LONG-TERM SEDIMENT ACCUMULATION IN THE MIDDLE JURASSIC EARLY EOCENE CORDILLERAN RETROARC FORELAND-BASIN SYSTEM

The late Middle Jurassic-early Eocene (similar to 120 m.y.) sediment-a ccumulation history of the Cordilleran foreland basin in northern Utah exhibits a sigmoidal pattern on a rate vs, time plot, with moderate r ates of accumulation during late Middle Jurassic, very low net rates d uring Late Jurassic-earliest Cretaceous, increasingly rapid rates duri ng Early-middle Cretaceous, and low rates during Late Cretaceous-early Eocene time, This pattern is consistent with deposition in a progradi ng foreland-basin system that comprised integrated back-bulge, forebul ge, foredeep, and wedge-top depozones, The upper Middle Jurassic repre sents the back-bulge depozone; the Upper Jurassic was deposited on the eastern flank of a flexural forebulge; the basal Cretaceous unconform ity is the result of eastward migration of the forebulge; the thick, L ower-middle Cretaceous succession represents the foredeep depozone; an d the Upper Cretaceous-early Eocene embodies the syndepositionally def ormed wedge-top depozone, Previous models that explain Middle-Late Jur assic stratigraphic patterns in terms of foredeep subsidence (alone) a nd a Late Jurassic hiatus in crustal shortening in the Cordilleran oro gen are shown to be neither necessary nor supported by evidence from t he Cordilleran hinterland.