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

Abstract

An outcrop dataset from the early Eocene Sunnyside Delta Interval of the Green River Formation in the Uinta Basin, Utah, USA, documents alluvial channel lithosomes. The abundance of Froude supercritical-flow sedimentary structures, together with an abundance of high-deposition-rate sedimentary structures, in-channel bioturbation and pedogenic modification, in-channel muds and thick soft-clast conglomerates, identify these lithosomes as deposits of variable-discharge rivers. These recognition criteria are part of an emerging facies model for variable-discharge rivers. This facies model, however, yet lacks robust recognition criteria for macro-scale or bar-scale stratal patterns of variable-discharge rivers. This study presents a dataset that corroborates some known stratal patterns and provides examples of hitherto unknown bar-scale stratal patterns of variable-discharge rivers, including: (i) low-angle downstream-accretion sets that may form as washed-out sheets in high sediment supply conditions or downstream of hydraulic jumps; (ii) high-angle upstreamaccretion sets that imply deposition from systematically upstream-migrating channel-scale hydraulic jumps (cyclic steps); (iii) concave-up, upward-flattening high-angle downstream-accretion sets that are consistent with aggradation in channel-scale hydraulic-jump scours; (iv) upstream-accretion and lateralaccretion sets that may be linked to high-magnitude flood reworking of point bars; and (v) aggradation or vertical-accretion sets of ambiguous origin. These unconventional stratal patterns are compared to the established bar strata, such as those formed by point bars and braid bars and a discussion is provided on formative conditions for the here documented unconventional strata. This work highlights a need for further studies on the effect of discharge variability on bar formation and on the link between river morphology and bar types.

Bedforms and sedimentary structures related to supercritical flows in glacigenic settings

“…Observations from glacigenic depositional environments can therefore be transferred to non‐glacigenic depositional environments, and vice versa. Glacifluvial systems can be compared to other high‐energy ephemeral alluvial or fluvial systems (Blair, 1999; Fielding, 2006; Froude et al ., 2017; Carling & Leclair, 2019; Wang & Plink‐Björklund, 2020). Glacigenic Gilbert‐type deltas display the same facies types and depositional architectures as non‐glacigenic Gilbert‐type or fan deltas (Massari, 2017; Kostic et al ., 2019; Okazaki et al ., 2020; Postma et al ., 2020), as do glacigenic density flows and non‐glacigenic density flows (Lang et al ., 2017a; Ono & Plink‐Björklund, 2018; West et al ., 2019).…”

“…Fielding (2006) proposed ‘upper‐flow‐regime sheets, lenses and scours fills’ (‘UFR’) as a possible architectural element in fluvial deposits supplementing the widely applied architectural‐element scheme of Miall (1985). Upper‐flow‐regime bedforms are considered as potentially indicative of fluvial sedimentation under strongly seasonal climate, with pronounced variability and discharge peakedness (Alexander & Fielding, 1997; Fielding, 2006; Froude et al ., 2017; Carling & Leclair, 2019; Wang & Plink‐Björklund, 2020). These conditions are met also in glacifluvial systems, where meltwater discharge is subject to strong seasonal fluctuations (Marren, 2005; Cuffey & Paterson, 2010).…”

Morphodynamics of supercritical flow in a linked river and delta system, Daihai Lake, Northern China

“…Architecturally, the here described river deposits are similar to other variable discharge rivers with common downstream or vertically accreting macroforms (Tunbridge, 1981; Stear, 1985; Olsen, 1989; North & Taylor, 1996; Zaleha, 1997; Thomas et al ., 2002; Kumar et al ., 2003; Allen et al ., 2013, 2014; Plink‐Björklund, 2015, 2019; Wang & Plink‐Björklund, 2020) that consist of erosionally bound beds interpreted as flood event beds (e.g. Tunbridge, 1981; Sneh, 1983; Turner, 1986; Abdullatif, 1989; Deluca & Eriksson, 1989; Fielding & Alexander, 1996; Nichols & Hirst, 1998; Shukla et al ., 2001; Hinds et al ., 2004; Hampton & Horton, 2007; Chakraborty & Ghosh, 2010; Chakraborty et al ., 2010; Fielding et al ., 2011; Mader & Redfern, 2011; Donselaar et al ., 2013; Plink‐Björklund, 2015, 2019; Wang & Plink‐Björklund, 2020). High deposition rates are another key characteristic of variable‐discharge rivers (Plink‐Björklund, 2015, 2019), where rapid deposition and aggradation are enhanced by rapid discharge variation of floods (Abdullatif, 1989; Fielding & Alexander, 1996; Esposito et al ., 2018), and high suspended sediment concentrations (Ono et al ., 2020).…”

“…Antidunes form low-amplitude positive bedforms at supercritical flow conditions due to the passage of internal waves migrating downstream or upstream (Cheel, 1990;Alexander et al, 2001;Spinewine et al, 2009). Steeper scours filled with backsets (F9) and broad scours with gravel backsets (F4) are characteristic for experimentally produced upstreammigrating cyclic steps (Cartigny et al, 2014;Ono et al, 2020), and are also documented in outcrops of fluvial deposits (Wang & Plink-Björklund, 2020;Slootman & Cartigny, 2020). Backset bedding forms conformably at the downstream margin of hydraulic jump scours in response to stationary upstream migration of cyclic steps (Ono et al, 2020).…”

Supercritical‐flow processes and depositional products: Introduction to thematic issue

“…For example, a growing number of papers are demonstrating that supercritical-flow sedimentary structures are abundant, if not even dominant, in certain kinds of alluvial successions (Fielding, 2006;Fielding et al, 2011;Plink-Bj€ orklund, 2015), in which they provide important palaeohydrological and palaeoclimatic information (e.g. Allen et al, 2011Allen et al, , 2014Gall et al, 2017;Wang & Plink-Bj€ orklund, 2020), and are informing possible new approaches to a genetic categorization of ancient fluvial channel fills more strictly tied to the hydrology and (palaeo)discharge regimes of formative channels, rather than planform patterns (Fielding et al, 2018).…”