Browsing by Author "Rogers, John"

Now showing 1 - 9 of 9
  • Thumbnail Image
    Item
    Open Access
    Geology of Aeolian and marine deposits in the Saldanha Bay region, Western Cape, South Africa
    (2003) Franceschini, Giuliana; Compton, John; McMillan, Ian K; Rogers, John
    The west coast of South Africa has been exploited for diamonds for most of the last century. Although some literature has accumulated over this period, the evolution of the coastal plain since the Cainozoic remains poorly understood. The aim of this thesis is to offer a multidisciplinary study of the evolution of coastal deposits in the Saldanha Bay area, along the southern part of the west coast. The environments investigated comprise lagoons, coastal dunes and shorelines. The ages of the deposits were obtained with radiocarbon dating, strontium isotope stratigraphy and biostratigraphy.
  • Thumbnail Image
    Item
    Open Access
    The holocene ostracods of the Agulhas Bank, South Africa : their classification, distribution and ecology
    (1995) Conway-Physick, Jessica Ann; Dingle, Richard; Rogers, John
    An analysis of the Holocene ostracod fauna of the Agulhas Bank has been carried out on seventy-three surficial sediment samples. Sixty-six species of Ostracoda have been recorded, of which fifty-nine species are accounted for in forty genera and the remaining seven species are of indeterminate classification. The species are described and their distribution and ecology is given. An. analysis of the sedimentology, as well as an oceanographic analysis of the bottom water on the Agulhas Bank, has provided environmental parameters for each sediment sample location, enabling relationships to be described between ostracod faunas and environmental conditions. Quantitative factor analysis has been carried out on the twenty-four most abundant species, generating seven factor associations relating ostracod assemblages to a set of environmental parameters. The independent variables analyzed were the temperature, salinity and dissolved-oxygen content of the bottom water, as well as the sand content of the sediment. Contour maps of these variables have been drawn up using SADCO data for the oceanographic variables, and the sediment samples to calculate the sand content. The overall oceanography of the Agulhas Bank has been analyzed by relating the environmental parameters generated at each location to the water masses present on the shelf, and to the oceanic currents affecting them. Finally, the seven factor associations generated have been related directly to the substrate types, the water masses, and the currents present on the Agulhas Bank.
  • Thumbnail Image
    Item
    Open Access
    A late quaternary history of Agulhas-Benguela interactions from two sediment cores on the western continental slope of South Africa
    (2002) Rau, Amanda Jane; Lee-Thorp, Julia A; Rogers, John
    Changes in circulation and productivity in the southeastern South Atlantic Ocean over the last 850 kyr are investigated through the multiproxy study of two giant piston cores, MD962080 and MD962084, retrieved from the Agulhas Bank and Olifants River continental slopes of South Africa. The stable oxygen isotope record of the benthic foraminifer, Cibicidoides wuellerstorfi, and the planktonic taxon, Globorotalia inflata, provide the stratigraphic framework from which the age models were created. The results indicate that biotic responses to surface hydrological changes in the study area are complex and involve both high- and low-frequency variations.
  • Thumbnail Image
    Item
    Open Access
    The marine geology of Walker Bay, off Hermanus, SW Cape, South Africa
    (1995) Lenhoff, Louis; Rogers, John
    The seafloor geology of Walker Bay on the southern Cape coastline is described by making use of geophysical information obtained over a period of 4 years, between 1986 and 1990. The data include side-scan sonar images, seismic profiles, seabed samples and observations by a Remotely Operated underwater Vehicle (ROV). Four sonograph facies were identified, based on their distinctly different reflectivity patterns. Using the seabed samples and R.O.V. observations, the physical characteristics of these facies are determined and presented in map format. Facies 1 consists of Bokkeveld Group rock outcrops with relatively high relief, occupying approximately 45 percent of the study area. Facies 2 represents similar outcrops but with low relief and partially covered by a thin veneer of unconsolidated sediment, including localized occurrences of loose cobbles and boulders. Facies 3 and 4 relate to sediment-covered areas displaying different bedform types. Facies 3 is dominated by well-defined patches of megarippled gravelly sand, whereas Facies 4 consists of small-scale rippled sand. The characteristics of the Facies 3 megarippled patches are discussed in detail and their relationships with the local wave pattern and nearby Facies 1 and 2 rock outcrops are investigated.
