BOX, DARREN, Department of Earth Sciences, University of Liverpool, U.K.

ABSTRACT: Variability of Incised Valley Fills: Examples from the Cretaceous of Wyoming, U.S.A.

Incised valley fills are critical elements in the sequence stratigraphic analysis of elastic successions and are increasingly being sought as prospective hydrocarbon reservoirs. However, to date there is only a limited

understanding of their variability in terms of several key parameters such as the amount of incision on the sequence boundary, the depositional systems and systems tracts preserved in the valley fill, the facies characteristics of the depositional systems, and whether the fill is related to single or multiple sea level cycles.

Ongoing comparative studies of several incised valley fill sandstone bodies in the Cretaceous of Wyoming, U.S.A. (Newcastle, Muddy, Ericson, Teapot, Fox Hills Sandstones) demonstrate considerable diversity. For example, incised valley fills in the Newcastle and Fox Hills Sandstones are small-scale, single bodies dominated by sand-rich, tidal-estuarine facies deposited during the transgressive systems tract. In contrast, incised valley fills in the Ericson Sandstone are large-scale, composite bodies comprising alternations of coarse-grained fluvial lowstand deposits and tidal- estuarine transgressive systems tract deposits. The composite nature of valley fills in the Ericson Sandstone is controlled, to a large degree, by higher order sequence boundaries, suggesting that the intial vall y was re-used during a series of sea level fall events. Identification ofthe variability of incised valley fills and the processes that control is an important factor in sequence stratigraphy interpretations and has implications for lithological prediction down-dip of valley systems.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.