ABSTRACT

The Paleocene to lowermost Eocene Zumaia series, situated along the Basque Coast (northern Spain), consists of pelagic limestones and marlstones with interbedded, mixed calcarenite turbidites. We have compared the composition and frequency of turbidite beds in time, as well as the clay-mineral and bulk-rock carbon isotope composition of the pelagic beds, to discern the factors controlling turbidite deposition. The pelagic beds are coccolithic mudstones with mainly planktic foraminifera. The sources of detrital material to turbidites were the carbonate-producing shelves (bearing Foramol associations) around the evolving Bay of Biscay, and to a minor extent the incipient Pyrenean chain for siliciclastic components. We have found close relationships between variations of turbidite facies and depositional rates with clay-mineral assemblages and carbon isotopes in the pelagic beds. The latter are in line with global trends. It appears that in the early Bay of Biscay, climate and nutrient availability controlled turbidite sedimentation. The bulk-rock late Paleocene (NP 5-NP 9) positive carbon-isotope excursion coincides with a period of relatively cool and dry climate characterized by the highest frequency of turbidity flow. We propose that during this period globally enhanced latitudinal temperature gradients and strong thermohaline ocean circulation increased bioproductivity in surface waters by upwelling and/or by wind-driven Ekman pumping. In contrast, warm, perennially wet periods such as the early Paleocene (NP 1-NP 4) and the Paleocene/Eocene transition (upper NP 9-NP 10) are characterized by low frequency of turbidity flow in the basin. This may have been caused by sluggish thermohaline circulation and low carbonate productivity during this time period.