Abstract: Identifying Sequence Boundaries and Flooding Surfaces using Ichnology: Examples from the Cretaceous Western Interior of North America

Simon A. J. Pattison

Recent studies in the Kenilworth Member (Campanian), Book Cliffs, Utah and the Viking Formation (Albian), central Alberta highlights the

importance of using ichnology in defining erosion surfaces. Sequence boundaries are commonly highlighted by abrupt changes in ichnology. In the Kenilworth, Cruziana is the dominant ichnofacies below the sequence boundary, while Skolithos is the dominant ichnofacies above. In most cases, significant facies changes highlight the sequence boundary. However, difficulties arise when the facies above and below the sequence boundary are similar. For example, incised valley-fill mudstones of the Viking rest stratigraphically on top of lower shoreface mudstones. This contact is marked by a Glossifungites ichnofacies consisting of Arenicolites and Skolithos. In this case, the identification of the Glossifungites ichnofacies is critical in determining the position of the sequence boundary.

Most flooding surfaces are clearly marked by facies changes, erosional lags and differences in ichnology. However, correlating these surfaces from the lower to the upper shoreface is often difficult. Kenilworth flooding surfaces correlate landward into an amalgamated package of upper shoreface and foreshore sandstones. Up to a maximum of three flooding surfaces have been traced into an individual sand body where they are expressed as thoroughly bioturbated intervals dominated by Ophiomorpha, Skolithos and Palaeophycus. Therefore, the identification of thoroughly bioturbated intervals in thick shoreface sandstones may represent parasequence boundaries or flooding surfaces.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994