Sequence Stratigraphic Significance of Sharp-Based Lowstand Shoreface Deposits, Kenilworth Member, Book Cliffs, Utah

Abstract

The Kenilworth Member of the Blackhawk Formation (Upper Cretaceous, Campanian) is nearly continuously exposed from Price to Hatch Mesa in the Book Cliffs, Utah. Five main depositional environments are recognized: coastal plain, foreshore and upper shoreface, lower shoreface, inner shelf, and incised valley-fill deposits. The shelf-to- shoreface deposits combine to form nine coarsening-upward, wave- to storm-dominated parasequences that are 7-32 m thick, and that split basinward into smaller coarsening-upward successions or bedsets that are 2-18 m thick. A maximum of four bedsets occurs within an individual parasequence. Bedset bounding surfaces are occasionally marked by thoroughly bioturbated, iron-rich carbonate-cemented sandstones within the parasequences. In the subsu face, similar bedset bounding surfaces may act as permeability barriers or baffles, leading to reservoir compartmentalization. Parasequences 1-6 form an aggradational-to-progradational parasequence set of the highstand systems tract, parasequences 7 and 8 occur as sharp-based lowstand shoreface deposits at Price River and Battleship Butte forming the attached lowstand systems tract, and parasequence 9 forms the transgressive systems tract. The base of each lowstand shoreface is marked by an organic-rich lag deposit that consists of granules, coarse-grained sandstone, wood fragments, coal intraclasts, and shell debris. The sharp base, basal lag deposit, and basinward shift in facies delineate a sequence boundary at the base of each lowstand shoreface deposit. The presence of two sharp-based lowstand shorefaces suggests that the Kenilworth lowstand was punctuated by a stillstand. The sharp-based lowstand shoref ce sandstones form the bulk of the attached lowstand systems tract and represent one end member in a continuum of lowstand deposits. The other end member is the detached lowstand systems tract. Revising existing sequence stratigraphic models to include both types of lowstand systems tracts is a first step toward improving the accuracy of these models, which are often used as prospecting tools in lowstand deltaic environments.