Formation of point bars through rising and falling flood stages: Evidence from bar morphology, sediment transport and bed shear stress

摘要

Flow processes and sediment transport in a channel bend and associated point bar have been studied in modern rivers, theoretical models and physical experiments: however, the relationship between flow process and point-bar morphology has rarely been explained due to the complex nature of open channel flow. Plan-view exposures of an ancient point-bar complex, exposed at the top of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation, south-central Utah, allowed reconstruction of bar morphology, sediment transport and bed shear stress, which were used to extrapolate flow processes. Studies of these outcrops show that compound point bars and scroll bars were probably formed during falling and rising flood stages, respectively. A simulation model of plan-view channel form shows that channel dimensions, such as radius of curvature and sinuosity of the point-bar complex, range between 205m and 351m and 104 and 122, respectively, throughout the evolution of the channel bend. Variations in strength of the helical flow were interpreted as the main control on facies architecture and bar morphology. Strong helical flow was related to the deposition of the scroll bars, while strength of helical flow is decreased for compound bars. The use of cross-beds as a common palaeocurrent indicator was found to be inconsistent with mean flow directions and channel margin orientation.