Interplay between longitudinal f luvial and transverse alluvial fan systems and growing thrusts in a piggyback basin (SE Pyrenees)

The integration of sedimentological and structural data in the western end of the Ripoll syncline allows a clearer understanding of the palaeogeography of the area between the oblique termination of the Pedraforca thrust sheet and the frontal Vallfogona thrust during middle Eocene times. In this region, a 2.5-km thick clastic succession corresponding to the Bellmunt and Milany sequences infilled the Ripoll syncline. This succession evolved from fluvio-deltaic to alluvial. The long-lived and west prograding longitudinal fluvio-deltaic system was parallel to the axis of the syncline. The fluvio-deltaic deposits are found interfingered with others, which correspond to transverse alluvial fans of northern provenance, which are increasingly important towards the top of the section. The persistent and uniform WSW palaeoflow direction of the fluvio-deltaic system together with the position of open marine conditions to the south indicate that the southern boundary of the system was constrained by a tectonic barrier which is attributed to the emergence of the Vallfogona thrust. The ramp of this thrust above the foreland basin strata formed the Ripoll piggyback basin from middle Lutetian times on, several millions years earlier than previously proposed. The western end of the Ripoll syncline is thus an example of a complex partitioning of a foreland basin over time and space by structures of differing growth duration and different trend. The interaction of the oblique system of thrusts, corresponding to the eastern termination of the Pedraforca thrust sheet and the Vilada anticline, and the frontal system of the Vallfogona thrust controlled the position of the Bellmunt river-dominated delta, which acted as the outlet of the Ripoll piggyback basin. D 2002 Elsevier Science B.V. All rights reserved.