Thesis (PhD)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: Foreland basins commonly fill with sediment derived from the adjacent fold/thrust belt,
providing a relatively simple source-to-basin configuration. However, that is not true for the
early southwestern Karoo Basin, since the composition of the Ecca Group sedimentary rocks
do not match the composition of the adjacent fold/thrust belt.
The southwestern Karoo Basin is bordered to the west and south by the Cape Fold Belt
(CFB) and provides the opportunity to study the linkage between its early structural evolution
and deposition in the two spatially and temporally distinct Tanqua and Laingsburg
depocentres. The CFB was formed when the early Palaeozoic passive continental margin,
which formed a large section of the southern edge of Gondwana, evolved into an active
convergent margin during the late Palaeozoic. Orogenesis resulted in a northwest-trending
Cedarberg branch and an eastwest-trending Swartberg branch. The oroclinal bend between
the two branches includes large-scale northeast-trending syntaxis structures, such as the Hex
River and Baviaanshoek anticlinoria, which influenced the sedimentation path into the basin.
Spectral gamma ray (SGR), mineralogical and geochemical studies of exposed rocks
from the Tanqua and Laingsburg depocentres indicate a near uniform provenance for both,
dominated by granitic and metamorphic material derived from a provenance seemingly far
beyond the CFB.
SGR data, combined with lithology, show that regional stratigraphic correlation is possible
in the Skoorsteenberg, Kookfontein and Waterford Formations in the Tanqua depocentre. The
same is true for the Laingsburg and Fort Brown Formations in the Laingsburg depocentre.
There are no major changes in the SGR data set between the successive sandstone or shale
units that could imply different origin, and no distinct signals in the SGR pattern of the shale
intervals that could potentially correspond to maximum flooding surfaces.
The Tanqua and Laingsburg depocentre sandstones are very fine- to lower mediumgrained,
tightly packed, poorly to well sorted, and have undergone mechanical compaction
and pressure solution. The mineralogical composition and texture of these sandstones
suggest that they have undergone high-grade diagenesis to low-grade regional burial
metamorphism to the lower greenschist facies (250 ± 50ºC; ~2 kbars). They are
mineralogically and geochemically classified as lithic arenites and greywackes, and the
Tanqua depocentre sandstones are slightly more mature than the Laingsburg depocentresandstones. REE patterns for the Tanqua and Laingsburg depocentre sandstones are similar,
suggesting that both form part of the same evolutionary pattern and that the sediments have
one common origin, i.e. a provenance predominantly composed of granitic material.
Homogenous εNd-values for all sandstone samples of around –5 at the time of deposition
indicate that there is little or no variation in provenance between the Tanqua and Laingsburg
depocentre sediments. TCHUR model ages of 0.70 to 0.95 Ga, and TDM model ages of 1.19 to
1.49 Ga, resulted from a mixture of Archaean and Proterozoic material in unknown
proportions. The most likely source terrane is thought to be the North Patagonian Massif. The
latter show Nd isotopic compositions corresponding to an average εNd-value of -5 at 265 Ma. / AFRIKAANSE OPSOMMING: Voorlandkomme word oor die algemeen gevul met sediment afkomstig van die
aanliggende plooigordel, wat lei tot ‘n redelik eenvoudige brongebied-tot-afsettingskom
konfigurasie. Dit is egter nie van toepassing vir die vroeë suidwestelike Karookom nie,
aangesien die samestelling van die Ecca Groep sedimentêre gesteentes nie ooreenstem met
die samestelling van die aanliggende plooigordel nie.
Die suidwestelike Karookom word aan die weste en suide begrens deur die Kaapse
Plooigordel en bied die geleentheid om die verwantskap tussen die vroeë strukturele evolusie
en afsetting in die twee ruimtelik en temporeel afsonderlike Tankwa en Laingsburg
subkomme te bestudeer. Die Kaapse Plooigordel het gevorm toe die vroeë Palaeosoïkum
kontinentale grens, wat ‘n groot deel van die suidelike grens van Gondwana was, ontwikkel
het tot ‘n aktiewe konvergerende grens gedurende die laat Palaeosoïkum. Orogenese het
gelei tot die vorming van ‘n noordwes-strekkende Sederberg tak en ‘n ooswes-strekkende
Swartberg tak. Die oroklinale buig tussen die twee takke sluit grootskaalse noordoosstrekkende
sintaksis strukture in, soos die Hex Rivier en Baviaanshoek antiklinoria, wat die
sedimentasie rigtings na die kom beïnvloed het.
Spektrale gammastraal (SGR), mineralogiese en geochemiese studies op die dagsome
van die Tankwa en Laingsburg subkomme dui ‘n byna identiese brongebied aan vir beide,
oorheers deur granitiese en metamorfe materiaal vanaf ‘n brongebied oënskynlik vêr vanaf
die Kaapse Plooigordel.
SGR data, gekombineer met litologie, dui aan dat dit moontlik is om regionale
stratigrafiese korrelasies in the Skoorsteenberg, Kookfontein en Waterford Formasies in die
Tankwa subkom te maak. Dieselfde geld vir die Laingsburg en Fort Brown Formasies in die
Laingsburg subkom. Daar is geen groot veranderinge, wat ‘n verskil in oorsprong kan aandui,
in the SGR datastel tussen die opeenvolgende sandsteen of skalie eenhede nie, en ook geenuitstaande tekens in the SGR patroon van die skalie-intervalle wat moontlik kan ooreenstem
met ‘n maksimum vloedingsvlak nie.
Die Tankwa en Laingsburg subkom sandsteenlae is baie fyn- tot laervlak mediumkorrelrig,
dig gekompakteer, swak tot goed gesorteer, en het meganiese kompaksie en drukoplossing
ondergaan. Die mineralogiese samestelling en tekstuur van hierdie sandsteenlae
dui daarop dat hulle hoë-graadse diagenese tot lae-graadse regionale begrawingsmetamorfose
tot laervlak groenskis fasies (250 ± 50ºC; ~2 kbars) ondergaan het. Hulle word
mineralogies en geochemies geklassifiseer as litiese areniete en grouwakke. Die Tankwa
subkom sandsteenlae is effens meer volwasse as die Laingsburg subkom sandsteenlae. Die
lantanietgroep patroon vir die Tanqua en Laingsburg sandsteenlae is eenders, wat aandui dat
beide deel gevorm het van dieselfde evolusionêre ontwikkeling en dat die sedimente een
gesamentlike oorsprong gehad het, naamlik ‘n brongebied bestaande hoofsaaklik uit
granitiese materiaal.
Homogene εNd-waardes van ongeveer –5 by tye van afsetting vir al die sandsteen
monsters dui daarop dat daar min of geen verandering in brongebied vir die Tankwa en
Laingsburg subkom sedimente was nie. TCHUR model ouderdomme van 0.70 tot 0.95 Ga, en
TDM model ouderdomme van 1.19 tot 1.49 Ga, is afkomstig van ‘n mengsel van Argeïese en
Proterosoïese materiaal in onbekende hoeveelhede. Die mees waarskynlike brongebied is
die Noord Patagoniese Gebergtes. Dit wys Nd isotopiese samestellings wat ooreenstem met
‘n gemiddelde εNd-waarde van –5 by 265 Ma.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16075 |
| Date | 12 1900 |
| Creators | Van Lente, Belinda |
| Contributors | Wickens, H. deV., Flint, S.S., University of Stellenbosch. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | ix, 339 leaves : ill. |
| Rights | University of Stellenbosch |
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