AAPG Annual Convention and Exhibition

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Fluvial Megafans, Terminal Fans, Distributive Fluvial Systems – A Stratigrapher's Nightmare?


Recent work on distributive fluvial systems has greatly elevated our awareness on laterally extensive “unconfined” fluvial systems. Yet, this is a morphological classification that includes everything from alluvial fans to multiple fluvial fan systems, and is not necessarily distributive at all. To a stratigrapher, sedimentologist and petroleum geologist, there is a very distinct and significant difference between alluvial fans and any fluvial (river) system. Differences between the fluvial megafans and terminal fans are however less clear. Moreover, the facies and stratigraphic models presented within the distributive fluvial system's concept, are the same as those published on fluvial megafans in 1990s. Specifically, fluvial megafans were recognized for the fan shape, and progradation across alluvial plains, building extensive sedimentary bodies, that show lateral and vertical changes in net/gross ratio, channel amalgamation degree and channel size and type. This raises the question on how are distributive fluvial systems (excluding the alluvial fans) different from fluvial megafans or terminal fans. The aim of this presentation is to compare the different fluvial fan types, analyze their similarities and differences what concerns facies, their distribution, stratigraphic and lateral trends, sediment delivery to the basin, and controls on deposition. Examples of detailed field and subsurface data are presented from Wasatch and Green River Formations, Uinta Basin, and Williams Fork Formation, Piceance Basin, and compared to literature.