MULTI-PROXY INDICATORS OF THE PALEOCENE/EOCENE (P/E) BOUNDARY IN THE WILLISTON BASIN OF NORTH DAKOTA

Rapid onset of an abrupt, transient global warming event, referred to as the Initial Eocene Thermal Maximum (IETM), occurred at the Paleocene/Eocene boundary ~55.0 Ma.  Coincident with the IETM is a worldwide decrease (~4‰) in the d¹³C compositions of marine and terrestrial inorganic carbon.  The dissociation of methane hydrate is currently the favored mechanism for the carbon isotope excursion (CIE).  Recognition of the CIE enables global correlation, constituting a powerful tool for studying the roles of differing carbon reservoirs in reducing greenhouse gas levels and curbing IETM warmth.

The Williston Basin in North Dakota provides an exceptional opportunity to study terrestrial carbon cycling.  We report mounting evidence that provisionally places the P/E boundary (=IETM) at the basal contact of the Golden Valley Fm., between the Bear Den Mbr. and the underlying Fort Union Fm.  Inspection of palynofloras show a succession of biostratigraphic FADs within the upper Bear Den Mbr. that are similar to those immediately post-dating the onset of the CIE in other Laramide basin IETM records.

A bulk d¹³C_org record spanning a lignite at the Fort Union Fm.-Bear Den Mbr. contact exhibits patterns of isotopic variation comparable to those from other terrestrial IETM d¹³C_org records in Western Europe.  The Williston Basin bulk d¹³C_org record shows an initial 4‰ increase followed by a sharp 5‰ decrease from –21‰ to –26‰.  The resemblance of our d¹³C_org data to those from distant locales may indicate that these deposits record the global CIE at the P/E boundary.  Alternatively, the isotopic pattern of variation seen in the Williston Basin d¹³C_org record may be an artifact stemming from the thermal alteration of preserved plant material.  Further study will help refine our interpretations.

Kaolinite becomes the dominant clay mineral in the Bear Den Mbr. ~4.5 meters above the negative d¹³C_org shift.  The influx of detrital kaolinite likely reflects intensified chemical weathering of continental rocks under elevated pCO₂ conditions, though a shift in spatial patterns of regional weathering cannot be ruled out at this point.  Deposition of large quantities of kaolinite in the Williston Basin is consistent with an emerging picture indicating that enhanced chemical weathering played an important role in the cessation of the transient IETM.