Reconstruction of a sandy point‐bar deposit: implications for fluvial facies analysis

The Salt Wash distributive fluvial system (DFS) of the Upper Jurassic Morrison Formation consists of stacked fluvial channel bodies interbedded with overbank deposits. The Salt Wash DFS has previously been recognised as a braided fluvial fan covering an area of over 100,000 km2. However, the addition of high‐resolution satellite imagery means planform exposures are now recognisable and point bar deposits can be identified in the sand‐rich proximal to medial reaches of the system, requiring a reassessment in this area of the DFS. An individual exhumed point bar deposit has been identified within a 30,000 km2 plan view area of meander belt in central Utah, which offers a unique perspective into the preserved internal distribution of facies and architecture. Field techniques have been utilised in conjunction with LiDAR and heli‐drone three‐dimensional outcrop datasets to compare measurements of the bedform and barform architectural elements within two contrasting outcrop exposure styles. One outcrop (Caineville Wash) has extensive plan view and vertical exposures, whereas the other (Caineville Road) is semi‐restricted to 2D canyon exposures with limited planform exposure. The contrasting exposure styles have been used to develop criteria for the interpretation of sandy meandering river deposits in 2D exposures, where plan view characteristics are not available. Internally, the point bar body exposed at Caineville Wash outcrop can be subdivided into three portions from the plan view imagery: upstream, central and downstream portions of a point bar. Mapping of the internal architecture of the fluvial bar allows recognition of downstream and laterally accreting components. The upstream and downstream portions of the point bar are predominately composed of downstream migrating barform deposits; and the central portion of the point bar consists of laterally accreting elements. Sedimentary logs taken from around the outcrop display vertical profiles commonly considered characteristic of a braided fluvial system. Through understanding the internal architecture of the Caineville Wash point bar deposit, it is possible to create a planform reconstruction of a stacked multi‐storey channel body from a two‐dimensional outcrop. Results indicate that point bar deposits in sand‐rich fluvial systems may have been incorrectly interpreted as braided deposits due to: 1) the presence of a significant proportion of downstream accreting elements within the point bar deposits; and 2) 2D sedimentary logs considered characteristic of a braided fluvial system (such as vertical grain‐size trends and repeating conglomerate lags). Subsequently, sandy meander‐belt deposits may be under‐represented within the proximal – medial portion of the Salt Wash DFS.