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Department of Geological Sciences, University of Southern California, University Park, Los Angeles, CA 90089-0740, USA
1 Africa and Latin America Area, Amoco Production Company, PO Box 3092, Houston, TX 77253, USA
2 Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
Models of braided stream deposition have largely been developed from studies of regionally degrading and laterally confined alluvial environments. Glacial outwash streams, in particular, have supplied important and widely cited descriptions of intra-channel processes. These fluvial systems are typically confined within quite narrow valleys. It is felt that such systems have low long-term preservation potential and are unlikely to be present in the geologic record in large quantities. Therefore, the study of these modern laterally confined degradational systems may not provide holistic analogs of the larger-scale alluvial architecture developed in braided river environments in the ancient. The Escanilla Formation of the Spanish Pyrenees provides a well-exposed example of an Eocene fluvial system flowing axially within the Pyrenean foreland basin. Sedimentologic study shows coarse channelized deposits of braided character wholly enclosed within large amounts of fine-grained overbank mudstones and siltstones (>40% by volume), with both being deposited coevally across the Escanilla floodplain.
A new depositional model is proposed that combines facets of existing models derived from other fluvio-morphologic systems. This consists of a laterally confined channel belt, internally preserving a braided stream character, capable of rapid vertical aggradation on short geological time-scales (about a thousand years). Avulsion processes are used to explain finer sediment deposition in interfluve settings, as well as the large-scale architectural geometries within the lower Escanilla Formation. This new model illustrates that discrete channel belt avulsion, and the preservation of thick sequences of overbank material are not exclusively characteristics of higher sinuosity fluvial systems.
