Oligocene-Miocene alluvial sedimentation in the northern Ebro Basin, NE Spain: Tectonic control and palaeogeographical evolution

Near the margin of the basin, the Tertiary series is folded and thrusted and several unconformities can be identified. Three tecto-sedimentary units (T3 to T5) have been characterised for the whole area. At the margin, they are bounded by angular or syntectonic unconformities. In more central areas, their correlative conformities can be registered as changes from coarsening- to fining-upward general trends or as sudden shifts in the vertical trend. Fining-upward trends are related to onlaps and record the retrogradational stages of the alluvial fans. In contrast, coarsening-upward trends record the progradation of the fans. Stratigraphic and sedimentological studies permit to establish the palaeogeographical evolution of the area during Oligocene and Early Miocene times. Also, it can be demonstrated that during periods of tectonic activity in the Pyrenees the alluvial fans prograded whereas during the stages of relative tectonic inactivity the fans retrograded.