Sequence Stratigraphy of a Lacustrine System: Upper Fort Union Formation (Paleocene), Wind River Basin, Wyoming, U.S.A.
Louis M. Liro, 1993. "Sequence Stratigraphy of a Lacustrine System: Upper Fort Union Formation (Paleocene), Wind River Basin, Wyoming, U.S.A.", Siliciclastic Sequence Stratigraphy: Recent Developments and Applications, Paul Weimer, Henry Posamentier
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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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Applying depositional sequence stratigraphic concepts to the interpretation of siliciclastic depositional systems is becoming an increasingly important tool in petroleum geology. After a succession of breakthroughs during the 1970s and 1980s, sequence stratigraphic concepts now have entered a phase of intense application and documentation, especially with regard to successful implementation in the field of petroleum geology. Workers have applied these concepts to a variety of databases, ranging from outcrop to cores to electric logs and to multifold seismic data. Clearly, sequence stratigraphic concepts embody–not a rigid model or template–but rather a way of looking at geology. This volume has two purposes: to compile some recent applications of siliciclastic sequence stratigraphic concepts, and to present new studies focused on refining conceptual models. This memoir grew out of a 1991 symposium, "Variations in Depositional Systems Within a Sequence Stratigraphic Framework: Applications to Exploration," organized by the authors at the AAPG annual meeting at Dallas. Robert Loucks and Rick Sarg have edited a companion volume, also published by AAPG,“Entitled Carbonate Sequence Stratigraphy: New Developments and Applications.”