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

[摘要] Until recently, there has been very limited academic palynological study of South Africa’soffshore west coast Cretaceous sequences. For this thesis, an in-depth study of the palynologicalsequence as well as a combination of other disciplines was undertaken for Offshore Sites C-B1and O-A1, situated off the west coast of Cape Town. The study has yielded copius amounts ofmostly well-preserved palynomorphs (trilete spores, pollen, diniflagellate cysts and foraminiferaltest linings) ranging from Early to Late Cretaceous in age.Both Offshore Sites consist mostly of sandstone and siltstone lithologies but it was thecarbonaceous mudstone-rich intervals that were targeted for palynomorph extraction. ThePetroleum Agency, SA (previously SOEKOR) has identified ten time periods (nine in theCretaceous and one in the Tertiary) from these lithologies but for this study, only eight of thesetime 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 thePetroleum Agency, SA representing offshore Cretaceous sedimentary strata from the west coastof South Africa. All of these samples have been analysed palynologically. A wide variety ofpalynomorphs have been described from these samples and are fully illustrated. These includeangiosperms, gymnosperms, spores of known and unknown botanical affinity, four species ofalgae and one species of fungi. These palynomorphs are compared with published descriptionsand 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 duringboth the Early and Late Cretaceous intervals of both Offshore Sites. Angiosperms only reallybecame a significant part of the vegetation during the Late Cretaceous. Gymnosperms found atboth Offshore Sites are dominated by the genera Classopollis, Podocarpidites, Zonalapollenites,Balmeiopsis and Exesipollenites. Cyathedites, Gleicheniidites and other trilete schizeaceoustypes dominated the spore population. The information gained from the identification of themicrofossils present in these two Offshore Sites contributes not only to amplifying thebiostratigraphy of this part of the geologic column but also to reconstruct the Cretaceouspalaeoflora and palaeo-environment.The age ranges of the palynomorphs present have provided a general Cretaceous age bracket forthese two Offshore Sites, supporting the Petroleum SA’s findings. However these palynomorphsdo not always age-define the level they were found in and on more than one occasion Tertiarypalynomorphs were located at the base of the Cenomanian and Turonian stages (Late CretaceousPeriod).The pollen and spore assemblage include many species that are known from both northern andsouthern hemispheres, although the northern hemisphere representatives are usually representedby only a few individuals. There is a noticeable difference between the Early and LateCretaceous floras. Early Cretaceous floras are dominated by gymnosperms and fern spores withonly a few angiosperm pollen grains present. Late Cretaceous flora is much richer inangiosperm pollen and in Tertiary flora. Botanical affinities were used to reconstruct thepalaeoflora of both Offshore Sites. The forests that existed during the Cretaceous were probablycool, moist and temperate, located in the highlands of a slowly developing rift valley. Thedominant trees in these forests produced palynomorphs attributable to Podocarpaceae,Microcachryidites, Balmeiopsis and Exesipollenites. The lowland woodlands consisted primarilyof 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 anyestablished pollen zonation. Therefore new pollen zones were established for this thesis and willhopefully contribute to the understanding of the palaeoflora of this part of the southernhemisphere. Zones are used to designate each Offshore Site into divisions that characterize theessential changes in the vegetational history as well as divide the history of the palaeoflora in theCape Basin. Offshore Site C-B1 consists of 5 zones (CI, CII, CIII, CIV and CV) and foursubzones namely Subzone CI-A, CI-B, CV–A & CV–B. Offshore Site O-A1 is comprised of 5zones namely OI, OII, OIII, OIV and OV.Palaeoenvironmental reconstructions were deduced by studying the palaeoflora and palynodebrisin each zone / facies of each Offshore Site. The Early Cretaceous was tropical, humid and warmwith palaeoflora indicators such as Classopollis and Ephedripites suggesting the start of a shifttowards warmer more semi-arid conditions. In comparison, the beginning of the Late Cretaceousassemblages suggested a shift to hotter, humid and then semi arid conditions. By the end of theLate Cretaceous the environment had once again reverted back to warm, wet and humidconditions.Using the core depth as scale, the relative frequencies of the organic matter types in OffshoreSites C-B1 and O-A1 were plotted using CONISS – the distance cluster analysis program whichidentified 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 toD4 and OA – D1 to D4) show the gradual and immediate changes in palynodebris and couldreflect environmental and geological changes. The palynodebris component for both OffshoreSites supports the origin of the sediments deposited into the Cape Basin as continent-derived andtransported into the Cape Basin via fresh water sources. The presence of sporomorphs and resinsin the palynodebris reflects a great diversity of hinterland vegetation. Plant and cuticlepalynodebris found at both Offshore Sites suggest stable dry conditions terrestrially and nearshore deposition. Amorphous organic material and the presence of green algae throughout bothOffshore Sites indicate a terrigenous source associated with shallow fresh to brackish water oflagoons and swamps. The presence of marine palynomorphs supports deposition in a marine orsaline esturine marsh environment.

[发布机构] University of the Witwatersrand