Abstract

Medano Creek in Colorado provides a unique opportunity to study upper-flow-regime deposition. It is a steep, aggrading fluvial system in which antidunes are the dominant bedform. Medano Creek is a straight, single-channel stream where flow is largely confined to a sequence of upstream-migrating topographic steps (pools and riffles), a stable channel geometry. Antidunes and upperflow-regime plane beds are arranged in an organized pattern on the surface of pool and riffle sequences as a response to spatially varied flow conditions. In the creek, antidunes form sedimentary structures consisting of low-angle backset and foreset beds and subhorizontal laminae grouped into lenses. Laminae in a lens are often subparallel to the lens-bounding surfaces on the downcurrent end and are truncated by the upper bounding surface on the upcurrent end. Upper-flow-regime plane bed and rhomboid ripples produce planar laminae. Through lateral shifting of the active channel and upstream migration of pools and riffles, Medano Creek produces a facies sequence consisting of alternating 10-40-cm-thick layers of (planar) laminae and low-angle, cross-stratified, lenticular bedding.

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