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 hin terland drainage basins with their depositional basins are an importan t, but neglected, element of fluvial systems. Studies of present-day e xamples are limited and, to date, no examples have been described from the geological record. Tertiary synorogenic fluvial successions in th e Spanish Pyrenees include regional-scale, unconformity-based, linear bodies of conglomerate that are interpreted as the fills of transfer-z one 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 basin s, One body, here termed the Sis conglomerate, is preserved in a growt h syncline between two thrust-related, lateral structures. Growth of t he syncline during sedimentation permitted a 1400 m succession of clas t-supported, pebble and cobble conglomerates to accumulate during a c. 18 m.y. period between the middle Eocene to Oligocene. The paleovalle y served not one but a series of evolving thrust-sheet-top and forelan d basins during this period. The stability of the paleovalley was gove rned by its location between long-lived, pre-existing structures that were inverted during compressional deformation. This example demonstra tes that fluvial paleovalleys located in transfer zones can, when stru cturally controlled, be long-term features capable of accumulating and preserving a significant thickness of strata.