A NEW VIEW OF FLUVIAL FACIES MODELS – AGGRADATIONAL DISTRIBUTARY NETWORKS

Most aggradational fluvial systems found in modern active tectonic basins form distributary networks rather than single thread channels, yet the facies models we use to evaluate fluvial deposits typically come from single thread channels in degradational or steady state regimes which are contributory in nature. The implications of this contradiction could be significant for our ability to reasonably interpret ancient fluvial deposits because the facies preserved in the sedimentary rock record were probably deposited in fluvial systems that are considerably different than the modern rivers we use to aid in their interpretation. Examples of aggrading systems in continental tectonic basins – such as the rivers in the Pantanal Basin of Brazil, the Andean, Himalayan and Alaskan Forelands, the San Joaquin Basin, and the Rio Grande rift basin – all show development of large-scale fluvial fans during periods of aggradation, where approximately 90-95% of the sedimentary basin is being filled with fluvial fans with the remainder filled by a trunk stream that may or may not display the distributary pattern. This view on fluvial networks implies that most of the preserved fluvial facies in the rock record come from broad distributary fans rather than single thread channels. The deposits of these systems are likely to consist of amalgamated channel sediments deposited near the fan apex passing distally to fine-grained dominated facies that hold discrete channel packages with an anastamosed or broadly distributary pattern. Facies distributions in many sedimentary basins appear to follow this pattern, including the Oligo-Miocene of the Ebro Basin, Spain, and the Devonian of Spitsbergen and the British Isles, the late Triassic Chinle and the late Jurassic Morrison Formations of the southwest US, and the Eocene Willwood Formation in Wyoming. Given this evidence, we need to develop new ‘facies' models that take into account the cyclicity and depositional patterns that in the context of distributary fluvial systems rather than focus on models developed from modern rivers in degradational settings.