ENGLISH ABSTRACT: The farm Geelwal Karoo is situated some 16km north of the Olifants River mouth on the West Coast of
South Africa and hosts fluvial, marine and aeolian deposits of post-Gondwana age. The oldest basal fluvial
succession, unconformably overlies Proterozoic and Palaeozoic basement rocks and is in turn capped by
aeolianite and littoral packages representing two transgressive cycles.
The fluvial channel clay succession is deposited in shallow bedrock-incised channels, has a wedge-shape
and is deposited parallel to the present coastline. The flow direction is along the coast and the northwardtapering,
angular, poorly sorted basal vein-quartz lag indicates a northward palaeo-flow direction. Less than
1% total heavy minerals (THM) is found in the matrix of these gravel units and the heavy mineral suite is
distinguished by zircon, pseudorutile and kyanite. The channel clay unit is dominated by an upper, mediumgrained
quartzose sand and kaolin clay facies, which shows advanced post-depositional weathering. The
fluvial unit is correlated with the channel clay unit of Hondeklip Bay and a Cretaceous age is proposed for the
initial channel incision.
The two shallow marine successions have been correlated with the Late Miocene, Early Pleistocene, +30m
and +50m packages respectively. These marine sediments were first described by John Pether (1994) in
the Hondeklip Bay area and were named according to their transgressive maxima. They are transgressive
successions arranged en echelon down the coastal bedrock gradient, from oldest and highest to youngest.
The offshore environment of the +50m package consists of fine silty sand, which is moderately sorted. The
mineral assemblage is dominated by quartz and the average THM is 18%. The inshore environment is
distinguished by a single poorly sorted basal cobble lag which shows an overall fining upward succession.
The beachface environment is composed of medium to fine-grained sand, which is moderate to well-sorted.
Mineral diversity is greatest in the inshore and beachface environments and the average THM for these two
units is greater than 35%. The +30m package has been extensively eroded due to its lower erosion and
outcrops were sporadic along the coast. The +30m offshore sediments are recognised by fine sediments
with high concentrations of glauconite and organic matter. The inshore environment is distinguished by
numerous poorly sorted pebble lags with fining upward successions. Both the inshore and beachface units
have higher feldspar concentrations than the corresponding +50m units. The average THM for these two
units is less than 3%.
The aeolianite unit, which comprises several distinct units, extends over the entire length of the study area
and is characterized by calcrete and red bed horizons. Colour variations in the otherwise homogeneous unit
are due to heavy mineral enrichment and/or different degrees of in situ weathering and cementation. The
unit is composed exclusively of fine- to medium-grained sand and the THM concentration averages 9%.
This unit is composed of more than one generation of aeolian sand and forms part of an aeolian transport
corridor which transported sand from the beach to the interior. The oldest unit has been equated with the
Upper Miocene Prospect Hill Formation, whereas the more recent yellow dune sand is equated with the
Pleistocene Springfontyn Formation. At Geelwal Karoo, only the heavy sand placer in the +50m package was deemed to be of any economic
significance. The average THM of this placer was calculated to be 40% and some 150 thousand tons of Tibearing
material can be expected from this succession. This relatively small volume of heavy minerals and
extensive cementation however, make this placer a less attractive prospect than the neighbouring Namakwa
Sands operation. / AFRIKAANSE OPSOMMING: Die plaas Geelwal Karoo is ongeveer 16km noord van die Olifantsriviermond aan die Weskus van Suid-
Afrika geleë en het voorkomste van fluviale, marine en eoliese afsettings van post-Gondwana ouderdom. Die
oudste eenheid, ‘n basale fluviale eenheid, oorlê Proterosoiëse en Paleosoiëse plaaslike vloer gesteentes
wat op hulle beurt weer bedek word deur eoliese en littorale eenhede verteenwoordigene ven twee
transgressiewe siklusse.
