Allocyclic and autocyclic controls on facies distribution in interdistributary bayfill deposits of the Campanian Neslen Formation, Eastern Book Cliffs, Utah, U.S.A.

ABSTRACT The Campanian Neslen Formation, in the eastern part of the Book Cliffs, is divided into three stratigraphic intervals. The middle Neslen interval consists of paralic bayfill deposits that average 23 m in thickness. This article describes the lateral and vertical variability within the bayfills deposited in this middle Neslen interval. Comprehensive mapping along 27 km of outcrops, including high-resolution description of 62 sedimentary logs and incorporation of wireline log data from 86 wells, has led to a detailed understanding of the processes related to infilling of interdistributary bays. Six vertically stacked heterolithic to sand-prone bayfill units are identified. A distinction is made between bayfill units and bayfill successions, based on their bounding autocyclic or allocyclic flooding surfaces. Units are bounded by high-frequency flooding surfaces, and show large lateral variations in thickness, facies associations, and lithology. These bayfill units are laterally subdivided into a varying number of bayfill successions, separated by interdistributary channel systems or associated bayhead-delta facies associations. Lateral variation in facies architecture, salinity, and depositional energy levels are mapped in each bayfill succession. This variation is attributed to several controlling factors, including proximity to sediment entry points, water depth, the sequence stratigraphic position of the bayfill succession, and the compaction of underlying sandbodies. The detailed facies patterns and multitude of tributary causes described and discussed here are applicable to an overall understanding of facies architecture in paralic environments.