  • Thumbnail Image
    Item
    Open Access
    Sedimentary facies from the Head of the Cape Canyon : insights into the Cenozoic evolution of the western margin of South Africa
    (2004) Wigley, Rochelle Anne; Compton, John; Rogers, John
    Cenozoic sedimentary successions have a restricted distribution and are largely incomplete due to erosion and non-deposition on the western margin of southern Africa. For this reason, much controversy and uncertainty remains on the geological evolution of the western margin. The wide western margin is largely devoid of bathymetric features, except for the deeply incised Cape Canyon that crosscuts the continental slope and shelf ~150 km to the northwest of Cape Town. The Head of the Cape Canyon forms a well-developeed trough landwards of the Western Ridge, which separates the middle and outer shelf. More than 50 cores, up to 6 m in length, at water depths between 190 and 450 m were recovered from the Head of the Cape Canyon region. Siliclastic, authigenic and biogenic sediments, varying in age from Cretaceous to Holocene provide the basis of a detailed sedimentary analysis. The diversity of lithostratigraphic units recovered from the condensed sedimentary record provides a unique opportunity to define in detail, for the first time, a late Cenozoic stratigraphic record for the western outer continental shelf.
  • Thumbnail Image
    Item
    Open Access
    Sedimentation on the continental margin off the Orange River and the Namib Desert
    (1977) Rogers, John
    This study is the first detailed reconnaissance survey of the bathymetry, bedrock geology and the superficial sediments of the continental margin off the Orange River and the Namib Desert. The study area is bounded by latitudes 25° and 3o0 s. The Orange shelf is up to 100 km wide and 200 m deep, and the Walvis shelf off Lllderitz is up to 80 km wide and 400 m deep, the inner shelf is underlain by. Precambrian bedrock and is usually less than lOi km wide and shallower than 100 m. Tripp Seamount penetrate the upper slope in a depth of 1000 m and rises to within 150 m of sea level, the level of the Orange Banks on the outer orange shelf. South of the Orange River unfossiliferous ferruginous sandstones and mudstones crop out as seaward-dipping scarps on the middle shelf. North of the Orange River, similar scarps are composed of quartzose lime wackestones, identical to a Palaeogene suite on the Agulhas Bank. The shallow outer orange shelf is underlain in places by Upper Middle Miocene nummulitic _limestones, which are overlain by glauconitic conglomeratic phosphorites. Quartz-free algal limestones are found both on the Orange Banks and on the tip of Tripp Seamount. Authigenic pyrite and gypsum were found in two samples of semi-consolidated Neogene clay on the slope off Lüderitz. Pyrite is formed by combining teErigen-. ous adsorbed iron with sulphur released by anaerobic reduction of sea-water sulphate. The dissolution of planktonic foraminiferal tests provides calcium ions which combine with sulphate to form gypsum, once the calcium/sulphate solubility product is exceeded. On the Walvis shelf off LUderitz residual glauconite was reworked from older Neogene sediments, particularly on Lllderitz Bank. North of latitude 26°S residual phosphorite pellets were probably formed in Neogene diatomaceous oozes and then concentrated during a Late-Tertiary regression. Erosion of semi-consolidated .Neogene clays and ?Palaeogene quartzose' limestones. on ·the middle and outer Walvis shelf led to deposition of very fine residual quartz sand south of LUderitz. Coarse quartz ~and was reworked from littoral sandstones on the middle shelf south of the Orange River. The effect on sedimentation in the hinterland of poleward shifts of climatic belts during Quaternary interglacials and the reverse movement during glacials is assessed. Recent terrigenous sediments are derived by erosion of poorly consolidated Karoo sediments in the catchment of the Upper Orange. Erratic but powerful floods transport the sediments to the coast in suspension each summer. At the mouth in tense wave action dominates the submarine delta of the Orange River and fractionates the sediment load. Sand is transported equatorwards by littoral drift and is blown off beaches towards the Namib Sand Sea during violent Trade-Wind gales each sunnner. In contrast, silt and clay are transported polewards by a countercurrent, particularly during westerly winter storms. Recent biogenic sedimentation is controlled by upwelling in the B~nguela Current System, which is most intense off the wind dominated Sperrgebiet, south of LUderitz. Weaker upwelling off the Orange River allows oceanic planktonic foraminifera to penetrate to the middle shelf, whereas on the Walvis shelf in the study area they only reach the outer shelf. Towards latitude 25°s the Benguela Current System decays and interacts with poleward-flowing warm oxygen-poor water. Periodic mass mortalities of oxygen-starved phytoplankton lead to some of the most organic-rich sediments in the world.
