Tectonic control of nested sequence architecture in the Sego Sandstone, Neslen Formation and Upper Castlegate Sandstone (Upper Cretaceous), Sevier Foreland Basin, Utah, USA

Abstract

Abstract The Sego Sandstone, Neslen Formation and Upper Castlegate Sandstone are laterally equivalent formations of the Upper Cretaceous Mesaverde Group, which crop out along the Book Cliffs of eastern Utah and represent sediment shed into a foreland basin from the Sevier orogenic belt to the west. The studied interval contains three regional unconformities which bound stratigraphic sequences around 100 m thick and of one-to-three million year duration, here termed high-order Sequences I–III. The high-order sequence boundaries display increasing depth of erosion northwestward toward the Sevier orogenic belt and mark changes in sediment dispersal patterns and provenance, none of which can be explained by eustatic sea-level falls. Their formation is attributed to erosion and isostatic rebound of proximal parts of the foreland basin following thrust events in the adjacent Sevier orogenic belt. In the southeastern part of the study area, high-order Sequence II (∼Sego Sandstone) contains several (>4) nested stratigraphic sequences around 20 m thick and of hundred thousand year 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 these low-order sequences consists of: (a) LST of incised valleys (up to 14 m thick) filled with estuarine sandstone, and (b) TST of marine shale and sandstone. No HSTs are preserved. The origin of these sequences (eustatic/tectonic) cannot be determined. When traced to the northwest, 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 of the Upper 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 orogenic belt. During emplacement of thrust sheets, subsidence rates in proximal areas of the basin were sufficiently high to offset any low-order base-level falls, 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.