Stratigraphic architecture within delta plain deposits in the Upper Cretaceous Neslen and Farrer Formations, Book Cliffs, Utah

The Upper Cretaceous Neslen and Farrer Formations record a complex interaction of delta plain, estuarine, and interdistributary bay environments deposited along the western margin of the Western Interior seaway. A data set based on 62 sedimentary logs along a 27-km outcrop transect, in combination with 2 cored wells and log data from 86 wells, enables detailed mapping of both the lateral and vertical distribution of facies associations. Autocyclic processes are typically represented by surfaces indicative of deepening events of local extent. These surfaces may reflect either delta-lobe or trunk channel avulsion, or a combination of this. Additionally, significant variation in the rate of generation of accommodation (as well as changes in sediment supply) during deposition is evident by basinward and landward shifts in paleoenvironment across stratigraphic surfaces of regional significance. These changes are attributed to allocyclic processes. Based on these large-scale shifts, a threefold subdivision of the Neslen Formation into a lower, middle, and upper interval has been applied. The lower and upper Neslen intervals, and the lowermost Farrer Formation, are predominantly composed of coastal plain deposits, whereas the intervening middle Neslen interval consists of stacked bayfill units. Five sequence boundaries are interpreted and are either characterized by incision and estuarine fill or by a change in fluvial channel architecture. The improved understanding of spatial distribution and strati graphic control within paralic facies of the Neslen and Farrer Formations would provide valuable input to reservoir characterization of subsurface formations in the North Sea (Husmo et al., 2003), along the Norwegian Continental shelf (Thrana et al., 2014), North America (Hurley et al., 1992), as well as Russia, Kuwait, Venezuela, Saudi Arabia, Iraq, Nigeria and Algeria (Reynolds, 2017), and the Northwest Shelf of Australia (Heldreich et al., 2017).