New Insights for Correlation in Shoreface-to-Shelf Systems Based on Observations in the Campanian Blackhawk Formation to Castlegate Sandstone Stratigraphic Interval, Book Cliffs, Utah

Pattison, Simon A.J.¹, Huw Williams², Paul Davies² (1) Brandon University, Brandon,

MB (2) Reservoir Geology Consultants Limited, Rhandirmwyn, United Kingdom

Isolated sandstone bodies of enigmatic origin and uncertain stratigraphic position are scattered throughout the Cretaceous Western Interior of North America. An unusually coarse example occurs in the Middle Mountain to Floy Wash region of the Book Cliffs, east-central Utah. The coarse-grained deposits are concentrated in erosively-based patches which are 0.5 to 4 m thick, up to 40 m2 in areal extent, and consist of fine- to very coarse-grained sandstones, mudstone clasts, granules, pebbles, bone fragments, shell debris, fish teeth, Teredolites-bored wood fragments, and oolitic ironstones. Fossils include Scaphites hippocrepis III, Baculites and Inoceramus. These coarse-grained patches abruptly pass into iron-rich siltstones, forming a laterally persistent marker horizon that can be recognized for tens of kilometers in eastern Utah. Previous studies have correlated this horizon to a variety of stratigraphic intervals ranging from the late Santonian Emery Sandstone to the early Campanian Kenilworth Member. The recognition of index fossil Scaphites hippocrepis III coupled with the high resolution sequence stratigraphic framework has revealed a definite Campanian age. This layer is comprised of transgressively-modified, detached falling-stage and lowstand deposits that mantle the top Aberdeen Member sequence boundary, thus defining the Aberdeen-Kenilworth boundary.

The main implication for correlation in shoreface-to-shelf systems is that parasequence- and member-scale rock packages bracketed by this coarse-grained layer and the top of the Castlegate Sandstone exhibit a gradual basinward thinning (i.e. 30 % reduction in thickness over 40+ km) not the abrupt clinoform-style pinch-outs popularized in previous studies. Other shoreface-to-shelf systems are likely to exhibit similar thickness trends, thus providing an alternative correlation style to existing models.