New, high-resolution, regional correlation and isopach maps provide evidence that (1) Laramide-style deformation began as early as ca. 77 Ma in central Utah, and (2) rapid (208 km m.y. -1 ) and extensive (400 km) progradation of a clastic wedge was facilitated by reduced subsidence during the transition from Sevier-to Laramide-style deformation. This study defines three Campanian, alluvialto-marine clastic wedges that traversed 200-400 km eastward across the Utah-Colorado segment of the Cordilleran foreland basin. Wedges A and C are thicker successions with rising-trajectory shoreline stacking patterns (Blackhawk Formation and Lower Castlegate Sandstone, Bluecastle Tongue and Rollins Sandstone) that reflect relatively slow overall progradation (50-81 km m.y. -1 ) of narrow (10-20 km wide), wave-dominated shorelines. In contrast, wedge B consists of lower-volume successions with a flat to falling shoreline stacking pattern (Middle Castlegate Sandstone, Sego Sandstone, Neslen Formation, Corcoran and Cozzette Members of the Iles Formation) that suggests rapid progradation (~208 km m.y. -1 ) of embayed (60-80 km wide), mixed-energy (wave-and tide-influenced ) shore lines. Wedges A and C prograded 200-250 km in more than ~3 m.y., whereas wedge B prograded 340-400 km in ~2 m.y. (~170 km m.y. -1 ). The anomalously extensive wedge B is unique in the Utah-Colorado segment because of its long extent, rapid progradation rate, dominance of tidally influenced facies, long-transit transgressions/regressions, and low-accommodation, nested sequence architecture. Stratigraphic relationships indicate development of wedge B coeval with both Sevier-and Laramide-style deformation in Utah. Assuming a constant sediment supply, the extensive (>300-400 km) "sheet-like" amalgamated wedge (wedge B) may have been caused by (1) reduced subsidence driven by a flexural interference pattern, whereby two short-wavelength (±200 km) flexural profiles are superimposed during the uplift of both basement-cored and thin-skinned thrust belt, (2) an increase in, or eastward migration of, dynamic subsidence during tectonic transitions associated with slab flattening or rollback (i.e., long-wavelength flexure), or (3) reduced subsidence due to short-wavelength flexural interference augmented by the reduction, or migration, of long-wavelength flexure.

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Despite advances in estuarine facies models, ancient bayhead delta deposits are not widely recognized or utilized, and their facies characteristics are poorly documented. Sedimentology of three well-exposed, bayhead delta deposits within the well-known stratigraphic framework of the Book Cliffs, Utah, and comparison to modern bayhead deltas provide insight into their variability, and a model for reservoir prediction. Bayhead deltas develop at the innermost part of bays within wave-dominated and mixed-energy estuaries on transgressive coastlines. The deposits of these deltas are valuable because they record the high-frequency turnaround from transgression to regression; in turn, this delineates the long-term transgression trajectory and informs reservoir prediction. This study uses outcrop data and synthesis of modern bayhead delta characteristics to define criteria for bayhead delta recognition, develop a detailed facies model and highlight their utility in a regional context. The following criteria for bayhead delta identification are proposed herein: (i) overall coarsening-up pattern; (ii) small-scale clinoform (5 to 15 m height; 100 to 1000 m length) showing a down-clinoform decrease in grain size and palaeocurrent energy; (iii) increasing abundance of mud interbeds towards the clinoform toe; (iv) basinward-directed palaeocurrents generated mainly by fluvial input with strong tidal overprint; (v) greatest tidal influence in the intertidal zone of the inactive delta front and better preservation of turbidites at clinoform toes; (vi) brackish trace and body fossils; and (vii) position within an overall backstepping stratigraphic succession. Ancient bayhead deltas described in this study are situated within a moderately rising to flatly rising transgressive interval over a 300 km transect. Steeply rising transgressive trajectories sequester sandy, thicker, better connected transgressive deposits in the inner part of the estuary where bayhead deltas tend to stack vertically. Flatter transgressive trajectories sequester sand in disparate, disconnected estuarine sandstone bodies with partial preservation of bayhead deltas. Proposed criteria aid recognition of bayhead delta deposits in other basins to reconstruct transgressive shoreline trajectories that inform reservoir models and volumetrics.