In young or currently active foreland basins of the world, along-orogen variations in structural deformation and/or depositional environments are common elements of the later phases of basin development. The late Eocene Escanilla Formation of the South-Central Pyrenean foreland basin represents an ancient drainage system in which such variability can be studied in detail using high-resolution magnetostratigraphy combined with a more traditional field-based approach. Downstream changes in the nature of the alluvial system were strongly influenced by the on-going Eocene structural partitioning of the foreland basin as it began to become incorporated into the southward-advancing South Pyrenean thrust system. Lower subsidence rates within these allochthonous ‘piggy-back’ sub-basins served to increase channel-body interconnectedness of sheet-like alluvial conglomerates, to inhibit the preservation of significant volumes of fine-grained overbank material, and to promote the extensive development of pedogenic calcretes.
During the phase of coastal progradation along the subsiding basin axis, a number of N-S-trending anticlines impeded the westward progradation of the alluvial system, producing a strong diachrony in the age of a Lutetian-Priabonian-aged deltaic system along the orogen. Fold growth across the western oblique ramp of the South-Central Unit thrust system dramatically influenced middle to late Eocene drainage patterns and lithofacies distributions. Within the portions of the drainage system upstream of these active folds, the alluvial deposits were periodically ponded, allowing the deposition of micritic lacustrine limestones. Fluctuations in regional base-level exerted control on the drainage system upstream into the alluvial drainage basin. Base-level rises caused short reversals in the longer-term westward regression of marine environments across the foreland basin, whereas base-level falls produced widespread sheet conglomerate deposition.