Controls on the depositional architecture of fluvial point‐bar elements in a coastal‐plain succession

The architecture and lithofacies organisation of fluvial point‐bar elements record the spatio‐temporal evolution of river channels. This study discusses the factors that control facies distributions and geometries of point‐bar elements present in a fluvial succession that accumulated on a low‐gradient coastal plain on the western margin of the Western Interior Seaway (Campanian Neslen Formation, eastern Utah, USA). Forty outcropping point‐bar elements located within an established sequence stratigraphic framework have been examined through facies, architectural and palaeocurrent analyses. Point‐bar elements increase in width‐to‐thickness aspect ratio vertically through the succession. Four point‐bar element types are identified based upon their lithofacies assemblages and geometry. Two point‐bar types conform to those depicted in traditional facies models; they are dominated by cross‐bedded sandstone, with subordinate amounts of ripple‐laminated and horizontally laminated sandstone. In contrast, the other two point‐bar types exhibit unusually low proportions of cross‐bedded sandstone and higher proportions of massive sandstone, horizontally laminated sandstone and ripple‐laminated sandstone. The occurrence of these atypical point‐bar assemblages is restricted to the marine‐influenced lower and middle parts of the Neslen Formation. An up‐succession increase in aspect ratio and degree of amalgamation of point‐bar elements through the Neslen Formation may reflect a decrease in the rate of accommodation generation, an increase in the rate of sediment supply, or autogenic processes that operated on an overall prograding coastal plain. The accumulation of point‐bar elements with lower proportions of cross‐bedded sandstone in the lower Neslen Formation can be attributed to decreased stream power. Database‐assisted analysis has been undertaken to compare the lithofacies and architecture of point‐bar elements from the Neslen Formation to those in other humid‐climate, coastal‐plain successions. This comparison reveals that the geometry and facies observed in point‐bar elements of the Neslen Formation might record an unusual set of combined allogenic (accommodation generation and fluvial discharge variations) and autogenic (backwater processes and presence of peat mires) process interactions.