Abstract

The Santonian Emery Sandstone Member of the Mancos Shale is a continental to shallow marine succession that prograded into the Cretaceous Western Interior Seaway of western North America. Sediment sourced from the uplifted Sevier Orogen to the west fed a shoreline in the adjacent subsiding north–south trending foredeep. These sandstones are now superbly exposed along a 120-km-long, strike-parallel, continuous outcrop in the cliffs of the Wasatch Plateau of central Utah. Shoreface parasequences are composed of coarsening-upward successions of hummocky cross-stratified and intensely bioturbated sandstones, indicating deposition at a storm-dominated shoreline. Two distinct depositional complexes record eastward and northeastward shoreline progradation at the northern and southernmost extremities of the Plateau respectively. In the subsurface, Emery strata comprise a thick succession of coal-bearing, coastal-plain deposits. The Emery Sandstone records a period of long-term paralic progradation to aggradation, containing at least 17 parasequences arranged into three, 600 kyr, regressive–transgressive cycles that are recognizable in both complexes. Each cycle comprises only highstand and transgressive systems tracts, and erosional sequence boundaries are absent. Absence of erosional sequence boundaries is attributed to high rates of subsidence in the foreland basin at this time. Santonian glacio-eustatic sea-level fluctuations are proposed to account for the high-frequency cyclicity, whilst longer-term aggradation may be a function of organic controls on shoreface stacking patterns by immobile raised coal mires lying landward of the shorefaces.

When placed in a temporal and spatial context, the Emery Sandstone provides a missing link in the understanding of long-term controls on Late Cretaceous depositional systems in the central Utah segment of the foreland basin, and allows inferences to be made regarding foreland basin dynamics. In particular, thrust tectonics is postulated to have played a major role in governing the architecture of Late Cretaceous strata in central Utah. Marked along-strike thickening of the Emery Sandstone in particular is attributed to long-lived elevated rates of subsidence in northern Utah and southern Wyoming during the Late Cretaceous that is adjacent to concentrated zones of thrust nappes. A north-dipping depositional slope is inferred from this subsidence distribution, aligned approximately parallel to the structural grain of the hinterland load. Northeastward progradational directions of successive Turonian, Santonian, and early Campanian regressive wedges in this part of the Western Interior Basin are attributed to the presence of this tectonically induced regional slope gradient. Sediment sourced to these Late Cretaceous shorefaces is likely to have been sourced through structural recesses within the thrust front, resulting in a spatial alignment of several successive regressive wedges and implicates an important, long-lived kinematic control on sediment supply. Of Late Cretaceous regressive wedges, only the Emery Sandstone does not temporally coincide with reported periods of active thrusting, suggesting that the regression was a consequence of factors other than uplift-related, elevated rates of sediment supply.