Depositional development in a dryland basin: The Morrison Formation, Eastern Utah, USA

Abstract

The study comprises the Morrison Formation with the three members, Tidwell, Salt Wash, and Brushy Basin members, Upper Jurassic, Utah, USA. The Tidwell Member represents floodplain, lacustrine and fluvial environments. The Salt Wash Member is fluvial dominated, and the Brushy Basin Member is mainly lacustrine. In eastern Utah, the deposits are well exposed both in cross section and planar view. Sedimentological data are collected together with Google Earth-images, used to establish the depositional development through the Morrison Formation. Dryland climate of the region in late Jurassic time affected the hydrology, pattern of sediment transport, sediment distribution and character of depositional environment in the Morrison Basin. The Morrison Basin was created as a back-bulge basin of the Cordilleran Orogeny, with the rate of subduction governing the rate of creation and destruction of accommodation. This is reflected and recorded by vertical sedimentary trend in the Morrison Formation in the study area, including paleosols and unconformities. The fluvial system in the Tidwell and Salt Wash members was earlier believed to be a prograding braided system within a large alluvial fan. Google Earth-images and palaeocurrent measurements reveals that the fluvial system are of a more sinuous character, as part of a major distributary low-gradient fluvial system, termed the Salt Wash distributary fluvial system by Owen et al. (2015b).