Middle Eocene calcareous nannofossils in the Jaca transect (South-central Pyrenees Eocene Basin, Aragón river valley, Huesca)

Abstract

The calcareous nanoplankton is studied from the base of the turbidite systems of the Upper Hecho Group (Jaca Basin, middle Eocene of the South-central Pyrenean Basin) up to the Gracionepel instabilities, within the Larrés slope Marls. This new chronostratigraphic contribution is of crucial importance for the improvement of the detailed temporal and spatial correlation framework of the genetically related depositional systems at basin scale. The calcareous nanofossil assemblage is largely dominated by Coccolithus pelagicus, C. formous as well as different species of Reticulofenestra, accompanied by occasional specimens of several species of Sphenolithus and Chiasmolithus. The results obtained, based on the detailed biostratigraphic study of a composite succession 2,500 m thick, allow us to characterize the main biohorizons of the middle Eocene on the basis of global biostratigraphic standards. The studied succession was deposited during the CNE11-CNE15 or NP15-NP16 biozones, within a time span of ~3.45 Myr, between 43.96 and 40.51 Ma (Lutetian/Bartonian). This age range is compatible with the existing scheme for the South-central Pyrenean Basin. The Upper Hecho Group, between the Roncal-Fiscal megaturbidite (MT-5) and the instability facies of Gracionepel, starts in the uppermost part of Zone NP15 continues into Zone NP16. The proposed biozonation provides new data for a correlation between stratigraphic events of other regional sections and facilitates the understanding of the lateral and temporal evolution of the studied systems, as well as the improvement of the general palaeogeographic framework of the basin.