VINCENT, S. J., and *TREVOR ELLIOT, Department of Earth Sciences, Univ. Liverpool, Liverpool, England

ABSTRACT: The Importance of Long-lived, Fluvial Transfer Zone Palaeovalleys in Understanding Basin-Scale Facies Patterns

Transfer zone palaeovalleys that link sediment producing hinterlands withdepositional basins are important, but neglected, features in basin studies. To date, there have been few studies of modern palaeovalleys and no examples have been rigorously described from the geological record. Tertiary syn-orogenic fluvial successions in the Spanish Pyrenees include spectacularly exposed, regional scale, linear bodies of conglomerate that are interpreted as transfer zone palaeovalleys. The bodies are aligned down the regional gradient of the mountain belt and are up to 20 km long, 7.5 km wide and more than 1 km thick. They are bound by synclinal, valley-shaped unconformities and exhibit axial to lateral facies changes and marginal syntectonic unconformities that testify to their origin as pala ovalleys. The palaeovalleys are sited in footwall synclines of lateral structures that were active during deposition and reflect re-use of long-lived lineaments during Alpine compressional deformation. The fill of the palaeovalleys spans 20-25 Ma (mid-Eocene to Oligo-Miocene) and they served not one, but a series of developing foreland basins. These examples suggest that transfer zone palaeovalleys can, when structurally located, be long term features capable of accumulating and preserving significant thicknesses of strata. The location of the palaeovalley determines sediment provenance (via its drainage basin) and the point at which sediment enters the depositional basin. Also, erosional unconformities in the palaeovalley fill reflect periods of sediment by-pass and are likely to corres ond to phases of deposition in the co-eval basin; conversely, periods of sediment accumulation in the palaeovalley (particularly fine-grained sedimentation) are likely to correspond with phases of reduced sedimentation in the co-eval basin. Understanding the location and evolution of palaeovalleys is, therefore, crucial to any thorough, predictive analysis of sediment budgets and basin-scale facies patterns.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.