Die fluviale kanaalklei-opeenvolging, afgeset in vlak ingesnyde rotsbedding-kanale, is wigvormig en is
afgeset parallel aan die huidige kuslyn in ‘n alluviale waaier-afsetting. Die vloeirigting was langs die kus en
die noorwaards toespitsende, hoekige, swak gesorteerde basale aar-kwarts bodemgruis dui op ‘n
noordwaards palaeo-vloeirigting. Minder as 1% totale swaarminerale (TSM) is gevind in die tussenmassa
van hierdie gruis-eenhede en die swaarmineraal reeks word onderskei deur sirkoon, pseudo-rutiel en kianiet.
Die kanaalklei eenheid word oorheers deur ‘n boonste, medium-korrelrige kwarts-bevattende sand en
kaolien kleifasies was dui op gevorderde verwering na afsetting. Die fluviale eenheid word gekorreleer met
die kleikanaal en ‘n Kryt-ouderdom word voorgestel vir die aanvanklike insnyding van die kanaal.
Die twee vlak marine opeenvolgings word gekorreleer met die Laat Mioseen, vroeg Pleistoseen, naamlik die
+30m en +50m eenhede onderskeidelik. Die aflandige omgewing van die +50m eenhied bestaan uit matiggesorteerde,
fyn slikkerige sand. Die mineraalversameling word oorheers deur kwarts en die gemiddelde
TSM is 18%. Die subgetysone word onderskei deur ‘n enkele swak-gesorteerde gruislaag en is oorwegende
opwaarts fynerwordend. Die strandomgewing is goed verteenwordige en bestaan uit matig tot goedgesorteerde
medium- tot fynkorrelrige sand. Die grootste mineraal-diversiteit kom voor in die subgety- en
strandomgewings en die gemiddelde TSM vir hierdie eenhede is hoër as 35%.
As gevolg van algemene erosie kom die +30m eenheid sporadies voor. Die aflandige omgeving is herken
deur fyn kleierige of slikkerige sedimente met hoe konsentrasies gloukoniet en organiese materiaal. Die
subgetysone omgewing is gekenmerk deur verskeie gruislae wat almal opwaarts fynwordend is. Altwee die
subgety- en strandomgewings het hoer feldspar konsentrasies as die +50m eenhede. Die gemiddelde THM
vir hierdie enhede is minder as 3%.
Die eolitiese eenheid, bestaan uit verskeie duidelik-onderskeibare eenhede, beslaan die totale lengte van die
studiegebied en word kenmerk deur uitgebreide kalkreet en rooi-laag horisonne. Kleurverskille in die
andersins homogene eenheid kan verklaar word in terme van lae swaarmineraal konsentrasies en/of as
gevolg wan verskillende grade van in situ verwering en sementering. Die eenheid bestaan uitsluitlik uit fyntot
medium-korrelrige sand en het ‘n gemiddelde TSM konsentrasie van 9%. Die eenheid bestaan uit meer
as een generasie eoliese sand en maak deel uit van ‘n eoliese vervoersisteem wat sand vanaf die strand na
die binneland vervoer het. Die oudste sande in hierdie eenheid is gekorreleer met die Laat Mioseen
Prospect Hill Formasie terwyl die meer onlangse geelduin sand vergelyk word met die Pleistoseen
Springbokfontein Formasie. By Geelwal Karoo is slegs die +50m eenhede beskou as economies van belang. Die gemiddelde TSM van
hierdie swaarmineraal-ertsligaam is bereken op 40% met ‘n verwagte 150 duisend ton Ti-draende material
van die opeenvolging. Die relatiewe klein volume swaarminerale en uitgebreide sementering het tot gevolg
dat dit ‘n minder aanloklike proposisie is as die aanliggende Namakwa Sands aanleg.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/21452 |
| Date | 03 1900 |
| Creators | Elferink, Lisa |
| Contributors | 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 | English |
| Type | Thesis |
| Rights | Stellenbosch University |
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