An Overlooked Play-Type on a Ramp Margin: Prodelta Turbiditic Lobes, Lower Kenilworth Member, Cretaceous Western Interior, Book Cliffs, Utah

Simon A.J. Pattison
Brandon University, Brandon, MB

Shelf turbidite bodies have long been recognized in ancient rock successions, but most examples are poorly constrained both by the limited outcrop exposures and by the lack of modern analogues or data. This study takes advantage of the exceptional outcrop exposures in the Book Cliffs of eastern Utah to answer some fundamental questions regarding the origin and distribution of inner shelf turbidites. The lower Kenilworth Member (Campanian, Blackhawk Formation) is characterized by turbidite-rich shelf deposits at a number of localities, including Gunnison Butte and Hatch Mesa, offering unparalleled three-dimensional transects across the inner shelf, both along depositional-dip and depositional-strike. These shelf deposits are comprised of a complex mixture of high-energy event beds, including wave-modified turbidites, hummocky cross stratified sandstones, hyperpycnites and classical turbidites, suggesting deposition above storm wave base. The inner shelf turbidite bodies are detached from their time-equivalent shoreface deposits, and were deposited 10 to 20 km offshore. Sediments bypassed the shoreface through a network of subaqueous channels which were cut by the turbid underflow of sediment and water generated by storm and/or river flood events. Shallow marine facies models should be revised to include turbiditic-rich channels and lobes in some inner shelf settings. Preliminary observations link the generation of these turbiditic shelf deposits to tectonic activity. Inner shelf turbidite lobes and channels are viable play-types in foreland basin strata, as demonstrated by existing gas production from the Mancos B deposits in western Colorado and eastern Utah, and are likely to have been overlooked in many basins around the world.