The late-cenozioc history and palaeo-environments of the coastal margin of the South-Western Cape Province

Tankard, A.J. (1976) The late-cenozioc history and palaeo-environments of the coastal margin of the South-Western Cape Province. PhD thesis, Rhodes University.

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Abstract

This thesis examines the Late Cenozoic history and palaeoenvironments of the coastal margin between Elands Bay on the west coast and Die Kelders on the south coast. This study is introduced with a detailed discussion of eustatic sea level oscillation. The history of the existing ice sheets, sea floor spreading, isotopic composition changes of the oceans, and isostatic responses of the crust to varying loads are reviewed with regard to their bearing on sea level changes. A detailed account of the Neogene stratigraphy of the south-western Cape Province is presented. The Middle to early Late Miocene Saldanha Formation is characterised by shallow marine phosphatic sandstone and phosphorite. It is thought to have been deposited in a warm transgressive sea. The Pliocene Varswater Formation was deposited during a secondary transgression induced by.seaward tilting of the coastal margin during a time of worldwide regression. The Varswater Formation is characterised by pelletal phosphorites. It includes marine, estuarine, and fluvial facies. The estuarine sands and peats contain a rich fossil mammal fauna. Depositional environments of the Pelletal Phosphorite Member are examined by means of conventional grain size analysis to show that deposition took place on a shallow sublittoral platform dominated on the outer edge by a breaker-bar. Accretion of the breaker-bar to form a barrier-island allowed the development of an estuarine complex on the leeward side. Post-depositional diagenetic changes were examined by means of scanning electron microscopy. A detailed account of the petrology and geochemistry of the phosphorite and pelletal phosphorite is presented. The apatite mineral is a carbonate fluorapatite. It is concluded that the phosphorite is related to upwelling of phosphorus-rich waters. Instability of the west coast in the Pleistocene reduced the Namaqualand 45-50 m transgression complex to 10 m a.s.l.in the Saldanha area. There were three sea level peaks in the last interglacial: 6,3 m a.s.l., 2-3,5 m a.s.l, 0 m a.s.l. In the area from the Cape Peninsula to Die Kelders elevated shorelines up to 30 m a.s.l. were described, but it was found impossible to correlate these even over short distances. Thermophilic molluscs found in the last interglacial estuarine-lagoonal facies are suggestive of a warmer hydroclimate than today. Palaeoclimatic inferences were made for the period during the Würm lowering of sea level by examining Die Kelders cave sediments. It was found that the period of glacial advance in high latitudes was accompanied by a cold, wet climate in the southwestern Cape when the coastal plain was unoccupied by man. An attempt is made to reconstruct the history of transgression, palaeoclimate, and tectonism for the South African coastal margin. It is shown that the tectonic history could best be explained by a combination of seaward tilting of the western and southeastern margins on a broader tilting of the entire subcontinent. The Neogene was characterised by a single eustatic transgression in the Miocene, with secondary transgressions in the Pliocene being the result of coastal warping. The Miocene strata are characterised by a cosmopolitan mollusc fauna suggesting deposition prior to development of the major Antarctic ice sheet. Ocean temperatures became colder through the Pliocene and Pleistocene. The last interglacial thermophilic molluscs are attributed to a southward shift of the South Atlantic anticyclone, and solar heating of the sheltered embayments.

Item Type:Thesis (PhD)
Uncontrolled Keywords:Paleogeography, Cenozoic, South Africa, Western Cape Geology
Subjects:Q Science > QE Geology
Divisions:Faculty > Faculty of Science > Geology
Supervisors:Eales, H.V. and Ruddock, A.
ID Code:4034
Deposited By: Ms Chantel Clack
Deposited On:13 Nov 2012 06:57
Last Modified:13 Nov 2012 06:58
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