Spatial variation in the sedimentary architecture of a dryland fluvial system

Abstract

Ancient dryland terminal fluvial systems are often recognised within the rock record for having a progressive downstream decrease in the size and amalgamation of channel elements and systematic downstream increase in sheet and overbank elements, alongside the downstream decrease in grainsize that is displayed by most fluvial systems. The spatial distribution and downstream trends displayed by the fluvial sediments of the Lower Jurassic Kayenta Formation of south-western USA, have been examined quantitatively. The results indicate many trends that are similar to those of a dryland terminal fluvial system, including, a lack of confinement of the fluvial system, a downstream decrease in channel and sheet element amalgamation and width-to-thickness ratios, a downstream decrease in grainsize, albeit very small, and an increase in the percentage of overbank elements downstream. However, the study highlights several downstream relationships that are atypical. While some of these relationships may be the result of external factors inherent in this study, others, including the thicknesses of channel-fill and sheet elements that display no significant relationships to distance downstream, and channel-fill elements that display no significant variation in average grainsize with distance downstream, may be a consequence of fluvial interaction with a competing and coeval aeolian system. This work demonstrates the inherent complexity in arid dryland fluvial systems and the downstream architectural and compositional relationships that they depict. Consequently, models for fluvial style may provide only a first-order approximation for downstream trends in dryland systems, as the controlling factors upon these systems are inherently difficult to unravel, and the sedimentary detail is strongly dependent upon external setting and internal complexity. Consequently, a generalised model may not always be applicable to these systems.