DISCERNING PALEOCENE/EOCENE PALEOENVIRONMENT IN THE WILLISTON BASIN USING CLAY MINERAL ASSEMBLAGES

The Williston Basin (WB) of western North Dakota has potential stratigraphic indicators of paleoenvironmental shift across the Paleocene-Eocene (P/E) boundary in the Fort Union (FtU) and Golden Valley (GV) Formations.  Sediments in the WB are commonly fine-grained and internal stratigraphic continuity suggests tectonic quiescence, creating a basin setting sensitive to changes in ambient environmental conditions.  A key component in understanding the paleoenvironment of this setting is the clay mineral assemblages.  X-Ray diffractometric analyses from several WB localities show a shift in clay mineralogy several meters above the contact of the FtU/GV, indicating a significant difference in weathering regimes.  Within a distinct bright orange-colored zone of the Bear Den Member (BDM) of the GV, the clay mineral assemblage changes from dominantly smectites to kaolinite, then shifts back to smectites and chlorites near the top of the GV.  Kaolinite is overwhelmingly prevalent in the orange zone (>80% of the total clay fraction) and this shift represents the base of a weathering front that formed in sub-aerially exposed sediments.  This zone also has unique physical attributes that suggest the formation of a pedogenic silicon-enriched pan in response to a fluctuating water table. The smectites of the previously deposited strata were probably cannibalized through weathering and pedogenesis to form the kaolinite and release excess silicon to form the cemented pans.  The collective evidence (high kaolin content, massive nature and lack of bedding structure, low organic-C content) indicates that intense in-situ weathering overprinted the BDM orange zone. Clay-mineral assemblage changes in the WB conform to a global pattern of increased kaolinite production/deposition across the P/E boundary, and support the hypothesis that intensified chemical weathering of continental rocks curbed climatic warmth by lowering atmospheric pCO₂.