Forebulge influence on deposition of the Cretaceous Castlegate Sandstone, Book Cliffs, Utah, U.S.A.

Abstract

Analysis of facies changes and paleocurrent directions of the incised valley fill of the Late Cretaceous Castlegate Sandstone supports a reinterpretation of the axis of the well-studied Castlegate Sandstone incised valley axis from an east-west trend to a north-south trend. Strata in the easternmost extent of the incised valley fill are reinterpreted to represent fill along the edges of the valley, rather than loss of fluvial energy at drainage terminations as previously interpreted. Drainage systems of the Castlegate Sandstone illustrate the influence of topography of the Sevier foreland basin on depositional patterns of the western extent of the Cretaceous Western Interior Seaway (KWIS) during lowstand events. The Castlegate Sandstone was mapped from Tuscher Canyon, Green River, Utah to its easternmost expression to gather paleocurrent data and investigate morphologic changes within the incised valley fill. Early-stage valley fill, consisting of braided-stream facies, is concentrated in the western portion of the study area and paleocurrents in these deposits record flow to the south. Late-stage valley fill consists of meandering-stream deposits. The eastern portion of the incised valley fill is dominated by late-stage deposits with paleoflow to the west. Detailed correlation of Castlegate Sandstone lowstand strata indicates forebulge exposure. Forebulge exposure decoupled the Sevier foredeep from the KWIS, allowing foreland basin topography to become the major influence on drainage systems during lowstand events. The north-south orientation of the axis of the Castlegate Sandstone incised valley reflects axial drainage of the foreland basin. Reexamining lowstands of the Castlegate Sandstone, emphasizing facies reconstruction and paleoflow directions, illustrates the effect of the basin topography on the drainage patterns of the KWIS. Recognizing that the incised valley expressed axial flow may lead to new interpretations about the stratal evolution of the foreland basin.