Jurassic-Cretaceous evolution of the central Cordilleran foreland-basin system

Abstract

During Jurassic and Cretaceous time deposition in the western interior basin was controlled by a combination of subduction-related dynamic subsidence and thrust-generated flexural subsidence. Changes in the angle of oceanic plate subduction along the western margin of North America and thrust deformation in the Cordillera governed the spatial and temporal influences of these mechanisms throughout basin history. Dynamic subsidence was the primary control on basin deposition during Early-Middle Jurassic and Late Cretaceous time. During these periods, shallow-angle oceanic plate subduction beneath the western margin of North America produced convective mantle circulation and long wavelength subsidence in the western interior. A cessation of dynamic subsidence during Early Cretaceous time, brought on by an increase in the angle of subduction, is partially responsible for the ∼20 m.y. unconformity that separates the Jurassic and Cretaceous sequences in the western interior. During Late Jurassic time, thrusting in the Cordillera resulted in flexural partitioning of the back-arc region. Statal geometries in the Upper Jurassic Morrison Formation in Utah and Colorado indicate deposition in the back-bulge and forebulge depozones of the Late Jurassic foreland basin system and suggest the coeval existence of a flexurally subsiding foredeep to the west. During Early Cretaceous time, >200 km of shortening in the thrust belt resulted in uplift and erosion of the Late Jurassic foredeep and the eastward migration of foreland-basin system flexural components. Areas occupied by the Late Jurassic forebulge were incorporated into the Early Cretaceous foredeep while the Late Jurassic back-bulge depozone became the location of the Early Cretaceous forebulge. In eastern Utah and western Colorado, migration of the forebulge enhanced the regional Early Cretaceous unconformity associated with the cessation of dynamic subsidence. During late Early Cretaceous time sediment accumulation across the entire foreland-basin system may have been facilitated by the reinitiation of dynamic subsidence in the western interior. During the Late Cretaceous, thrusting in the Cordillera resulted in continued flexural subsidence of the foredeep in east-central Utah. However, increased dynamic subsidence throughout Late Cretaceous time allowed thick accumulations of strata to be deposited in the forebulge and back-bulge depozones of the foreland-basin system.