Grain‐size trends, basin subsidence and sediment supply in the Campanian Castlegate Sandstone and equivalent conglomerates of central Utah

Abstract

Reconstructions of grain‐size trends in alluvial deposits can be used to understand the dominant controls on stratal architecture in a foreland basin. Different initial values of sediment supply, tectonic subsidence and base‐level rise are investigated to constrain their influence on stratal geometry using the observed grain‐size trends as a proxy of the goodness of fit of the numerical results to the observed data. Detailed measurements of grain‐size trends, palaeocurrent indicators, facies and thickness trends, channel geometries and palynological analyses were compiled for the middle Campanian Castlegate Sandstone of the Book Cliffs and its conglomerate units in the Gunnison and Wasatch plateaus of central Utah. They define the initial conditions for a numerical study of the interactions between large‐scale foreland basin and small‐scale sediment transport processes. From previous studies, the proximal foreland deposits are interpreted as recording a middle Campanian thrusting event along the Sevier orogenic belt, while the stratal architecture in the Book Cliffs region is interpreted to be controlled by eustatic fluctuation with local tectonic influence. Model results of stratal geometry, using a subsidence curve with a maximum rate of ≈45 m Myr−1 for the northern Wasatch Plateau region predict the observed grain‐size trends through the northern Book Cliffs. A subsidence curve with a maximum rate of ≈30 m Myr−1 in the Gunnison–Wasatch Plateaus best reproduces the observed grain‐size trends in the southern transect through the southern Wasatch Plateau. Eustasy is commonly cited as controlling Castlegate deposition east of the Book Cliffs region. A eustatic rise of 45 m Myr−1 produces grain‐size patterns that are similar to the observed, but a rate of eustatic rise based on Haq et al. (1988) will not produce the observed stratal architecture or grain‐size trends. Tectonic subsidence alone, or a combined rate of tectonic subsidence and a Haq et al. (1988) eustatic rise, can explain the stratal and grain‐size variations in the proximal and downstream regions.