No evidence for an unconformity at the base of the lower Castlegate Sandstone in the Campanian Book Cliffs, Utah–Colorado, United States: Implications for sequence stratigraphic models

Abstract

Regional high-resolution correlations of the Campanian Desert Member of the Blackhawk Formation to the lower Castlegate Sandstone interval, Book Cliffs, Utah-Colorado show no evidence for a single, through-going unconformity separating the amalgamated channel sandstones of the Castlegate from the underlying Blackhawk Formation coastal plain deposits. This is a facies contact, which interfingers both laterally and vertically. Channel incisions are discrete and mostly confined to individual parasequences. They do not coalesce landwards into gigantic valleys as shown in the conventional model but are restricted to the proximal shoreface and are rarely traceable for more than a few kilometers landwards. Time lines extend uninterupted from the shallow marine into the neighbouring coastal plain deposits, connecting flooding surfaces with coals. Shoreface sandstones emerge from the adjacent coastal plain. This has significant implications for modelling in similar clastic settings worldwide. Conventional sequence stratigraphic models of fluviodeltaic systems show radically different stacking patterns compared to those of the high-resolution model presented herein. Each model carries a significantly different prediction regarding the three-dimensional distribution and architecture of reservoir and nonreservoir bodies in a fluviodeltaic system. Hydrocarbon exploration and development activities, such as optimally locating seismic surveys and/or drilling locations could be impacted if the wrong model is applied. As such, it is recommended that fluviodeltaic systems worldwide should be critically re-examined in the light of the high-resolution sequence stratigraphic model presented herein.