SYNCHRONOUS DETACHMENT FOLDS AND COEVAL SEDIMENTATION IN THE PREPYRENEAN EXTERNAL SIERRAS (SPAIN) - A CASE-STUDY FOR A TECTONIC ORIGIN OF SEQUENCES AND SYSTEMS TRACTS

This paper reports on the structural and sedimentary evolution of the middle to late Eocene of the Prepyrenean External Sierras (southern Py renees, Spain). The initiation, duration and kinematics of a set of gr owth structures that developed in a shallow marine depositional settin g is documented. The detailed analysis of the syntectonic marine sedim ents not only confirms the already known east to west progression of d eformation, but also reveals the continued growth of the early formed structures as later ones propagate towards the foreland. The sedimenta ry units coevally deposited with these growth structures are arranged in four depositional sequences. Their boundaries correspond to floodin g surfaces which grade basinwards into correlative conformities. They are also indicated by the presence of both angular unconformities and onlap geometries. Each depositional sequence generally consists of two systems tracts. The lower one, or transgressive systems tract, is for med by up to 400 m of azoic marls deposited in outer ramp areas. The u pper one, or highstand systems tract, mainly consists of shallow silic iclastic and carbonate facies, up to 200 m thick, deposited in middle to inner ramp areas. These depositional sequences are interpreted to b e controlled by regional tectonic pulses. An increase of tectonic acti vity resulted in the flooding of the basin and in the subsequent depos ition of a thick succession of nearly azoic blue marls (i.e. transgres sive systems tract). The overlying highstand systems tract developed f ollowing periods of diminished tectonism, with the consequent growth a nd progradation of shallow carbonate platform facies.