Abstract

Recent work on the evolution of the northern margin of the Ebro basin in its central sector, in Aragon, is reviewed, particularly in the light of new work on the mammal biostratigraphy, palaeomagnetism, tectonic structure and sedimentology of the Late Eocene to Early Miocene fluvial sediments. Three Sequences are distinguished in these sediments. During the deposition of Sequence A (Late Eocene to Early Oligocene) an ‘eastern fluvial system’ transported sediment axially westwards, a ‘northern fluvial system’ transported sediment from the Pyrenean axial zone towards the south, and there is no evidence of any feature, structural or topographic, along the line of the present Ebro basin margin. During the deposition of Sequence B (Early Oligocene to Late Oligocene) the ‘northern fluvial system’ replaced the ‘eastern fluvial system’, and the south Pyrenean piggy-back or thrust-sheet top basins became isolated from the Ebro basin by the first emergence of the Sierras Exteriores thrust ramp, which generated conglomerates to the south, and influenced sedimentation to the north. During the deposition of Sequence C (Late Oligocene to Early Miocene), sedimentation largely ceased north of the new thrust ramp, and the Ebro basin sedimentation became differentiated into the two major (Luna and Huesca) systems round the lateral edges of the ramp, and a series of small fan bodies in between.

Introduction

The central section of the northern margin of the Ebro basin is clearly defined structurally by the southern limit of Pyrenean deformation.