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Peter D. Warwick1, Romeo M. Flores2, Douglas J. Nichols2, Edward C. Murphy3
(1) U.S. Geological Survey, Reston, VA
(2) U.S. Geological Survey, MS 939, Denver, CO
(3) North Dakota Geological Survey, Bismarck, ND

Abstract: Depositional sequences within the Fort Union Formation (Paleocene), Williston Basin, North Dakota, South Dakota, and Montana

The Paleocene Fort Union Formation (FUF) in the southwestern part of the Williston Basin (ND, SD, and MT) is composed of several time-transgressive depositional sequences that are dated by palynology (biozones P1-P6) and radiometric ages (40Ar/39Ar) derived from tonstein partings in coal beds. Outcrop and borehole studies indicate that the lower Paleocene strata (biozones P1-P2) are defined by at least three eastward-prograding marine parasequences (each 20-45 m thick) that can be identified in the interfingering Ludlow (tidally influenced coastal plain) and Cannonball (marine) Members of the basal FUF. The parasequences are composed of lower shoreface and coarsening-upward middle and upper shoreface deposits. Flooding surfaces are marked by ravinement lags. Middle Paleocene strata (about 350 m thick, biozones P3-P4) thin to the east and consist of (1) deposits of regressive shallow marine and lower delta tidal systems forming the upper parts of the Cannonball and Ludlow Members, and (2) the lower delta plain fluvial deposits of the overlying lignite-bearing Tongue River Member. The upper Paleocene (about 200 m thick, biozones P5-P6) contains fluvial deposits of the lignite-bearing Sentinel Butte Member, which forms the upper part of the FUF. Four radiometric ages obtained from sanidine crystals in tonstein layers within the lower and middle Paleocene strata, indicate that the multiple highstands of the Cannonball Sea occurred between 64.68±0.15 Ma and 61.23±.38 Ma. These observations indicate that the last remnants of the intercratonic seaway were associated with frequent base-level fluctuations prior to the departure of the seaway from the midcontinent area.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana