Contrasting styles of shelf sediment transport and deposition in a ramp margin setting related to relative sea-level change and basin floor topography, Turonian (Cretaceous) Western Interior of central Utah, USA

Abstract

The Turonian lower Ferron Sandstone and Juana Lopez Members of the Mancos Shale in the Western Interior foreland basin form a series of regressive shoreface to shelf depositional complexes sourced from the Sevier fold-thrust belt to the west. Continuous, dip-parallel outcrop exposures around the northern and eastern edges of the Tertiary San Rafael Anticline (SRA) in central Utah provide an insight into the origins of anomalous, marine mudstone-encased sandstones. Lower Ferron strata consist of both coarsening-and-thickening upward and sharp-based shoreface successions, interpreted to have been deposited as sequential highstand and falling stage systems tracts. Overlying these shallow marine deposits is a surface that marks sediment bypass that is correlated down-dip into coarse-grained, cross-bedded sandstones, interpreted as the deposits of channelised turbidity flows. These turbidites are considered to be the products of sustained hyperpycnal flows exiting rejuvenated lowstand rivers based on sustained flow indicators such as dune-scale cross-bedding, absence of genetically related slumps and delta front deposits in up-dip areas, surfaces of sediment bypass, coarser grain-sizes than the underlying shoreface deposits, and regional palaeogeography. The flows are interpreted to have been ignitive in proximal areas down a tectonically-induced slope in the uplifted area of the Farnham Dome. The change in depositional style from falling-stage to lowstand systems tracts is attributed to increased rates of sediment supply to the shoreline, promoting hyperpycnal flows. A flooding surface caps the interval and separates the lower Ferron Sandstone from the overlying Juana Lopez Member. This interval in contrast, consists only of heterolithic, parallel and ripple-laminated graded sandstone–mudstone couplets interpreted to have been deposited by turbulent, storm-induced geostrophic flows. The couplets are arranged into bundles and are interpreted as parasequences, which in turn are arranged into a progradational parasequence set deposited during a period of sea-level highstand. Divergent sole mark and bedform palaeocurrents from both members suggest slight eastward deflection of the turbulent flows off the proto-SRA. Further evidence for this topography is inferred from an absence of turbidites along the western side of the structure. The inferred influence of local basin floor structures on sedimentation of these units has important implications for structural models for this basin, for generalized sequence stratigraphic models applied to ramp-type foreland basin sequences and for elucidating the origin of anomalous marine mudstone-encased sandstone bodies.