FITZSIMMONS, R., Department of Earth Sciences, University of Liverpool, Liverpool, England

ABSTRACT: High Resolution Sequence Stratigraphy of the Eagle Formation (Campanian) in the Mesaverde Group of Big Horn Basin, Wyoming, U.S.A.

The Campanian Eagle Formation (Mesaverde Group) in the Big Horn Basin, Wyoming is an easterly prograding clastic wedge which overlies the Cody Shale and is, in turn, overlain by the Clagget Shale. The wedge reflects deposition in a sand-rich, storm-wave and tide-influenced delta system. High resolution sequence stratigraphic analysis of this System reveals a complex history comprising two depositional sequences defined by regionally widespread, 3rd order erosional unconformities. Stratal architecture within the sequences is complicated by 4th order sequence boundaries, identified by forced regressions in prograding shoreface systems. Highstand, transgressive and, to a lesser extent, lowstand systems tracts are identified within the wedge. The only notable development of lowstand syste s tracts is above a previously unrecognized sequence boundary at the base of the wedge in the upper part of the Cody Shale. The sequence boundary is overlain by nearshore, storm-wave and tidal parasequences that are distinctive in being characterized by exceptionally high sedimentation rates. This unit is interpreted as a prograding lowstand wedge, being the distal, down-dip expression of an inferred incised valley system. It is overlain by further nearshore parasequences characterised by lower sedimentation rates and a backstepping stacking pattern. These parasequences are interpreted as transgressive system tract deposits and are overlain by a maximum flooding surface in the upper part of the Cody Shale. This lower sequence boundaryand associated prograding lowstand wedge are critical s they are considered to record the initiation of the Mesaverde depositional system in this region.

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