Long-lived transfer-zone paleovalleys in mountain belts: an example from the Tertiary of the Spanish Pyrenees

Major fluvial valleys located in sediment-transfer zones that link hinterland drainage basins with their 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 synorogenic fluvial successions in the Spanish Pyrenees include regional-scale, unconformity-based, linear bodies of conglomerate that are interpreted as the fills of transfer-zone paleovalleys. These paleovalleys 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 body, here termed the Sis conglomerate, is preserved in a growth syncline between two thrust-related, lateral structures. Growth of the syncline during sedimentation permitted a 1400 m succession of clast-supported, pebble and cobble conglomerates to accumulate during a c. 18 m.y. period between the middle Eocene to Oligocene. The paleovalley served not one but a series of evolving thrust-sheet-top and foreland basins during this period. The stability of the paleovalley was governed by its location between long-lived, pre-existing structures that were inverted during compressional deformation. This example demonstrates that fluvial paleovalleys located in transfer zones can, when structurally controlled, be long-term features capable of accumulating and preserving a significant thickness of strata.