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.
Mineral composition and carbon isotope signals of pelagic limestones and hemipelagic marlstones reveal the driving processes of interbedded turbidites. The Palaeocene to lower Eocene lower bathyal Zumaia series in northern Spain between Zumaia and Getaria (W of San Sebastian) offers this opportunity. This deep-marine series records the transition from a carbonate system (Palaeocene) to a siliciclastic system (early Eocene), mainly controlled by the initiation of Pyrenean uplift in the latest Palaeocene. During the tectonically quiet Palaeocene, varying carbonate turbidite deposition was controlled by climate-dependent production of (foramol association) carbonate debris on the carbonate shelves bordering the basin. The late Palaeocene period was characterized by a cycle of increasing and then decreasing turbidite sedimentation. Increased carbonate production was driven by comparably cool/dry climates in the late Palaeocene Atlantic region, and global marked temperature gradients which enhanced wind-driven Ekman-upwelling of nutrients. In contrast, during warm/perennially humid periods (early Palaeocene and the Palaeocene-Eocene transition) the dynamics of the carbonate turbidite deposition was sluggish, due to decreased carbonate production. In the carbonate system, frequency-size statistics of bed thicknesses show that turbidite generation occurred non-scale invariant. From the earliest Eocene, increasing siliciclastic input from the rising and west advancing Pyrenean chain is observed. After a transitional period, the highly dynamic lower Eocene siliciclastic system evolved on a prograding deep-sea fan under a perennially humid and seasonally wet climate. Several periods of increased tectonic activity and sediment input are recognized by clay mineral assemblages in interturbidite beds. Turbidite bed frequency-size statistics show a power-law distribution, implying that during such periods (particularly NP 12), the depositional system temporarily reached a self-organized critical state. The evolution of the system to critical state appears to be mainly driven by siliciclastic sediment input.
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