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The marginal mountain chains that frame the Ebro Basin in the northern and northeastern parts of the Iberian Peninsula show the existence of certain areas of sedimentation which are distinctive because of the local development of particularly thick, complete, and continuous stratigraphic sections. These are of particular importance with respect to the current renewal interest in the petroleum geology of northern Spain. The structure of these areas is probably related primarily to localized Hercynian folding, but is most clearly the result of movements in Upper Jurassic time. The final tectonic setting was achieved only during the Alpine folding during Oligocene time.
A manifest relation is apparent between some of these anomalous areas and the extensive diapiric phenomena surrounding them. In some cases the forces involved appear to be mainly gravitational and the diapirism is more clearly due to conditions of sedimentation than to tectonic processes, thus giving diapirs of the columnar or salt-plug type. In other cases on the contrary the diapirs are more clearly related to lines of faulting. A recently discovered and hitherto undescribed diapir near Pamplona has a very elongate laminar outcrop of saline materials of Keuper age and constitutes a novelty among the more classic diapirs of the region.
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