AN ALTERNATIVE MODEL FOR SHORELINE-PARALLEL, DETACHED SANDSTONES IN THE CRETACEOUS WESTERN INTERIOR SEAWAY BASED on THE FERRON SANDSTONE OF SOUTH-CENTRAL UTAH

Facies analysis of the Upper Cretaceous (Turonian) Ferron Sandstone in the western Henry Mountains of south-central Utah, USA, indicates sediment accumulation in a series of flood-dominated, marine current- and wave-influenced, deltas that were deflected in a shore-parallel (southward) direction. Twelve lithofacies are recognized and interpreted to record an array of delta plain, delta front, prodelta and offshore shelfal depositional environments. Deltas prograded into shallow water, forming mainly sharp-based mouth-bar sand bodies. Mouth bar and delta front sandstone bodies are broad lenses to sheets in the north-south (depositional strike) direction, and many are laterally amalgamated and extensive over distances of several km. The upper delta front was evidently fluidal and prone to failure, leading to the development of rotational slope failures, debris flow-filled gullies, and, in places, growth faults. Paleocurrent data indicate that the regional sediment dispersal direction was eastward. Data from delta-front facies, however, suggest that outflow plumes and associated bottom currents were deflected towards the southeast, giving rise to an asymmetric delta planform. The Holocene and modern Burdekin River Delta of NE Australia is considered a close planform, process, and facies analog for the Ferron Notom deltas. The Burdekin Delta and Ferron Sandstone facies assemblages are vertically and laterally heterogeneous, despite being the products of consistent arrays of environmental controls. Adopting a model that incorporates such a degree of heterogeneity may negate the need for multiple depositional models for complex stratigraphic intervals such as the Ferron Sandstone. The facies model also suggests that deltas showing planform asymmetry may be produced by directional growth of delta lobes, rather than by deflection of beach ridges about the river mouth. Such a model of asymmetric delta lobe growth and extension in the direction of dominant longshore sediment transport may help to explain the common occurrence of apparently isolated, elongate and broadly shoreline-parallel sandstone bodies, detached from the contemporaneous shoreline by 10s of km, in the Cretaceous of the North American Western Interior Seaway and elsewhere.