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Sequence Stratigraphy and Tectonic Influences on a Fluvial-Deltaic Lacustrine System: Fort Union Formation, Wind River Basin, Wyoming

LIRO, LOUIS M., Texaco Inc., Houston, TX

The nonmarine Fort Union Formation in the Wind River basin of Wyoming is divided into a fluvial lower part and a lacustrine upper part. The separation between these intervals is a tectonically enhanced sequence boundary recording increased and pronounced subsidence in the Fort Union depocenter owing to Laramide thrusting. This sequence boundary appears to be recorded in a thin but widespread shale rather than the base of the lacustrine Waltman Shale Member. A sand between the shale marker and the base of the Waltman Shale likely represents a maximum flooding surface.

The upper part of the Fort Union is composed of the lacustrine Waltman Shale that is encroached by two coeval but distinct deltaic systems. The persistent progradation of these deltas suggests that the volume of sediment supplied was relatively constant. The similarity in relative rates of progradation and aggradation in the deltaic sequences suggests possible eustatic control. Dominant sigmoidal offlap indicates a gradual decrease in accommodation space with time. Well logs through this interval record an overall regressive unit (a progradational parasequence set) that can be subdivided into a series of progradational parasequences. The upper Fort Union is characterized by repetitive transgressive and highstand systems tracts, with some lowstand wedges present. Individual transgressi e-regressive cycles are numerous in the upper Fort Union.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)