Abstract

Transverse ribs — a gravel bedform in shallow, high-energy fluvial systems — are a series of narrow, current-normally orientated accumulations of large clasts. On Spring Creek alluvial fan, Yukon, they are common and occur in braided channels and on longitudinal bars. Aspects of their occurrence indicate that rib sequences form very rapidly (via abrupt stages) upon waning flow over surfaces that are often immobile.

Previous flume experiments showed that ribs can form beneath an upstream-migrating hydraulic jump. On the grounds that super-critical flow is rare in Spring Creek, and prompted by observations in a large flume, this study supports and extends the viewpoint that transverse ribs are relict antidune bedforms. This alternative hypothesis receives support from the nature of bivariate relationships of rib wavelength versus rib width and coarseness of rib material, as well as from a successful prediction of paleoflow parameters, using data from Spring Creek and another study. Paleovelocity is calculated on the basis of antidune wavelength and an equation for competence. Two methods, based on clast size versus shear stress relations, are selected for estimating paleodepth: the results indicate that large error arises if reconstruction is attempted for flow over abnormally low-sloped sections of the alluvial surface. The analysis concludes that a large majority of rib-forming flows are sub-critical, and it is suggested that the development of kinematic waves assists rib formation.

The preservation potential and/or ease of recognition of rib sequences in ancient alluvium appears to be low. However, because direct hydrologic investigation in rib-forming environments during high-stage flow is hampered by practical difficulties, ribs assume considerable importance by supplying this information indirectly, with reasonable accuracy.