Thermal diagenesis of Cretaceous sediment recovered at the Côte d'Ivoire Ghana Transform Margin

Abstract

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.

Significance of ODP results on deepwater hydrocarbon exploration – Eastern equatorial Atlantic region

“…A second alternative is that the basal section has been exposed to elevated temperatures. Holmes (1998) observed that the clays in this portion of the sedimentary sequence also appear to have undergone thermal alteration, i.e., smectite is absent having been replaced by order I-S clays. She assumed that the necessary elevated temperatures were caused by the passing of a spreading center along the transform margin.…”

No major thermal event on the mid‐Cretaceous Côte d’Ivoire–Ghana Transform Margin

“…Clay mineral assemblages consist of illite, kaolinite, I–S clay and chlorite; smectite is absent. Holmes (1998) interpreted the absence of smectite as a result of its replacement by ordered I–S clays at burial temperatures on the order 120–170 °C. A different picture emerges when data from the underlying interval (1148–1158 mbsf) are considered.…”

Rift–shear architecture and tectonic development of the Ghana margin deduced from multichannel seismic reflection and potential field data

“…The passage of this hot spreading centre along the Cote d'Ivoire-Ghana Ridge by Middle Albian, is consistent with results of ODP Leg 159 which show abnormal paleogeothermal gradients in tectonically deformed sediments associated with the earliest uplift of the ridge (e.g., Holmes 1998). These paleogeothermal gradients have been ascribed to a major thermal event along the ridge which date from pre-latest Albian-Cenomanian time in the east (Site 959), to during or after latest Albian-Cenomanian time in the west (Site 962) (Holmes, 1998). Our interpretation therefore corroborates the findings of Holmes (1998) who also linked the thermal event to a spreading centre which began affecting the eastern-end of the ridge (Site 159) before latest Albian-Cenomanian time, but traversed the western-end of the ridge (Site 162) during or after latest Albian-Cenomanian time.…”

“…These paleogeothermal gradients have been ascribed to a major thermal event along the ridge which date from pre-latest Albian-Cenomanian time in the east (Site 959), to during or after latest Albian-Cenomanian time in the west (Site 962) (Holmes, 1998). Our interpretation therefore corroborates the findings of Holmes (1998) who also linked the thermal event to a spreading centre which began affecting the eastern-end of the ridge (Site 159) before latest Albian-Cenomanian time, but traversed the western-end of the ridge (Site 162) during or after latest Albian-Cenomanian time. The main thermal event along the Cote d'Ivoire-Ghana Ridge therefore resulted in the thermal uplift of the ridge by Middle Albian time when the spreading centre was on its westward migration from the eastern-end of the ridge.…”

“…Various mechanisms have been proposed to explain the causes of the main uplift event of the Cote d'Ivoire-Ghana Ridge, including tectonics Basile et al, 1993;Attoh et al, 2004), thermal heating (Mascle and Blarez, 1987;Todd and Keen, 1989;Holmes, 1998) and the coupling of lithospheric plates across the COB . All earlier proposed mechanisms underscore the fact that the early uplift history of the ridge is still not clearly understood.…”