Documenting an Ancient Large Fluvial Fan System: A Comparison to Modern Examples and Current Facies Models

Abstract

The Roan Cliffs in Utah provide a 150km long outcrop exposure of a radially distributive lobate body of Early Eocene fluvial sediments we interpret as a large fluvial fan system. This work presents the observations made in outcrop, compares these to observations in modern fluvial fan systems, and considers the implications when compared to prevailing fluvial facies models. The channel-fills in this river system are dominated by upper flow regime and high deposition rate sedimentary structures and are organized into downstream accreting barforms. In-channel paleosols are commonly developed and multiple-meter long continental bioturbation traces are found below bar accretion surfaces. These observations are consistent with a highly variable discharge regime, a requirement for development of large fluvial fans in the modern (Leier et al., 2005). A high degree of lateral channel amalgamation and splay-rich floodplains suggest frequent channel avulsions, again consistent with development of large fluvial fans in the modern (Chakraborty et al., 2010). Paleocurrents show a radially distributive pattern across the basin, and we use a statistical methodology (Owen et al., 2014) to constrain the apex of the fan system, likely located 200 km to the south near the current Utah-Arizona border. We record decreases in net-to-gross, total package thickness, and maximum channel depth away from the interpreted axis of the fluvial system, near the modern Green River, that are consistent with axial to distal changes in the architecture of modern fluvial fan systems (Shukla et al., 2001). These observations and the resulting interpretation challenge some existing paradigms in fluvial stratigraphy. This pattern of highest net-to-gross deposits coinciding with the thickest stratigraphic interval is the opposite of what would be expected if accommodation was driving fluvial architecture (i.e. Shanley and McCabe, 1994). Also, many of the best studied examples of large fluvial fans in the modern are from the Himalayan and Andean Mountain Belts where these fans feed into an axial river system down-dip. The configuration observed in the Early Eocene Uinta Basin is an axial river system feeding down-dip into a large fluvial fan, separated from any thrust front by 100s of kilometers, and terminating into a lacustrine basin. This example demonstrates that large fluvial fans do not need to be adjacent to a thrustbelt and can be fed by (rather than feeding into) an axial river system.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017