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Abstract

Seismic and wireline-log data through the nonmarine Paleocene upper Fort Union Formation in the Wind River basin of Wyoming reveal cyclic sedimentation that can be placed in a sequence-stratigraphic framework.

The Fort Union Formation is divided into a fluvial lower part and a lacustrine upper part. The separation between the intervals records increased accommodation and lake formation in the Fort Union depocenter due to orogenic (Laramide) thrusting on the margins of the basin. The initial increase in subsidence appears to be recorded in a thin, but widespread, marker bed below the base of the first lacustrine shale. A sand between the marker bed and the base of the lacustrine shale may represent a transgressive sand. The base of the overlying lacustrine shale records maximum transgression of the lake, and represents the maximum flooding surface for the overall upper Fort Union Formation.

The upper part of the Fort Union Formation is composed of lacustrine shales and fluvial and deltaic sands resulting from encroachment into the lake system by two geographically distinct deltas. Seismic data display dominant offlap geometry, indicating a gradual decrease in accommodation space with time. Sequences and systems tracts are defined on seismic data by the presence of sequence boundaries and downlap surfaces. Wireline logs through the deltaic interval verify the overall regression observed on seismic data, further displaying a series of progradational parasequence sets in the highstand systems tracts. The regressive deltaic interval records complete sequences composed of member lowstand, transgressive, and highstand systems tracts.

Lacustrine settings of sufficient size, such as the Paleocene of the Wind River basin, likely are affected by cyclic processes similar to those identified in marine environments. Most likely, these cycles are controlled by pulses of contemporaneous tectonic activity and Milankovich-style climate modification. Petroleum exploration methods based on marine sequence-stratigraphy concepts appear equally applicable in such lacustrine settings.

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