7. Thermal Diagenesis of Cretaceous Sediment Recovered at the Cote Dâ•Živoire â•fi Ghana Transform Margin

The early opening of the South Atlantic ocean along the eastern Romanche Fracture Zone, including an initial phase of transform faulting and pull-apart basin formation, followed by the generation of a seafloor spreading center, was investigated during Ocean Drilling Program Leg 159 to the Côte d’Ivoire-Ghana continental margin. Clay minerals in the <0.5-μm fraction of tectonically disturbed sediment recovered during this leg were analyzed by X-ray diffraction to determine the extent of thermal diagenesis caused by the passing of the South Atlantic spreading center along this transform margin, and to date the passing. Results from Sites 959-961 reveal mild thermal alteration of clays, to paleotemperatures of 120°–170°C, in older, mostly undated sediment, in response to an elevated paleogeothermal gradient, as indicated by the absence of randomly interstratified illite/smectite group clays (R = 0 I/S clay), and the presence of dominant regularly interstratified (R = 1) I/S clay. Thermally altered sediment at these sites underlie unaltered sediment with likely erosional contacts. Based on a date of nannofossil biozone CC9b for the oldest unaltered sediment at Site 959, the thermal event there must have been pre-CC9b. At Site 962, drilled on a minor marginal ridge to the west of Sites 959–961, heat has altered clay in basal sediment to R = 1 I/S, and the transition uphole to thermally unaltered sediment, bearing R = 0 I/S clay, was recovered over a 175-m-thick interval, indicating the thermal event was intense but short-lived. Near-vertical beds at the top of the mid-Cretaceous section were not heated, while folded and unfolded sediments, 175 m deeper, were heated. Based on estimations of paleotemperatures of these sediments, a paleogeothermal gradient of near 350°C/km would be required to form the observed clay mineral assemblages. This heating must postdate the age of the sediments, which is latest Albian–Cenomanian (nannofossil biozone CC9b). Thus, the spreading center passed between Sites 959/960 and Site 962 sometime during the latest Albian–Cenomanian.