Until recently, there has been very limited academic palynological study of South Africa’s
offshore west coast Cretaceous sequences. For this thesis, an in-depth study of the palynological
sequence as well as a combination of other disciplines was undertaken for Offshore Sites C-B1
and O-A1, situated off the west coast of Cape Town. The study has yielded copius amounts of
mostly well-preserved palynomorphs (trilete spores, pollen, diniflagellate cysts and foraminiferal
test linings) ranging from Early to Late Cretaceous in age.
Both Offshore Sites consist mostly of sandstone and siltstone lithologies but it was the
carbonaceous mudstone-rich intervals that were targeted for palynomorph extraction. The
Petroleum Agency, SA (previously SOEKOR) has identified ten time periods (nine in the
Cretaceous and one in the Tertiary) from these lithologies but for this study, only eight of these
time periods that span the Barremian to the Campanian stages of the Cretaceous were utilized..
Over 550 samples that include cuttings, sidewall and core samples, have been provided by the
Petroleum Agency, SA representing offshore Cretaceous sedimentary strata from the west coast
of South Africa. All of these samples have been analysed palynologically. A wide variety of
palynomorphs have been described from these samples and are fully illustrated. These include
angiosperms, gymnosperms, spores of known and unknown botanical affinity, four species of
algae and one species of fungi. These palynomorphs are compared with published descriptions
and illustrations to aid and effect identifications. Several new palynomorphs were found,
however formal naming will only be undertaken at a later stage.
Palynological data indicates that the gymnosperms and ferns dominated the plant groups during
both the Early and Late Cretaceous intervals of both Offshore Sites. Angiosperms only really
became a significant part of the vegetation during the Late Cretaceous. Gymnosperms found at
both Offshore Sites are dominated by the genera Classopollis, Podocarpidites, Zonalapollenites,
Balmeiopsis and Exesipollenites. Cyathedites, Gleicheniidites and other trilete schizeaceous
types dominated the spore population. The information gained from the identification of the
microfossils present in these two Offshore Sites contributes not only to amplifying the
biostratigraphy of this part of the geologic column but also to reconstruct the Cretaceous
palaeoflora and palaeo-environment.
The age ranges of the palynomorphs present have provided a general Cretaceous age bracket for
these two Offshore Sites, supporting the Petroleum SA’s findings. However these palynomorphs
do not always age-define the level they were found in and on more than one occasion Tertiary
palynomorphs were located at the base of the Cenomanian and Turonian stages (Late Cretaceous
Period).
The pollen and spore assemblage include many species that are known from both northern and
southern hemispheres, although the northern hemisphere representatives are usually represented
by only a few individuals. There is a noticeable difference between the Early and Late
Cretaceous floras. Early Cretaceous floras are dominated by gymnosperms and fern spores with
only a few angiosperm pollen grains present. Late Cretaceous flora is much richer in
angiosperm pollen and in Tertiary flora. Botanical affinities were used to reconstruct the
palaeoflora of both Offshore Sites. The forests that existed during the Cretaceous were probably
cool, moist and temperate, located in the highlands of a slowly developing rift valley. The
dominant trees in these forests produced palynomorphs attributable to Podocarpaceae,
Microcachryidites, Balmeiopsis and Exesipollenites. The lowland woodlands consisted primarily
of cheirolepideacean conifers with the undergrowth including varieties of ferns and bryophytes.
Results of the pollen and spore analysis suggest that neither west coast sites fits into any
established pollen zonation. Therefore new pollen zones were established for this thesis and will
hopefully contribute to the understanding of the palaeoflora of this part of the southern
hemisphere. Zones are used to designate each Offshore Site into divisions that characterize the
essential changes in the vegetational history as well as divide the history of the palaeoflora in the
Cape Basin. Offshore Site C-B1 consists of 5 zones (CI, CII, CIII, CIV and CV) and four
subzones namely Subzone CI-A, CI-B, CV–A & CV–B. Offshore Site O-A1 is comprised of 5
zones namely OI, OII, OIII, OIV and OV.
Palaeoenvironmental reconstructions were deduced by studying the palaeoflora and palynodebris
in each zone / facies of each Offshore Site. The Early Cretaceous was tropical, humid and warm
with palaeoflora indicators such as Classopollis and Ephedripites suggesting the start of a shift
towards warmer more semi-arid conditions. In comparison, the beginning of the Late Cretaceous
assemblages suggested a shift to hotter, humid and then semi arid conditions. By the end of the
Late Cretaceous the environment had once again reverted back to warm, wet and humid
conditions.
Using the core depth as scale, the relative frequencies of the organic matter types in Offshore
Sites C-B1 and O-A1 were plotted using CONISS – the distance cluster analysis program which
identified four main groups of samples in each core (Facies CB- D1 to D4 and OA – D1 to D4)
on the basis of their organic content. These eight palynofacies assemblages (Facies CB- D1 to
D4 and OA – D1 to D4) show the gradual and immediate changes in palynodebris and could
reflect environmental and geological changes. The palynodebris component for both Offshore
Sites supports the origin of the sediments deposited into the Cape Basin as continent-derived and
transported into the Cape Basin via fresh water sources. The presence of sporomorphs and resins
in the palynodebris reflects a great diversity of hinterland vegetation. Plant and cuticle
palynodebris found at both Offshore Sites suggest stable dry conditions terrestrially and near
shore deposition. Amorphous organic material and the presence of green algae throughout both
Offshore Sites indicate a terrigenous source associated with shallow fresh to brackish water of
lagoons and swamps. The presence of marine palynomorphs supports deposition in a marine or
saline esturine marsh environment.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/5685 |
| Date | 19 September 2008 |
| Creators | Sandersen, Andrea |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf |
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