Geologic evolution of conjugate volcanic passive margins: Pelotas Basin (Brazil) and offshore Namibia (Africa). Implication for global sea level changes

[摘要] Volcanic passive margins are a major type of large igneous provinces, characterized by seaward dipping reflectors (SDRS), normally associated with subaerially emplaced basalt flows and intercalated at least in part with continental sediments. In the South Atlantic, volcanics extent laterally for hundreds of kilometers and can reach a thickness of about 15 kilometers. A number of questions related to their formation. Among them, the influence of hotspots, the timing of volcanic emplacement with respect to continental breakup and the nature of the crust associated with the volcanic wedge remain uncertain. The Walvis and Pelotas basins are particularly well imaged examples of volcanic passive margins, providing an unique opportunity to investigate the evolution of volcanic margins. The sedimentary infill of the Pelotas and Walvis basins directly resulted from the Jurassic to lower Cretaceous breakup of Pangea. In the Pelotas Basin along the Brazilian margin, forty eight sequence boundaries were identified based upon the analysis of 1,500 km of 2D reflection seismic profiles and four hydrocarbon exploration wells. These sequences form the sequence stratigraphic framework for the lower Cretaceous (Albian) to Cenozoic sedimentary succession and their age control is based upon calcareous nannofossils and planktonic foraminifera biostratigraphies. This local sequence stratigraphic framework is compared with the global chart (Haq et al., 1987) and the new cycle chart of Hardenbol et al. (in press). Positive oxygen isotope events defined in several deep-water DSDP/ODP sites are compared with the sequence boundaries defined in the Pelotas Basin, Exxon record (Haq et al., 1987), and new cycle chart (Hardenbol et al., in press). From the sequence stratigraphic analyses, a chronostratigraphic chart and an onlap curve were built. A relative sea-level curve for the upper Cretaceous and Cenozoic records was developed in the Pelotas Basin based on the variations in the coastal onlap. Magnitude of sea-level fluctuations were estimated in measuring the downward shift of coastal onlap in seismic profiles for each sequence boundary. A composite oxygen isotope record were built based upon benthonic foraminifera from DSDP/ODP sites and compared with the Antarctica glacial history, the eustatic curve of Haq et al. (1987), and the relative sea-level curve of the Pelotas Basin. A reasonable correlation results from the comparison between the relative sea-level curve derived from sequence stratigraphic studies and the composite oxygen isotope record. At least since the middle Eocene, positive isotope events correlate well with sequence boundaries. These correlation indicate that glacial eustasy has been the principal factor regulating stratal stacking patterns on a global scale since at least the middle Eocene. Oxygen isotope values for deep-water benthonic foraminifera during the Aptian to lower Albian stages and Campanian to Maastrichtian stages are similar to those observed during the middle Eocene. Due to the evidence for Antarctic glaciation during the middle Eocene, similarity between Cretaceous and Eocene isotope values could indicate the presence of polar ice as early as the Aptian.

[发布机构] Rice University