Palaeoclimatic influence on the evolution of a lower Priabonian mixed carbonate-siliciclastic system in the Ebro Basin (NE Spain)

This study presents a detailed facies analysis of the lower Priabonian La Tossa Formation, a mixed carbonate-siliciclastic succession deposited in the eastern sector of the Ebro basin (South-Pyrenean foreland basin). Through integrated macro- and micro-facies interpretations, we reconstruct the palaeoecological evolution of a shallow-water carbonate ramp system and asses its response to environmental and climatic shifts during the lower late Eocene. Sedimentation was strongly influenced by coeval fan-delta systems along the basin margins, which directly influenced the facies belt distribution and composition. A total of 13 different facies were identified, arranged along a transect from proximal to distal ramp settings. Their spatial and stratigraphic organisation reflects the interplay of global climatic trends, regional tectonics and local environmental controls.

The facies model for the lower interval of the succession is characterized by proximal ramp facies consisting of siliciclastic-influenced packstones, dominated by Campanile and acervulinid-gypsinid foraminifera. These deposits transition laterally into Nummulites packstones, with localized Nummulites banks. Basinwards, the facies grade into coral frame- and cluster-reefs, which in some sections exhibit coralline algal crusts, and rhodoliths associated with encrusting foraminifera facies. The distal ramp facies are distinguished by the presence of orthophragminids and bryozoan-rich limestones, interbedded with marls devoid of macrofossils, as well as bryozoan and hexactinellid sponge’s marls.  Interspersed with this facies belt pattern are two episodes of coralline algal maërl environment. The two maërl levels extend from proximal zones, where they overlie the Campanile sandstones, to distal zones, where they overlie the orthophragminid facies. The upper interval reflects a distinct evolution into a purely carbonate-dominated system. The low-energy proximal ramp setting, consists of porcelaneous foraminifera-rich grainstones with abundant Nummulites, bivalves and echinoids. These units grade into coral frame reef facies with encrusting foraminifera, which in the distal ramp settings transition into marls and orthophragminid-rich limestones.

The changes in facies indicate that climatic oscillations influenced the distribution of benthic communities in this region of the western Tethys. The development of maërl environments and the stabilization of the platform into a purely carbonate system in the upper interval of the succession, suggest cooler climatic periods. This evolution of facies thus reflects the climatic shift from greenhouse to icehouse conditions during the onset of the late Eocene, which led to the Antarctic glaciation in the earliest Oligocene.

 

Keywords: Ebro basin, Tossa Formation, palaeoecology, late Eocene, facies analysis