Long-lived fluvial palaeovalleys sited on structural lineaments in the Tertiary of the Spanish Pyrenees

Major fluvial palaeovalleys located in transfer zones that link sediment-producing hinterlands with depositional basins are an important, but neglected, element of fluvial systems. Studies of present-day examples are limited and, to date, no examples have been described from the geological record. Tertiary syn-orogenic fluvial successions in the Spanish Pyrenees include regional-scale, unconformably-based, linear bodies of conglomerate interpreted as transfer zone palaeovalleys. These palaeovalleys are sited mainly in the external zones of the mountain belt, between the internal, Axial Zone on which the drainage basin was largely established and the depositional basins. One palaeovalley, here termed the Sis palaeovalley, is located in a growth syncline between two thrust-related, lateral structures. Subsidence within the syncline permitted a 1400 m plus succession of clast-supported cobble conglomerates to accumulate during an 11-21 Ma period (Middle Eocene to Oligo- Miocene). This palaeovalley served not one, but a series of evolving thrust-sheet-top and foreland basins during this period. The stability of the palaeovalley was governed by its structural siting which, using evidence from the stratigraphy immediately underlying the palaeovalley, is considered to be a long-lived lineament which was re-used during compressional deformation. An appreciation of the location and evolution of the palaeovalley has the potential to contribute significantly towards a better understanding of the development of the Pyrenean orogen.