A POSSIBLE TERRESTRIAL RECORD OF THE INITIAL EOCENE THERMAL MAXIMUM IN NORTH DAKOTA

The geologically sudden warming that began at the Paleocene-Eocene boundary (IETM=PETM) had a profound effect on marine and terrestrial ecosystems, but there are few sequences of continental rocks that have been demonstrated to record the event. We will present new data from one such sequence, the Golden Valley Fm. of western North Dakota. The Golden Valley Fm. is a 20-30 m thick sequence of fluvial mudstones, carbonaceous shales, and lignites. The lower Bear Den Mbr. is highly kaolinitic (up to ~80% of clay minerals) and preserves a typical late Paleocene megaflora. The Camels Butte Mbr. has less kaolinite and a typical early Eocene megaflora and vertebrate fauna. Previously the Paleocene-Eocene boundary has been correlated with the Alamo Bluff lignite at the base of the Camels Butte Mbr. Previous pollen work on the GOlden Valley Fm. detected the first appearance datum (FAD) for Intratriporopollenites instructus in the lower part of the Bear Den Mbr. and the FAD of Platycarya just below the top of the Bear Den Mbr. In IETM sections in the Bighorn and Powder River Basins of Wyoming, the FAD of I. instructus is either within or immediately after the IETM, and the FAD of Platycarya is a few tens of meters higher. Thus, the order of FADs is the same in all three areas. Palynomorph FADs suggest that the IETM may begin at the base of or within the Bear Den Mbr., and thus that the high kaolinite clays in this terrestrial section are coincident with the IETM. Although the deposition of kaolinite in the Bear Den Mbr. likely occurred during the IETM, it is unlikely that the kaolin formed in situ, because the kaolinitic sediments are not extensively modified by pedogenesis, and contain leaf compressions that would have been destroyed by intense soil formation.