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AAPG Bulletin, V.
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
Simon A. J. Pattison
Department of Geology, Brandon University, Brandon, Manitoba, Canada; [email protected]
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, throughgoing 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 landward 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 landward. Timelines extend uninterrupted from the shallow marine into the neighboring coastal-plain deposits, connecting flooding surfaces with coals. Shoreface sandstones emerge from the adjacent coastal plain. This has significant implications for modeling 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 reexamined in light of the high-resolution sequence stratigraphic model presented herein.
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