From cross‐strata sets to allogenic controls: Paleohydraulics of a distal channel‐belt complex in the Kayenta Formation (Sinemurian), southwestern Utah

ABSTRACT The depositional architecture of fluvial deposits reflects various controls (channel style, tectonics, climate and sediment supply), making it challenging to constrain individual influences. The Kayenta Formation (Sinemurian) offers a unique opportunity to assess such controls by removing three (channel style, bedload grain size and eustasy). A well‐exposed channel‐belt complex at Warner Valley (SW Utah) enables analysis of whether the distal Kayenta – characterised by greater overbank preservation in contrast to amalgamated sandstones proximally – was dominated by channel splitting or sinuosity, and why its architecture changes distally while channel style does not. Combined paleohydraulics analysis, numerical bar accretion reconstruction, outcrop architecture and bedform characterisation at Warner Valley allowed the sought factor reduction. Results reveal a vertical succession of sandsheet and channel‐fill cross‐set thickness and grain size with covariance that is positive (0.81) in the Springdale Sandstone Member and negative (−0.69) in upsection channel belts of the Kayenta, along with a decreasing sand/mud ratio for sandstones of similar grain size. Cross‐set thicknesses indicate dunes 1 m high‐climbing sub‐critically at <2.2° on the lee side of bars, which were inclined at <10°. Bar accretion was predominantly downstream. Paleohydraulics‐derived widths of channels (150 to 535 m) and channel belts (995 to 3208 m) underestimate their widths in the field (at least 400 to 2000 m and 1180 to 11 000, respectively). Paleoflow was 6 to 10 m deep, decreasing upsection. Bedload deposits and bar accretion indicate a braided system with occasional flashy discharge. Vertical trends suggest suspended load increase upsection along with greater flow channelisation, from poorly channelised bars and sandsheets (Springdale Sandstone) to narrower channels and channel belts. Persistent braiding in the distal Kayenta implies that greater overbank preservation reflects higher tectonic subsidence rates than in the proximal sector. The quantitative approaches used here prove helpful in untangling controls on fluvial deposition.