Reversed-Climbing Ripples and Sand-Wave Deposition in Arkansas River: ABSTRACT

Abstract

This study of modern river deposits substantiates and extends flume studies of regressive and sand waves by Jopling (1961) and Allen (1965). As first proposed by Jopling, regressive are a sequence of climbing oriented in a direction opposite that of the general current flow. At the risk of introducing another new term, they are herein termed climbing ripples rather than regressive ripples, because regression has a specific meaning in geology. Four types of sedimentary features were observed in an exposure of a large (4.5 ft high and 35 ft long) sand wave in a cut bank of the Arkansas River near Tulsa, Oklahoma. In downstream sequence they are: (1) accretionary foresets with tangential bases; (2) avalanche foresets with abrupt basal contacts; (3) reversed climbing ripples; and (4) normal or ripples. The accretionary foresets show a progressive decrease in mean grain size down the foresets, have an angle of slope (28°) less than that of the avalanche foresets (31.5°), and contain no reversed climbing ripples. In contrast to this, the avalanche foresets show a progressive increase in mean grain size down the foresets, a steeper slope, and development of reversed climbing ripples. The change from accretion to avalanche foresets apparently was the result of a decrease in current velocity. As velocities decreased, excessive deposition took place near the head of the foresets resulting in oversteepening and avalanching. At the same time a reverse eddy formed near the toe of the sand wave and produced the reversed climbing ripples. The reversed climbing are finer grained (2.82 vs. 2.43^phgr), poorer sorted, and more strongly fine-skewed than the foreset deposits. In the accretion foreset sands the sorting decreases progressively from top to bottom. The accretionary foreset sands tend to be more positively skewed near the base than they are near the top, whereas the avalanche foreset sands tend to be more positively skewed near the top and more nearly symmetrical near the base. End_of_Article - Last_Page 552------------