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Units 6 and 7 of DSDP Site 361 in the southern Cape Basin consist of over 900 meters of Cretaceous clastic sediments. These record changes in the physical conditions of southern African continental margin sedimentation, and in the chemical and physical oceanography of the South Atlantic Ocean from a time shortly after the separation of South America from Africa until deep marine conditions prevailed in a much wider Late Cretaceous ocean basin. This chapter summarizes evidence that Unit 7 was deposited in an anoxic, at times nearly stagnant water mass, and that Unit 6 represents the onset both of oxygenating bottom conditions and vigorous bottom currents. Unit 7 consists of coarse turbidites and debris-flow sandstones, sandy mudstones, and shales, whereas Unit 6 is well-sorted finely cross-laminated siltstones and shales. Both a decline in the relief of source regions (primarily by subsidence in response to lithospheric cooling following continental rifting) and the onset of currents are responsible for the lithologic changes. X-ray fluorescence and atomic absorption chemical analyses of sediments show that Cape granites and associated metamorphic rocks are the predominant source of Unit 7, whereas a significantly more mafic source is required for some Unit 6 shales and siltstones. This represents a shift from an extremely local provenance to one with input from more northerly sources such as the Orange River which drains basaltic and andesitic terranes in the Karroo Basin and Namaqualand. Transition trace metals in Unit 7 sandstones, however, are higher than in any of their potential source rocks, and apparently have been added to the sediments by chelation and other biological processes affecting or resulting in organic material and pyrite, both abundant in the sediments. Analyzed red shales interbedded with Unit 6 siltstones differ from pelagic clays of the latest Cretaceous (?)-Paleocene Unit 5 of Site 361 in having low MnO and few or no ichthyoliths (fish debris). They are thus inferred to have a terrigenous source. Scanning Electron Microscope photographs and X-ray diffraction mineralogical data are used to discuss the diagenesis of Unit 7 sandstones in the unusual anoxic environment in which they were deposited. An unusually saline bottom water composition appears required to explain the authigenic mineral assemblage kaolinite-illite-phillipsite. The same authigenic mineral assemblage occurs in carbonaceous shales cored above the evaporites in the Angola Basin at Site 364. In Unit 7 of Site 361, in situ dissolution of calcium carbonate appears to have been driven by the necessity to maintain pore fluid alkalinity as hydroxyl, Ca, K, Na, and other cations entered the authigenic minerals. These minerals, and framboidal pyrite associated with organic material suggest that diagenesis occurred at a pH of at least 8 and probably more than 9. Shipboard analyses of Unit 7 sandstone pore waters (Sotelo and Gieskes, this volume) show higher chlorinity and alkalinity than expected, suggesting an evaporite body is deeply buried within the southern African continental margin sediment wedge. This is the only evidence for still an earlier phase of more restricted circulation in the newly opened Cape Basin uncovered on Leg 40. The physical and chemical factors influencing the provenance, composition,and diagenesisof the sediments are used to formulate a

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Composition, Provenance, and Diagenesis of Cretaceous Clastic Sediments Drilled on the Atlantic Continental Rise off Southern Africa, DSDP Site 361Implications for the Early Circulation of the South Atlantic

Composition, Provenance, and Diagenesis of Cretaceous Clastic Sediments Drilled on the Atlantic Continental Rise off Southern Africa, DSDP Site 361Implications for the Early Circulation of the South Atlantic

Composition, Provenance, and Diagenesis of Cretaceous Clastic Sediments Drilled on the Atlantic Continental Rise off Southern Africa, DSDP Site 361Implications for the Early Circulation of the South Atlantic