Chapter 10 - Facies and architecture of unusual fluvial–tidal channels with inclined heterolithic strata: Campanian Neslen Formation, Utah, USA

Abstract

Abstract The sedimentary facies and architecture of Late Campanian fluvial–tidal channels and channel belts of the Neslen Formation are described from the Floy area of the upper Book Cliffs in northeastern Utah using outcrop data. Vertical sedimentary sections in multiple closely spaced canyons, combined with photomosaics and LIDAR data, describe in detail the deposits over an area of about 3 km by 2 km. The tidal–fluvial channels and channel belts fed sediments to embayments with bay-head deltas and to shorelines to the east. The Neslen distributary system (known to deposit diachronously, younger deposits towards east) fed the mixed energy (tidal and wave) shorelines of the Sego, Corcoran, and Cozette systems of eastern Utah and western Colorado. Above the inclined heterolithic strata (IHS) units of the Neslen Formation, there are more proximal alluvial plain deposits of the Blue Castle Tongue Sandstone, so that the succession is overall progradational. Focus is on the up to 8-m-thick channel units of IHS of Neslen Formation, that stack in successions with four to five stories and are interpreted as tidally influenced fluvial channel belts. The channel interpretation is supported by the presence of sharp erosional boundaries between the IHS units and overall “lenticular” geometry of these units. The tidal interpretation is based on facies characteristics such as bioturbation (Skolithos, Palaeophycus, Teichichnus, Teredolites), tidal rhythmites, occasional bidirectional paleocurrents, mud drapes, and examples of ripple and dune cross-strata that climb high upward on the point-bar accretion surfaces. The architecture and facies of the Neslen fluvial–tidal point bars show some specific characteristics that are different from previously described IHS strata such as a (1) dominance of dm-thick rippled sandstone beds within the heterolithic strata throughout the channel-fill deposits (little vertical or lateral variability), (2) no consistent trend of upward fining or coarsening from bottom to top of the point-bar units, and (3) lack of any channel abandonment “mud plug” deposits. The resulting model for Neslen fluvial–tidal channels is an erosive basal boundary with a “circular” or “draping” fill of the channels under similar flow conditions from bottom to top. Despite almost continuous exposure, correlation of the individual IHS units over a distance > 2–3 km is difficult because of multiple incisions and change in the character (thickness variability of sandstone beds) of individual IHS units.