The Upper Castlegate Sandstone and Sego Sandstone are laterally equivalent units of the Upper Cretaceous Mesaverde Group which crop out along the Book Cliffs of eastern Utah, and represent sediment shed into the Sevier foreland basin from the Sevier thrust belt to the northwest. The Upper Castlegate Sandstone consists of fluvial sandstone and shale, and grades southeastward into the estuarine Sego Sandstone and intercalated marine tongues of the Mancos Shale.

The studied interval contains three regional angular unconformities which bound stratigraphic sequences around 100m thick and of 1-3 m.y. duration, here termed high order sequences l-III. The base of Sequence I is at the base of the Castlegate Sandstone, the base of Sequence II at the base of the Sego Sandstone, and the base of Sequence III at the base of the Bluecastle Tongue of the Castlegate Sandstone. The high order sequence boundaries cut down-section northwestward toward the Sevier thrust belt and mark changes in sediment dispersal patterns and provenance, none of which can be attributed to eustatic sea-level falls. The formation of the high order sequences is attributed to isostatic rebound and erosion of proximal parts of the foreland basin following cessation of thrust events in the adjacent Sevier thrust belt.

In the southeastern part of the study area, high order Sequence II (~Sego Sandstone) contains several (>4) nested stratigraphic sequences around 20m thick and of ~100 k.y. duration, here termed low order sequences. The low order sequence boundaries are disconformities rather than angular unconformities and are not marked by changes in sediment dispersal patterns or provenance. Each of the low order sequences consists of: (a) a lowstand systems tract of incised valleys (up to 14m thick) filled with estuarine sandstone, and (b) a transgressive systems tract of marine shale. No highstand systems tracts are preserved. The origin of these sequences (tectonic/eustatic) is indeterminate. When traced northwestward toward the Sevier thrust belt, the low order sequence boundaries become conformable and unrecognizable, and no low order stratigraphic sequences corresponding to those in the Sego Sandstone can be recognized within the alluvial Upper Castlegate Sandstone.

The observed nested sequence architecture in this part of the Mesaverde Group is considered to be controlled by variation in subsidence rate related to the emplacement and erosion of thrust sheets in the adjacent Sevier thrust belt. During emplacement of thrust sheets, subsidence rates in proximal areas of the foreland basin were sufficiently high to offset any low order base-level fails, resulting in the deposition of a conformable fluvial succession. In distal parts of the basin where long term subsidence rates were lower, low order base-level falls produced relative falls in sea level and generated spatially restricted low order sequences. At times of maximum rebound, the entire study area was affected by erosion, resulting in the formation of the regionally extensive high order sequence boundaries.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah