The architecture of fluvial-deltaic sequences in the Upper Mesaverde Group (Upper Cretaceous), Book Cliffs, Utah

Abstract

Abstract The Mesaverde Group is a clastic wedge that prograded eastward into the Western Interior Basin from the Sevier Orogen of central Utah during the Late Cretaceous. The Blackhawk Formation and the overlying Castlegate Sandstone represent tongues of fluvial-deltaic and shoreface strata that prograded far into the basin in response to pulses of tectonism. The Desert Member of the Blackhawk Formation includes two fourth-order stratigraphic sequences and the Castlegate Sandstone represents the lower part of a third sequence. The upper two sequences begin with regional erosion surfaces, above which widespread fluvial blankets represent backfill deposits formed during periods of relative base-level rise. Transgressive and highstand deposits are poorly represented at the top of the Blackhawk Formation, having probably been removed by erosion prior to Castlegate sedimentation. The overriding tectonic control of this fluvial-deltaic wedge is demonstrated by regional changes in paleoslope and detrital sources. However, the sequence architecture indicates that this regional tectonism was modulated by short-term base-level changes, of tectonic or eustatic origin. Architectural-element analysis of the fluvial units indicates deposition predominantly by large bars (macroforms) in braided river systems. The bars developed by lateral, oblique and downstream accretion. They combine laterally and vertically to form widespread sandstone sheets typically capped by thin mudstones and bounded by flat erosion surfaces. The sheets and their component bars thicken from an average of 4–6 m in the Price area to 6–9 m east of Green River, 140 km down palaeoslope, probably as a result of tributary amalgamation and consequent deepening of the river system. In the subsurface such sheets would define flow units for production purposes. However, they are not internally homogeneous, consisting of bar sandstones <400 m in lateral dimensions, and bar increments and small channels >120 m long and wide. Many of these heterogeneities are bounded by mudstone units, and contain dipping accretion surfaces which could act as baffles to lateral fluid movement.