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

Abstract

AbstractAn 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 upstream‐accretion 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 lateral‐accretion 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.