Browsing by Subject "Marine Geology"

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    Benthic foraminifera from the Orange-Luderitz shelf, southern African continental margin
    (1981) Martin, Ruth Ann
    An investigation of the benthic foraminifera in the surficial sediments of the continental shelf and upper slope off the Orange River mouth and the Namib Desert was undertaken as an adjunct to sedimentological and geophysical investigations being conducted by members of the Joint Geological Survey/University of Cape Town Marine Geoscience Group. 117 species in 65 genera of benthic foraminifera are described and illustrated, and five bathymetric zones are distinguished on the basis of faunal differences. A cluster analysis yielded clusters which correlated well with the three deeper bathymetric zones, but were ambiguous in the two shallower zones. Fauna interpreted as relict or reworked were present at a number of stations and were distinguished on the basis of glauconite or phosphorite infillings, anomalous, distributions, and tests which are worn, battered, and partially dissolved. Bibliography: p. 62-65.
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    Geological mapping of the inner shelf off Cape Town's Atlantic Seaboard, South Africa
    (2018) Van Zyl, Frederik Wilhelm; Compton, John
    The Atlantic Seaboard is an 18 km stretch of coastline located on the Cape Peninsula, South Africa, roughly between the Cape Town suburbs of Mouille Point in the north and Hout Bay in the south. It borders heavy shipping traffic and contains a mix of urban and natural environments including up-market seaside neighbourhoods and is part of the Table Mountain National Park. The predominantly rocky coastline has a northeast–southwest orientation with interspersed sandy pocket beaches. A narrow, low-lying coastal plain (marine terrace) in the north merges with coastal cliffs further south. The geomorphology and sedimentology of the coast are closely linked to the underlying geology, influencing the shape of coastal embayments and promontories, as well as the composition and distribution of sediment. Hydrographic, geophysical and sedimentological techniques were used to collect high-resolution bathymetry, seafloor geology and sediment distribution data to better understand modern coastal processes. The results indicate a low-relief seafloor consisting of Malmesbury Group rocks in the north. To the south the seafloor consists of high-relief Cape Granite reefs interspersed with fine to medium grain sand and bioclastic (shelly) gravel. Sediment transport is generally northward by longshore drift. In the south, the high-relief granite reef and headlands form sediment traps resulting in several large pocket beaches and offshore sediment deposits. In the north, the low-relief Malmesbury bedrock is largely free of sediment, except within narrow erosional gullies. Most sediment rapidly passes through to the north resulting in a sediment-starved rocky seafloor. The three principal sources of beach sand are aeolian fine sand transported by the Karbonkelberg headlands bypass dune entering the sea at Sandy Bay, biogenic carbonate production along the coast, and weathering of Table Mountain Group sandstone and granite bedrock. A fourth source is sediment entering the system via longshore drift from the south of Duiker Point. The water depth around the Duiker Point headland is presently too deep for sediment to be transported easily through longshore drift, other than during large storm events, but during past sea-level low stands this would have played an important part in supplying sediment to the coast. Changes in sea level play an important part in shaping the geomorphology of the coastline. Beach deposits, both sandy and boulder beaches have been left at various elevations along the coast, both offshore and onshore. Although today the Sea Point area is protected by sea walls and man-made structures, a higher sea level was responsible for shaping the narrow coastal plain. Increasing rates of global sea-level change are becoming an important issue all over the world and the Atlantic Seaboard coast is not immune to the effects of sea-level rise. The frequency and magnitude of storm events that breach the sea defences erode beaches and sea cliffs and cause damage to private and public property are likely to increase in the future
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    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.
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    The petrography and major element geochemistry of the phosphorite nodule deposits on the Agulhas Bank, South Africa
    (1971) Parker, Robin James; Fuller, A O; Willis, J P
    Dredging operations carried out on the Agulhas Bank have proved the existence of a widespread phosphorite nodule deposit, considered to be essentially in situ. The pebble to boulder sized nodules recovered have been classified into two conglomeratic and three non-conglomeratic classes. The latter classes comprise (i) phosphatized microfossiliferous limestones (N I class); (ii) phosphatized highly ferruginous microfossiliferous limestones (N II class); and (iii) nodules composed of a poorly sorted mixture of quartz, glauconite and microfossil grains set in a micrite/collophane cement (N III class). Surface to centre phosphatization effects have been observed in some N I nodules. The first conglomeratic variety (C I class) is noted for abundant, often highly irregularly shaped, enclosed N I class phosphorite pebbles set in a matrix that is similar to the N III phosphorite type. The second conglomeratic variety (C II class) is similar to the first, but it is characterised by the inclusion of pebble sized microfossiliferous internal cast of macrofossils, as well as the presence of macrofossil shell debris. X-Ray diffraction studies have shown that the prime phosphate mineral present is francolite, a carbonate fluorapatite, while optically this mineral has been identified as cellophane. An X-Ray diffraction peak-pair technique has indicated an average 5.5% CO₂ concentration in the apatite phase of the phosphorites. Studies on the major element geochemistry of the various phosphorite classes has shown that the bulk geochemistry of the nodules corresponds to the dominant mineralogy and that variations in the bulk geochemistry of the nodules from within a given group reflects variations in the concentration of diluent allogenic minerals. A sympathetic relationship exists between the Na and S concentrations in the phosphorites, and this has been related to substitution effects in the phosphate mineral phase. The average P₂O₅ concentration determined for the Agulhas Bank phosphorites is 16.18%. The N I and N II phosphorite classes are considered to have originated as a result of limestone phosphatization involving a calcite-to-francolite replacement process. Many of the sedimentological features exhibited by the texturally heterogenous N III, C I and C II class nodules are incompatible under normal hydrodynamic conditions, suggesting an unusual depositional environment. In order to explain these features it has been proposed that the nodules were originally lime mud rich sediments and that the conglomeratic varieties were deposited by submarine transporting agencies such as mud-flows, turbidity currents and/or tidal/storm wave surges. Bioturbation may have been responsible for the mixing of lime mud and coarser material to produce the poorly sorted non-conglomeratic N III phosphorite class. Replacement of the calcitic lime mud by francolite .s considered to be the prime mechanism responsible for the phosphate mineralization and lithification of these N III, C I and C II class phosphorites.