of
the larger Bredasdorp sub-basin, which is the westernmost of the southern offshore sub-basins. To
understand the margin with respect to its present-day structure, isostatic state and thermal field, a
combined approach of isostatic, 3D gravity and 3D thermal modelling was performed by integrating
potential field, seismic and well data. Complimenting the resulting configuration and thermal field
of the latter by measured present-day temperature, vitrinite reflectance and source potential data,
basin-scale burial and thermal history and timing of source rock maturation, petroleum generation,
expulsion, migration and accumulation were forwardly simulated using a 3D basin modelling
technique. This hierarchical modelling workflow enables geologic assumptions and their associated
uncertainties to be well constrained and better quantified, particularly in three dimensions.
At present-day, the deep crust of the WBB is characterised by a tripartite density structure (i.e. prerift
metasediments underlain by upper and lower crustal domains) depicting a strong thinning that is
restricted to a narrow E-W striking zone. The configuration of the radiogenic crystalline crust as
well as the conductivity contrasts between the deep crust and the shallow sedimentary cover
significantly control the present-day thermal field of the study area. In all respects, this present-day
configuration reflects typical characteristics of basin evolution in a strike-slip setting. For instance,
the orientations of the deep crust and fault-controlled basin-fill are spatially inconsistent, thereby
indicating different extension kinematics typical of transtensional pull-apart mechanisms. As such,
syn-rift subsidence is quite rapid and short-lived, and isostatic equilibrium is not achieved,
particularly at the Moho level.
Accompanied syn-rift rapid subsidence and a heat flow peak led to petroleum preservation in the
basin since the Early Cretaceous. Two additional post-rift thermal anomalies related to the Late
Cretaceous hotspot mechanism and Miocene margin uplift in Southern Africa succeeded the syn-rift
control on maturation. This thermal maturity of the five mature source rocks culminated in four
main generation and three main accumulation phases which characterise the total petroleum systems
of the WBB. The Campanian, Eocene and Miocene uplift scenarios episodically halted source
maturation and caused tertiary migration of previously trapped petroleum. Petroleum loss related to
the spill point of each trap configuration additionally occurs during the Late Cretaceous-Paleocene
and Oligocene-Early Miocene. The timing and extent of migration dynamics are most sensitive to
the geological scenario that combined faulting, intrusive seal bypass system and facies
heterogeneity. In fact, for models that do not incorporate facies heterogeneity, predicted past and
present-day seafloor leakage of petroleum is largely underestimated. This complex interplay of
generation and migration mechanisms has significant implications for charging of petroleum
accumulations by multiple source rocks. Due to early maturation and late stage tertiary migration,
the syn-rift source rocks particularly Mid Hauterivian and Late Hauterivian source intervals
significantly control the extent of petroleum accumulation and loss in the basin.
Lastly, the modelled 3D crustal configuration and Mezosoic to Cenozoic thermal regime of the
WBB dispute classic uniform lithospheric stretching for the southern South African continental
margin. Rather, this PhD thesis confirms that differential thinning of the lithosphere related to a
transtensional pull-apart mechanism is the most appropriate for accurately predicting the evolution
of basin and petroleum systems of the margin. Also, the presented 3D models currently represent
the most advanced insights, and thus have clear implications for assessing associated risks in basin
and prospect evaluation of the margin as well as other similar continental margins around the world. / AFRIKAANSE OPSOMMING: Hierdie studie ondersoek die korsstruktuur en evalueer die kwalitatiewe en kwantitatiewe impakte
van kors-mantel-dinamika op insinkingspatroon, die termiese veld en petroleumstels evolusie aan
die suidelike Suid-Afrikaanse kontinentale grens, in die hede en die verlede, deur die toepassing van
’n multidissiplinêre en multiskaal-geomodelleringsprosedure wat beide konseptuele en numeriese
benaderings behels. Die modelleringsprosedure veral is belangrik aangesien hierdie kontinentale
grens ’n komplekse interaksie van uitbreidings- en strekkingsparallelle tektoniek gedurende die
Mesosoïese vastelandskeurprosesse daarvan dokumenteer. Omdat dit op die suidelike platvorm van
Suid-Afrika geleë is, maak die Westelike Bredasdorp Kom (WBK) die fokus van hierdie studie uit,
en verteenwoordig dit die westelike deel van die groter Bredasdrop-subkom, wat die verste wes is
van die suidelike aflandige subkomme. Om die grens met betrekking tot sy huidige struktuur,
isostatiese staat en termiese veld te verstaan, is ’n kombinasie benadering bestaande uit isostatiese,
3D-gravitasie- en 3D- termiese modellering gebruik deur potensiëleveld-, seismiese en boorgatdata
te integreer Ondersteunend totot die gevolglike konfigurasie en termiese veld van die laasgenoemde
deur middel van hedendaagse temperatuur, soos gemeet, vitriniet-refleksiekoëffisiënt en bronpotensiaal
data, komskaal-begrawing en termiese geskiedenis en tydsberekening van
brongesteentematurasie, is petroleumgenerasie, -uitwerping, -migrasie en -akkumulasie in die
toekoms gesimuleer deur gebruik te maak van ’n 3D-kommodelleringstegniek. Hierdie hierargiese
modelleringswerkvloei maak dit moontlik om geologiese aannames en hulle geassosieerde
onsekerhede goed aan bande te lê en beter te kwantifiseer, veral in drie dimensies.
