Depositional Sequences and Correlations within the Fort Union Formation, Williston Basin, North Dakota and Montana

Peter D. Warwick, Romeo M. Flores, Douglas J. Nichols, and Edward C. Murphy

Stratigraphic sections obtained from outcrops and boreholes, combined with palynostratigraphic zonations (biozones P1-P6) and radiometric ages, permit regional correlations within the strata of the Paleocene Fort Union Formation, Williston Basin, southwestern North Dakota and southeastern Montana. With these data the strata can be divided into several time-transgressive depositional sequences. 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 Fort Union Formation. The parasequences are composed of lower shoreface and coarsening-upward middle and upper shoreface eposits. 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 composing 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 Fort Union Formation. Radiometric ages obtained from sanidine crystals from two tonsteins, one layer from near the transition between biozones P2 and P3 (64.4±1.8 Ma), and the other layer from the lower Tongue River Member (upper part of biozon P3, 61.23±.38 Ma), help define the age ranges of the parasequences within the lower and middle Paleocene deposits. These combined data allow the first application of sequence stratigraphy to the tidally influenced coastal sections of the Paleocene Cannonball Sea, a remnant of the Cretaceous seaway.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California