Fluvial fan architecture, facies, and interaction with lake: lessons learned from the Sunnyside Delta interval of the Green River Formation, Uinta Basin, Utah

Abstract

The Early Eocene Sunnyside Delta interval of the middle Green River Formation in the Uinta Basin changes from fluvial channel and floodplain deposits to interbedded fluvial, deltaic, and lacustrine deposits across 20 km in the Nine Mile Canyon. Outcrop measured sections and photomosaics with GPS survey of excellent cliff face exposures are integrated with areal mapping of channel dimensions, channel to floodplain ratio, and sedimentary facies variability. This study identifies the Sunnyside Delta interval as a fluvial fan system, composed of variable discharge river and floodplain deposits that basinward transitions into deltaic, mixed and lake facies. Variable discharge signatures include the abundant Froude supercritical flow and high deposition rate sedimentary structures, in-channel mudstones, in-channel bioturbation and desiccation, and the low abundance of cross stratification. This work identifies upstream and downstream bar-scale (macroform) accretion styles that were not previously recognized, as variable discharge river macroforms are commonly referred to as poorly developed or even non-existent. This work identifies low angle downstream dipping accretion, steep upstream accretion, and vertical aggradation as some of the characteristic accretion styles. The fluvial fan stratigraphy is characterized by multiple scales of upward sandying and thickening successions with upward increasing channel to floodplain ratio, channel size, the degree of channel amalgamation, and the proportion of floodplain splay sandstones. The smallest scale upward sandying and thickening successions is recognized as avulsion packages that form the building blocks of the stratigraphy. Larger scale successions are likely to indicate lobe and fan progradation. The common avulsions also determines how the fan interacts with Lake Uinta, as this work documents lateral transitions between channel and floodplain, deltaic and lake facies across just a few hundred meters to a few kilometers. Furthermore, some mouth bar deposits consist of alternating carbonate grainstones and siliciclastic sandstones, and some abandoned channels are filled with dolomitic mudstones. All these transitions indicates a highly irregular shoreline, where fluvial and deltaic deposits build out locally at the active channel locations, laminated dolomitic mudstones accumulate in protected embayments or abandoned channels, and lime grainstones where lake’s wave and current energy is high. We interpret these fluvial-lacustrine interactions in Sunnyside Delta interval as a result of river avulsions and contemporaneous carbonate productions.