Stratigraphic complexity in fluvial fans: Lower Eocene Green River Formation, Uinta Basin, USA

Abstract

In this study, measured outcrop sections and geolocated photomosaics are integrated with areal mapping of channel dimensions, degree of amalgamation, calculations of channel‐to‐floodplain ratios and sedimentary facies variability to study and quantify the channel and floodplain deposits in the Sunnyside Delta Interval of the Lower Eocene Green River Formation in the Uinta Basin, Utah. Vertically, sand content and bed thickness increases, due to an increase in the channel‐to‐floodplain ratio, channel size and the degree of channel amalgamation. Laterally, the channel‐to‐floodplain ratio, channel size, the degree of channel amalgamation and the sand content in channel facies decreases in the paleo‐downstream direction. Such vertical and lateral transitions identify the Sunnyside Delta Interval as a fluvial fan (or distributive fluvial system). However, the vertical and lateral transitions occur at multiple spatial scales, demonstrating considerable stratigraphic complexity as compared to the existing facies and architectural models suggested for fluvial megafans and distributive fluvial systems. The smallest‐scale transitions are identified as avulsion‐related packages that form the building blocks of the stratigraphy, whereas the intermediate‐ and largest‐scale transitions are suggested to be related to lobe and whole fan progradation respectively. This documented complexity indicates the significance of self‐organization in building fluvial fan stratigraphy, and demonstrates that changes in the degree of channel amalgamation or in channel‐to‐floodplain ratio are not linked to accommodation changes. On facies scale, an abundance of Froude supercritical‐flow and high‐deposition‐rate facies, in‐channel mud deposits, and in‐channel bioturbation and desiccation indicate deposition in rivers with highly variable discharge. Such discharge conditions suggest seasonally and inter‐annually variable precipitation conditions in the US Western Interior in the Early Eocene.

Variable‐discharge‐river macroforms in the Sunnyside Delta Interval of the Eocene Green River Formation, Uinta Basin, USA

“…The northward‐flowing California palaeoriver is inferred to have formed a large fluvial‐fan system, represented today by deposits of the Colton/Wasatch and Green River formations (Fig. ), that interacted with the Lake Uinta further to the north (Plink‐Björklund et al., ; Gall et al., ; Jones, ; Wang & Plink‐Björklund, ). The previously documented palaeo‐flow directions in the Colton/Wasatch and Green River Formations corroborate a general northward sediment transport direction with some variability from north‐west to north‐east (Fouch et al., ; Dickinson et al., ; Remy, ; Schomacker et al., ; Ford et al., ; Gall et al., ; Jones, ).…”

“…), a ca 150 m thick fluvial‐deltaic and lacustrine succession stratigraphically bound below and above by widespread lake intervals, the ‘D Marker’ and ‘C Marker’, respectively (Remy, ; Keighley et al., ; Morgan, ). Despite its name, the Sunnyside Delta Interval consists of an alternation of riverine channel and floodplain deposits with deltaic and lake deposits (Keighley et al., ; Pusca, ; Wang & Plink‐Björklund, ). The age of the Sunnyside Delta Interval is constrained by an absolute date from the C Marker of 49·58 Ma (Smith & Carroll, ; Birgenheier et al., ), as well as the 49 Ma and 48·4 Ma dates below and above the Mahogany interval higher in stratigraphy (Smith et al., ) (Fig.…”

Facies and depositional model of alluvial fan dominated by episodic flood events in arid conditions: An example from the Quaternary Poplar Fan, north‐western China

“…Climate‐controlled depositional patterns are usually considered high‐frequency and can overprint depositional patterns developed in response to tectonics, which are typically low‐frequency (Waters et al ., ; Jones et al ., ; Chen et al ., ). In particular, climatically induced water and sediment supply is supposed to be the dominant control (Jones et al ., ; Salcher et al ., ; Harvey, ; Wang & Plink‐Björklund, ). Especially during the Pleistocene, distinct oscillations in climate exerted a large influence on the sediment supply to discharge ratio, which can have an important effect on whether the fans undergo aggradation or degradation (Wells & Harvey, ; Vandenberghe, ; Salcher et al ., ; Harvey, ).…”

“…Especially during the Pleistocene, distinct oscillations in climate exerted a large influence on the sediment supply to discharge ratio, which can have an important effect on whether the fans undergo aggradation or degradation (Wells & Harvey, ; Vandenberghe, ; Salcher et al ., ; Harvey, ). In addition, any abrupt changes in sediment supply to a reach will result in changes in the channel pattern and become the main trigger of channel bifurcation, avulsion and confluence (Pelletier et al ., ; Weissmann et al ., ; Chakraborty et al ., ; Salcher et al ., ; Wang & Plink‐Björklund, ). Moreover, ephemeral discharge regime under arid conditions can result in downstream disappearance of river systems before reaching a standing body of water (for example, terminal fans) (Kelly & Olsen, ; Nichols & Fisher, ; Cain & Mountney, ).…”

Experimental distributive fluvial systems: Bridging the gap between river and rock record

“…Whilst individually these criteria can relate to various fluvial styles (Sambrook Smith et al, 2010), combined they make a strong case for a depositing DFS. However, increasingly high-resolution studies of the ancient stratigraphic record of DFS recognize a greater stratigraphic complexity across the preserved fan system than previously appreciated (Cain and Mountney, 2009;Owen et al, 2015a;Wang and Plink-Björklund, 2019). The mechanisms facilitating some facies trends (e.g.…”