The andalusian stage (late miocene): Biostratigraphy, biochronology and paleoecology

Abstract

The foraminiferal and calcareous nannoplankton biostratigraphy and paleoecology of the Andalusian stratotype section, western Guadalquivir Basin, SW Spain, have been studied. About 125 foraminiferal (95 benthonic, 30 planktonic) and 29 calcareous nannoplankton taxa are recorded from the section which spans the Late Miocene to the earliest Pliocene. The base of the Andalusian coincides with the calcareous nannoplankton Discoaster calcaris/Discoaster quinqueramus (NN10/NN11) Zone boundary and is within the planktonic foraminiferal Globorotalia plesiotumida (N17) Zone. The Andalusian spans the time interval of approximately 9 to 5 m.y. and is correlative with the mid-Tortonian through Messinian stages. The Tortonian/Messinian boundary, denoted by the first indication of a marked faunal change due to eustatic sea level occurs about 50 m below the Caliza Tosca and is dated here at about 6.5 m.y. The base of the Caliza Tosca is correlated with the base of the main evaporitic sequence of the upper Messinian Stage in the Mediterranean and dated at about 5.5 m.y. here. Analysis of the benthonic foraminiferal fauna and the P/B ratio reveal a distinct decrease in bathymetry between the bottom and top of the section. The basal part of the sequence was deposited in a middle bathyal environment (water depths of 600–1000 m), the middle part of the section in upper bathyal depths (200–600 m), with a distinct tendency towards sequential shallowing in the upper part, immediately below the Caliza Tosca, from about 300 m to less than 100 m. The Caliza Tosca, a detrital calcarenite, was probably deposited in water depths no greater than 20–30 m. Within a short distance (40 m) above the top of the Caliza Tosca the basal Pliocene fauna indicates that water depth was similar to that somewhat below the Caliza Tosca, i.e., about 175–200 m. The estimated change in water depths from about 70–100 m to about 20–30 m within the upper part of the Andalusian indicate that a significant eustatic fall in sea level (estimated on the basis of oxygen isotope data in SW Pacific cores to have been about 40 m) may have occurred rather abruptly about 5.5 m.y. ago. The base of the Caliza Tosca in SW Spain and the base of the main evaporite sequence in the Mediterranean are probably reflections of a sudden eustatic sea-level fall, the former in a basin which maintained open connections with the Atlantic, the latter in a basin which temporarily lost connection with the Atlantic and became a basin of interior drainage and evaporation. The implications of this data from the Andalusian stratotype section are significant for understanding Late Miocene historical geology.