Detailed facies anatomy of transgressive and highstand systems tracts from the Upper Cretaceous of Southern Utah, U.S.A.

In the northern part of the Kaiparowits Plateau of southern Utah, excellent outcrops allow the detailed examination of Upper Cretaceous offshore, shoreface, estuarine, and coastal plain strata within an established sequence-stratigraphic framework. A lower sequence boundary, referred to as the Calico sequence boundary, is overlain by a 25- to 60-m-thick succession of strata interpreted as a transgressive systems tract. These strata consist of amalgamated braided river sandstones that pass upward into heterolithic estuarine strata and, in turn, into distal shoreface sandstones capped by a widespread fossiliferous horizon. Separating the heterolithic estuarine deposits from the overlying shoreface sandstone is a widespread erosion surface covered by a conglomerate of sub- to well-rounded pebbles. This surface is a ravinement surface or transgressive surface of erosion within the transgressive systems tract. The transgressive systems tract is overlain by up to 80 m of progradationally stacked parasequences, consisting of offshore and shoreface strata interpreted as the deposits of a highstand systems tract. In a landward direction the shoreface strata pinch out and highstand deposits are represented by coal-bearing coastal plain strata. A second sequence boundary, referred to as the Α-sequence boundary, erosionally overlies these shoreface strata and, in turn, is overlain by an estuarine unit up to 38 m thick. These estuarine strata comprise a second transgressive systems tract that is capped by an erosion surface overlain by offshore strata. This study provides a useful analog for the interpretation and correlation of strata in less well exposed areas and particularly in the subsurface. This paper also provides facies dimensions of shoreface parasequences that may allow for better modeling of variations in reservoir compartmentalization within a sequence-stratigraphic framework.