In die hede word die diep kors van die WBK gekarakteriseer deur ’n drieledige digtheidstruktuur
(met ander woorde voorrift-metasedimente onderlê deur bo- en benedekors domeine) wat dui op ’n
baie wesenlike verdunning, beperk tot ’n dun O-W-strekkingsone. Die konfigurasie van die
radiogeniese kristallyne kors, sowel as die konduktiwiteitskontraste tussen die diep kors en die vlak
sedimentêre dekking, beheer grotendeels die hedendaagse termiese veld van die studiearea. Hierdie
hedendaagse konfigurasie weerspieël in alle opsigte tipiese eienskappe van kom-evolusie in ’n
skuifskeur omgewing. Byvoorbeeld, Die oriëntasies van die diep kors en verskuiwingbeheerde
komsedimentasie byvoorbeeld is ruimtelik inkonsekwent en dui daardeur op verskillende
ekstensiekinematika, tipies van transtensionale tensiemeganisme. As sulks, is sin-rift-versakking
taamlik vinnig en kortstondig, en word isostatiese ekwilibrium nie by die Moho-vlak, in die
besonder, bereik nie.
Samehangende sin-rift vinnige versakking en hittevloeihoogtepunt het gelei tot petroleum behoud in die kom sedert die vroeë Kryt. Twee bykomende post-rift termiese anomalieë wat verband hou met
die laat Kryt-“hotspot” meganisme en die Mioseense kontinentale grensopheffing in Suidelike
Afrika het die sin-rift-beheer met maturasie opgevolg. Hierdie termiese maturiteit van die vyf
gematureerde brongesteentes het in vier hoofgenerasie- en drie hoofakkumulasie fases, wat die
totaliteit van die petroleumstelsels van die WBK karakteriseer, gekulmineer. Die Campaniese,
Eoseense en Mioseense opheffings senarios het episodies bronmaturasie gestop en tersiêre migrasie
van petroleum wat vroeër opgevang was veroorsaak. Addisioneel vind petroleumverlies gekoppel
aan die spilpunt van elke opvanggebiedkonfigurasie tydens die laat Kryt-Paleoseen en Oligoseenvroeë
Mioseen plaas. Die tydstelling en omvang van migrasiedinamika is die sensitiefste vir die
geologiese scenario wat verskuiwing, seëlomseilingstelsel en fasiesheterogeniteit kombineer.
Trouens, vir modelle wat nie fasiesheterogeniteit inkorporeer nie, is voorspellings van vroeëre en
huidige seebodemlekkasie van petroleum grotendeels onderskattings. Hierdie komplekse
wisselwerking van generasie- en migrasiemeganismes het beduidende implikasies vir die laai van
petroleumakkumulasies deur veelvoudige brongesteentes. Vanweë vroeë maturasie en laatstadiumtersiêre
migrasie, oefen die sin-rift-brongesteentes, veral middel Hauterivium- en laat Hauteriviumbronintervalle,
beduidende beheer oor die omvang van petroleumakkumulasie en -verlies in die
kom uit.
Laastens weerspreek die gemodelleerde 3D-korskonfigurasie en Mesosoïese-tot-Senosoïesetermiese
regime van die WBK ’n klassieke uniforme litosferiese rekking vir die suidelike Suid-
Afrikaanse kontinentale grens. Inteendeel, hierdie PhD-proefskrif bevestig dat ’n differensiële
verdunning van die litosfeer, gekoppel aan ’n transtensiemeganisme, die beste geskik is om ’n
akkurate voorspelling oor die evolusie van kom- en petroleumstelsels van die kontinentale grens
mee te maak. Verder, verteenwoordig die 3D-modelle, wat hier aangebied word, tans die mees
gevorderde insigte, en het hierdie modelle dus duidelike implikasies vir die assessering van
verwante risiko’s in kom- en petroleum teikene valuering van die kontinentale grens, so wel as van
ander soortgelyke kontinentale grense regoor die wêreld.
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