Abstract

The late Middle Jurassic–early Eocene (∼ 120 m.y.) sediment-accumulation history of the Cordilleran foreland basin in northern Utah exhibits a sigmoidal pattern on a rate vs. time plot, with moderate rates of accumulation during late Middle Jurassic, very low net rates during Late Jurassic–earliest Cretaceous, increasingly rapid rates during Early-middle Cretaceous, and low rates during Late Cretaceous–early Eocene time. This pattern is consistent with deposition in a prograding foreland-basin system that comprised integrated back-bulge, forebulge, foredeep, and wedge-top depozones. The upper Middle Jurassic represents the back-bulge depozone; the Upper Jurassic was deposited on the eastern flank of a flexural forebulge; the basal Cretaceous unconformity is the result of eastward migration of the forebulge; the thick, Lower-middle Cretaceous succession represents the foredeep depozone; and the Upper Cretaceous–early Eocene embodies the syndepositionally deformed wedge-top depozone. Previous models that explain Middle-Late Jurassic stratigraphic patterns in terms of foredeep subsidence (alone) and a Late Jurassic hiatus in crustal shortening in the Cordilleran orogen are shown to be neither necessary nor supported by evidence from the Cordilleran hinterland.

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