  • Thumbnail Image
    Item
    Open Access
    Sedimentology and micropalaeontology of gravity cores from the N.E. Atlantic continental slope south west of Ireland
    (1988) Buck, P J; Rogers, John
    Eleven gravity cores from the continental margin off Eire and Land's End (SW England) were examined and found to document the major trends of the Late Pleistocene climate. Several stratigraphic indicators; - carbonate content, sediment texture, grain size, composition, nature of terrigenous components, ice-rafted debris and foraminiferal diversity were examined and show that the glacial history of the study area can be closely correlated with the palaeoclimatic evolution of the adjacent European shelf. Sediments deposited during Late Pleistocene glacial conditions show the following characteristics when compared to the surface sediments deposited under Holocene interglacial conditions: an increase in the quantity of ice-rafted debris and percentage of mica, and a notable increase in the degree of frosting and pitting of the quartz grains. Overall grain size was finer resulting in a silty sediment package. Sedimentologically the cores fall into two groups (1 and 2). The major difference being that Group 1 (located on the Pendragon Escarpment) received increased quantities of fine silts from a 'shelf spill-over' mechanism operating on the Fastnet and Western Approaches Basins, during glacial regressions. All sediment samples displayed polymodal characteristics reflecting the interaction of several different physical processes e.g. ice-rafting, contour currents etc. Striking variations in the populations of planktonic foraminifera were noted, alternating between Arctic and Sub-Arctic assemblages, reflecting the waxing and waning of glacial activity. The coccolith-carbonate minima correlate with the Arctic-fauna maxima and the ¹⁸O/¹⁶O maxima of the oxygen-isotope curves. Foraminiferal-test analysis (ratio of whole foraminifera fragmented foraminifera) revealed that no correlation existed with any of the other parameters analysed. However, the cores were severely affected by the presence of bottom currents which were strong enough to remove the fragmented tests. Parallellaminated contourites and evidence of erosion were noted in all cores. Ten cores penetrated sediments deposited during the last glacial maximum of 20,000 B.P - 18,000 B.P. near the 75cm depth mark (Core 1865 was too short to reach such sediments). However sediments reflecting the 11,000 B.P glacial readvance, detected at around the 25cm mark, were not as clearly represented. Bioturbation has smoothed the climatic record throughout the lengths of these cores and has also suppressed the high-frequency oscillations (<10³ B.P).
  • Thumbnail Image
    Item
    Open Access
    The sedimentology, palaeontology and stratigraphy of coastal-plain deposits at Hondeklip Bay, Namaqualand, South Africa
    (1994) Pether, John; Rogers, John
    The exposures in diamond mines on the Namaqualand west coast of South Africa provide a rare opportunity to examine a record that is normally inaccessible beneath a thick cover of aeolian sands. This study presents the main results of fieldwork in mine excavations on the farms Hondeklip and Avontuur-A, near Hondeklip Bay. Sections in the deposits were described in detail and the vertebrate and invertebrate faunas were sampled. The buried topography of the gneiss bedrock, obtained by prospecting, is complex, with the main feature consisting of a coast-parallel ridge flanking a wide palaeochannel on its landward side. Advanced kaolinitic weathering affected both the bedrock and a diamondiferous, basal kaolinitic sediment patchily preserved in the channel. The incision of the channel is related to the Oligocene regression and the basal kaolinitic sediment is interpreted as a fluvial arkose deposited in the channel. Both the bedrock and the deposit in the channel were then kaolinized during humid climatic conditions in the late Oligocene and early Miocene. Weathering-profile silcrete also developed in the basal kaolinitic sediment. It is tentatively proposed that this weathering period may be represented in the Namib Desert by the thick laterite capping Eocene sediments at Kakaoberg. Subsequently, the palaeochannel was exhumed and was ultimately filled by late Tertiary marine deposits. The marine deposits were laid down in shallowing-upwards sequences of the shore face environment. Two regressive, progradational packages (alloformations) are recognized. The older extends seawards from at least -50 m asl. and is the "45-50 m Complex" of Carrington and Kensley (1969), now called the 50 m Package. East of the channel, on the exposed coast, high-wave-energy storm-deposition in the lower shoreface dominates the preserved record. With lowering of sea-level, the bedrock ridge emerged to the seaward of the prograding palaeoshoreline, reduced the level of incident wave energy and profoundly influenced the development of sub-environments within the progradational regime. Ultimately, low-energy bay deposits filled the palaeochannel in the bedrock. On the basis of vertebrate evidence and correlation with global sea-level trends, the age of the 50 m Package is middle Pliocene. The upper facies of the 50 m Package (foreshore and upper shoreface) have been extensively removed by later subaerial erosion. The subsequent transgression truncated the seaward extent of the 50 m Package, reached ~30 m asl. and prograded seaward from that elevation. It is called the 30 m Package and combines the "29-34 m Beach" and "17-21 m Complex" of Carrington and Kensley (1969). A late Pliocene age is envisaged. The upper-shoreface facies of the 30 m Package is usually preserved, but may be disguised by pedogenesis. The diamondiferous marine gravels mined in the area are mainly lower-shoreface storm deposits and pre-existing transgressive lags and shelf deposits have generally been reworked during regression. Enigmatic, muddy and/or phosphatic units, previously called "E-stage," are patchily preserved in the base of the 50 m Package and are revealed to be distal storm deposits laid down in the transitional shoreface to offshore environment. They are part of the overlying regressive sequence, but may include a fragmentary , petrified, mixed, vertebrate remanie. Nevertheless, eroded remnants of older deposits must also occur in places.
