Electron spin resonance dating of the culminant allostratigraphic unit of the Mondego and Lower Tejo Cenozoic basins (W Iberia), which predates fluvial incision into the basin-fill sediments

Abstract

The Cenozoic basins of western Iberia have a culminant allostratigraphic unit (designated UBS13), which records the beginning of Atlantic drainage and predates the fluvial incision that led to the development of the present drainage networks. However, the available numerical dating is quite limited and mainly restricted to the lower-level terrace deposits. Therefore, this study uses for the first time the electron spin resonance (ESR) method to date this culminant unit in the Mondego and Lower Tejo Cenozoic basins of Portugal. The depositional age of this unit is supposed to lie between ~3.7 Ma (basal deposits) and ~1.8 Ma (uppermost deposits). The Al-centre provided reliable ESR data, but the dates obtained by using the TiLi centre clearly underestimate the burial ages. With reference to the existing independent dating of the Vale Farpado site (3.7–3.6 Ma) at the lowermost basal level of the UBS13 deposits, the ESR (Al-centre) ages of 3.0 to 2.3 Ma obtained for the UBS13 basal and middle deposits give reliable estimates of the burial age. The ESR ages (Al-centre) obtained for the UBS13 uppermost deposits indicate a probable age of ~1.8 Ma. Thus, these results are of international significance, in that they constitutes the first numerical ages obtained for the uppermost levels of the Cenozoic basin-fills of western Iberia, which predate the fluvial incision that took place in response to lower Quaternary base levels.