ENHANCED CONTINENTAL WEATHERING AND PEDOGENESIS ACROSS THE PALEOCENE/EOCENE BOUNDARY IN THE U.S. WESTERN INTERIOR (NORTH DAKOTA)

An abrupt carbon isotope excursion (CIE) recorded by inorganic and organic carbon, and pronounced carbonate dissolution in the oceans indicates that vast quantities of C were rapidly released into the ocean-atmosphere reservoir during the Paleocene-Eocene thermal maximum (PETM, ca. 55 Ma). The transient nature of the PETM is attributed to the enhancement of such negative-feedback mechanisms as an accelerated hydrological cycle and intensified continental weathering. The fluvio-palustrine deposits of the Bear Den (BDM) and Camels Butte members (CBM) of the Golden Valley Formation in North Dakota (Williston Basin) span the Paleocene-Eocene boundary and may provide critical evidence that such a negative-feedback process was operative. The tripartite color of the BDM distinguishes it from the underlying late Paleocene Fort Union Formation and the overlying early Eocene CBM. Of particular interest is a 2-4 m thick “orange zone” within the BDM that is a leached, blaze white paleosol composed mostly of kaolinitic (>80%) claystone with meager amounts of poorly-preserved organic C. The base of this BDM subunit is marked by an orange, Fe-stained horizon that in places is underlain by a silicified, Fe/Mn pan; the latter feature is analogous to modern fragipans that form in weathered soils under humid, subtropical conditions. The orange zone is overlain by the BDM “carbonaceous zone” which culminates into the Alamo Bluff Lignite (ABL.) Fossil pollen and stable isotope analyses of bulk-organic C are used to construct composite bio-chemo-stratigraphies through the BDM at three localities in the basin. Palynofloras in two of these sections show that the orange zone is a barren interval containing highly degraded organic matter, which poses a major obstacle to detecting the CIE. Palynofloras are better preserved in the third locality and reveal that Platycarya spp.—a palynomorph now known to first occur within the PETM interval of some Wyoming sections (Wing et al., 2004)—first appears within this distinctive BDM subunit. Though identification of the CIE is hampered by preservational vagaries, we provisionally consider the BDM orange zone to be a product of intense PETM pedogenesis. Moreover, the overlying carbonaceous zone and ABL may reflect a shift to a wetter, more humid climatic regime during the later stages of the PETM.