Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Basin subsidence analysis, employing the backstripping method, indicates that
fundamentally two different basin-generating mechanisms controlled Tanqua depocentre
development in SW Karoo Basin. The subsidence curves display initial dominantly
decelerating subsidence, suggesting an extensional and thermal control possibly in a strikeslip
setting during the depocentre formation; on the other hand, subsequent accelerating
subsidence with time suggests that the dominant control on the depocentre formation in SW
Karoo was flexure of the lithosphere. Based on these observations on the subsidence
curves, it is possible to infer that the first stage of positive inflexion (~ 290 Ma) is therefore
recognised as the first stage of Tanqua depocentre formation.
Petrographic study show that most of the studied sandstones of the Tanqua depocentre at
depth of ~ 7.5 Km were subjected to high pressure due to the overlying sediments. They are
tightly-packed as a result of grains adjustment made under such pressure which led also to
the development of sutured contacts. It is clear the high compaction i.e. grain deformation
and pressure solution occurred on the sediments; leading to total intergranular porosity
reduction of the quartz-rich sediments and dissolution of the mineral grains at intergranular
contacts under non-hydrostatic stress and subsequent re-precipitation in pore spaces.
Furthermore, siliciclastic cover in the Tanqua depocentre expanded from minimal values in
the early Triassic (Early to Late Anisian) and to a maximum in the middle Permian (Wordian
-Roadian); thereby accompanying a global falling trend in eustatic sea-level and favoured by
a compressional phase involving a regional shortening due to orogenic thrusting and positive
inflexions (denoting foreland basin formation). The estimate of sediment volume obtained in
this study for the Permian Period to a maximum in the middle Permian is therefore
consistent with published eustatic sea-level and stress regime data. In addition, this new
data are consistent with a diachronous cessation of marine incursion and closure of Tanqua depocentre, related to a compressional stress regime in Gondwana interior during the late
Palaeozoic. / AFRIKAANSE OPSOMMING: Die ontleding van komversakking met behulp van die terugstropingsmetode bring aan die lig
dat die ontwikkeling van die Tankwa-afsettingsentrum in die Suidwes-Karoo-kom hoofsaaklik
deur twee verskillende komvormende meganismes bepaal is. Die versakkingskurwes toon
aanvanklike, hoofsaaklik verlangsaamde versakking, wat daarop dui dat ekstensie- en
termiese beheer gedurende die vorming van die afsettingsentrum plaasgevind het,
waarskynlik in strekkingwaartse opset. Aan die ander kant toon daaropvolgende
versnellende versakking wat mettertyd plaasgevind het dat die vorming van die
afsettingsentrum in die Suidwes-Karoo eerder oorwegend deur kromming van die litosfeer
beheer is. Op grond van hierdie waarnemings met betrekking tot die versakkingskurwes, kan
mens aflei dat die eerste stadium van positiewe infleksie (~ 290 Ma) dus as die eerste
stadium van die vorming van die Tankwa-afsettingsentrum beskou kan word.
Petrografiese studie toon dat die meeste van die sandsteen wat van die Tankwaafsettingsentrum
bestudeer is, op diepte van ~ 7,5 Km aan hoë druk onderwerp was
weens die oorliggende sedimente. Die sandsteen is dig opmekaar as gevolg van die
korrelaanpassing wat onder sulke hoë druk plaasvind, wat op sy beurt ook tot die
ontwikkeling van kartelnaatkontakte aanleiding gegee het. Dit is duidelik dat die sediment
aan hoë verdigting, dit wil sê korrelvervorming en drukoplossing, onderwerp was, wat gelei
het tot algehele afname in interkorrelporeusheid by die kwartsryke sedimente; die
ontbinding van die mineraalkorrels in interkorrelkontaksones onder niehidrostatiese
spanning, en daaropvolgende herpresipitasie in poreuse ruimtes.
Voorts het silisiklastiese dekking in die Tankwa-afsettingsentrum toegeneem van minimale
waardes in die vroeë Triassiese tydperk (vroeë tot laat Anisiaanse tydperk) tot hoogtepunt
in die mid-Permiaanse tydperk (Wordiaans–Roadiaans). Dié ontwikkeling het
gepaardgegaan met algemene dalingstendens in die eustatiese seevlak, en is verder
aangehelp deur saamdrukkingsfase wat gekenmerk is deur regionale verkorting weens orogeniese druk en positiewe infleksies (wat met voorlandkomvorming saamhang). Die
geraamde sedimentvolume wat in hierdie studie vir die Permiaanse tydperk bepaal is, met
die hoogtepunt in die middel van dié tydperk, is dus in pas met gepubliseerde data oor die
eustatiese seevlak en spanningstoestand. Daarbenewens strook hierdie nuwe data met
diachroniese staking van mariene instroming en die afsluiting van die Tankwaafsettingsentrum
wat met spanningstoestand in die Gondwana-binneland gedurende die
laat Paleosoïkum verband hou.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/20277 |
| Date | 03 1900 |
| Creators | Alao, Abosede Olubukunola |
| Contributors | Mikes, Daniel, Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 152 p. ; ill., maps |
| Rights | Stellenbosch University |
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