  • Thumbnail Image
    Item
    Open Access
    The stratigraphy and sedimentology of the Bredasdorp group, southern Cape Province
    (1990) Malan, Jean Arnaud; Rogers, John
    The Cainozoic Bredasdorp sediments along the south coast of the Cape Province, South Africa, have come under investigation through a few intermittent studies undertaken since the early part of the century. In this presentation the literature is reviewed and a stratigraphic subdivision, based on both lithostratigraphic and biostratigraphic principles, is applied to the sediments of the Bredasdorp Group. The Bredasdorp deposits can be classified, according to origin, as shallow marine and aeolian. The marine deposits are subdivided into the Pliocene De Hoopvlei and the Middle to Late Pleistocene Klein Brak Formations. The Late Pliocene to Early Pleistocene Wankoe Formation, the Late Pleistocene Waenhuiskrans Formation and the Holocene Strandveld Formation constitute the coastal aeolian deposits. These marine and marine-related (aeolian) formations, characterised by calcareous elastics have been grouped together in a newly defined Bredasdorp Group. In order to construct a depositional model for the Bredasdorp Group, various facies have been identified on the basis of geometry, lithology, fossil contents, palaeocurrent data, biogenic and sedimentary structures. The facies are related to environments constituting a shoreline setting with offshore, transitional, shoreface, foreshore and backshore zones, followed by back-barrier lagoons, estuaries, backshore dunes and coastal dunefields associated with transgressive/regressive shorelines. The various deposits, as well as distinctive geomorphological features, are correlated with the relative sea-level movements throughout the Cainozoic, which have shaped the southern Cape coastal plain. Sea-level curves for Southern Africa, drawn by several authors are compared. A relative sea-level curve is constructed for the south coast of South Africa. Several Early Cainozoic transgression/ regression cycles are recognised at places along the South African coast. The earliest cycle started in the Palaeocene and was followed by a less pronounced transgression/regression cycle occurring in the Late Eocene to Early Oligocene. Remnants of surfaces related to these cycles are recognised in the Southern Cape Province, but these are interpreted as products of subaerial processes. The next cycle, reaching a transgressive maximum of c. 180m, started in the Miocene and terminated in the Early Pliocene. Again, no evidence of marine deposits is preserved on this marine-planed surface. The Early Pliocene transgression reached a maximum present-day elevation of c. 120m. Marine planation of the coastal platform took place during the transgression, whereas the De Hoopvlei Formation situated below 120m, was deposited during the subsequent Late Pliocene regression. The Wankoe Formation was deposited during the same regression as backshore dunes and coastal dunefields. During the Quaternary transgression/regression cycles, of which at least three are indicated, the transgression reaching a maximum of about 50m, in places eroded part of the Neogene Bredasdorp Group. The Klein Brak Formation (preserved below 20m) was deposited during Middle to Late Pleistocene regressions. The Waenhuiskrans Formation, which is extensively developed on the present-day continental shelf, was deposited during this regression with sea-level receding to about -130m below present sea-level. The aeolian Strandveld Formation, which is still being deposited, originated from the Flandrian transgressive maximum at the start of the Holocene.