Abstract

Few studies reconstruct paleohydraulic parameters, such as meander amplitude and sinuosity, of ancient rivers based on plan-view outcrop exposures, but rather rely on interpolation from cliff exposures. In this study, extensive plan-view exposures of amalgamated ancient meander belts in the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation, Utah, USA, allow paleohydraulic reconstruction, evaluation of grain-size variability in meander belts, and their component unit and compound bars by incorporating the variability of paleocurrents, facies architectural-element analysis, and bar migration patterns.

Three channel belts were identified, marked by scour surfaces, an increase in grain size, and abrupt changes in paleocurrent orientations. The youngest channel was about 2.1 m deep and 118 m wide, and had a meander wavelength of 1 km and a sinuosity of 1.1–2.9. The middle channel was about 3.4 m deep, with a meander wavelength of 2.6 km, and a sinuosity of 1.2. The oldest channel was insufficiently exposed to document its plan-view style. Grain-size trends showed systematic upwards fining in each channel story. In plan view, coarsening occurs towards the bend apex along the bend axis, and fining occurs downstream within some meander scrolls. Plan-form grain-size trends were not obvious, partially due to grain-size variation within unit bars, perhaps also reflecting the fact that the exposures capture variable vertical position in each channel belt. Paleocurrents showed systematically varying trends within individual channel belts (from NE, to SW, to E), and abrupt changes between belts. Grain size and vertical facies associations vary as a function of the style of bar migration, as well as position within a bar (upstream vs. downstream). The outcrop shows a dominance of a meandering-river style, with significant lateral amalgamation of successive point bars within each belt. Compound braid bars, built by overlapping unit bars, constitute the youngest channel deposits, and are probably associated with channel abandonment. Independent measurements of meander wavelength, based on plan-view exposures, match results estimated from empirical equations. The Ferron rivers are small to medium in scale according to calculated paleohydraulic parameters (Q_w = 1.6 ∼ 2.8 × 10² m³/s).

Architectural-element analysis shows that the last stage, and the most sinuous channel belt (A1), filled largely with downstream accreting unit and compound braid bars, similar to those seen in high-sinuosity meander loops of the modern Red River at the Texas–Oklahoma border. This suggests that both braid bars and point bars may occur within a single meander channel and that braid bars may evolve into compound point bars as they migrate and attach to a channel margin.