A palynological investigation of the offshore Cretaceous sequence on the south-west coast of South Africa

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dc.contributor.author Sandersen, Andrea
dc.date.accessioned 2008-09-19T11:33:47Z
dc.date.available 2008-09-19T11:33:47Z
dc.date.issued 2008-09-19T11:33:47Z
dc.identifier.uri http://hdl.handle.net/10539/5685
dc.description.abstract 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. en
dc.language.iso en en
dc.subject Palynology en
dc.subject Cretaceous en
dc.subject Wesr Coast of South Africa en
dc.title A palynological investigation of the offshore Cretaceous sequence on the south-west coast of South Africa en
dc.type Thesis en

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