Near Bed Flow Process Inferred From Bar Morphology, Sediment Transport and Grain Size Distribution of a Plan-View Exposed Ancient Point Bar Complex

Abstract

Flow process and sediment transport within a channel bend and associated point bar have been studied in modern streams, theoretical models and physical experiments. However, the accuracy of these models are hard to evaluate for long-term evolution of natural rivers. It is also difficult to compare the resulting plan-form bar morphology and facies architecture of modern and modelled systems with what is preserved in the ancient rock record due to the lack of plan-view exposed outcrops. Moreover, compound point bars and scroll bars that are typically found in meandering rivers show different facies architecture, which is essentially the result of different flow processes that have rarely been distinguished. This study examined a point bar complex based on plan-view exposures of channel belts in outcrop of the Ferron Sandstone, south-central Utah. Flow process, sediment transport and bed shear stress show that compound point bars and scroll bars were formed during falling and rising flood stages respectively. The simulation of sine-generated streams showed that channel dimension parameters, such as radius of curvature and sinuosity, have small ranges of 351–205 m and 1.04–1.22 respectively throughout the evolution of the bend. Variation in flow process was interpreted as the main control on facies architecture and bar morphology. In this case, strong helical flow with a maximum strength of 1.05 was developed during the deposition of scroll bars but not the compound bars. The widely used paleocurrent indicator-dip direction of cross beds-was found to be inconsistent with the mean flow direction or the channel margin orientation.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015