LAUSR

Tectonic forcing of delta progradation is increasingly being invoked to explain stratal stacking patterns in foreland basins. Nonetheless, the recognition of different types of tectonic forcing and their consequences for the spatial and temporal distribution of accommodation often rely on incomplete datasets and indirect sequence stratigraphic criteria. Previous work has concluded that the Cenomanian-Turonian Frontier Formation of northern Utah, north-west Colorado, and south-west Wyoming (‘Vernal Delta’) owes its origin largely to tectonic overprinting of depositional patterns, although the lack of a comprehensive sequence stratigraphic framework for the unit has hampered evaluation of this claim. The present study provides detailed facies and sequence stratigraphic analyses based on outcrop sections and wireline log suites from the Uinta, Piceance and Green River basins. Four genetically-related intervals were defined and mapped by using regionally traceable stratigraphic horizons (flooding surfaces and sequence boundaries). Internally, intervals are composed of distal and proximal delta front lithologies, and coastal plain facies. Overall, Intervals 1 to 4 form a major basinward projection of coarse clastic strata generated in response to four separate, high-frequency regressions. Furthermore, a change through time from southward projection of elongate lobes (Intervals 1 and 2) to eastward dispersal and development of a broad, arcuate planform (Intervals 3 and 4) can be explained in terms of changes in prevailing tectonic forcing mechanisms. North–south trending Sevier Orogeny forebulge structures controlled Intervals 1 and 2. West–east progradation (Intervals 3 and 4) was probably controlled by Proterozoic basement lineament reactivation due to Laramide foreland uplifts. Therefore, this study provides direct geological evidence for the initiation of local Laramide deformation as early as 90 Ma. These findings contribute to a more complete understanding of tectonic forcing of coastal to shallow marine successions in foreland basins and the tectonic evolution of the western USA. This article is protected by copyright. All rights reserved.