Interdistributary bayfills, Neslen Formation (Upper Cretaceous), Book Cliffs: Processes and controls

Abstract

The Campanian (Upper Cretaceous) Neslen Formation which crops out in the Book Cliffs, Utah (USA) has been the focus area for this study where 32 sedimentary logs have been collected to identify the sedimentary processes and controls related to the infilling of interdistributary bays. The Neslen Formation is deposited as a clastic wedge sourced from the Sevier Orogeny which prograded eastwards and into the Western Interior Seaway. The middle Neslen Formation consists of six bayfill units where the two upmost bayfills (Bayfill 5 and 6) have been the area of interest. Detailed analyses of the spatial distribution of lithofacies and facies associations has enabled description and understanding of lateral variations in processes and sub-environments internally within the interdistributary bays. The depositional environment in the two bayfills were dominated by wave action, but bayhead deltas, sub-bays, distributary channels, and marshes has also played an important role in the evolution of the bays. Ternary diagrams were used to identify trends which made it possible to predict the lateral distribution of facies associations within a bay. The observed stratigraphic architecture is related to allogenic and autogenic processes. Allogenic processes have created two important and lateral extensive surfaces between the base of the Bayfill 5 Unit and the top of the Bayfill 6 Unit. Eustatic sea-level rise have played a significant role in the development of these two allogenic flooding surfaces observed. Autogenic processes have created many minor and local flooding surfaces internally in the Bayfill 6 Unit. The autogenic flooding surfaces is only observed in the Bayfill 6 Unit probably due to an interplay between allogenic and autogenic processes. An increase in sediment supply and subsidence rate (allogenically controlled) led to a higher frequency of delta lobe switching and avulsion (autogenically controlled) in the bay.