STRATIGRAPHIC AND CLAY MINERAL EVIDENCE OF PALEOWEATHERING AT THE PALEOCENE-EOCENE BOUNDARY, WILLISTON BASIN, NORTH DAKOTA

The Williston Basin in western North Dakota contains non-marine strata spanning the Paleocene-Eocene boundary. A distinct lithologic unit, named the Bear Den Mbr. (BDm) of the Golden Valley Fm., has been biostratigraphically constrained to the latest Paleocene/earliest Eocene. Three subunits comprise the BDm: a lowermost “gray zone” characterized by gray, smectitic mud- and siltstones preserving plant fossils, a middle “orange zone” typified by increased kaolinite content, iron staining, and low organic carbon content, and an uppermost “carbonaceous zone” characterized by decreased kaolinite and increased organic matter preservation upsection. Some authors have argued that the source of the kaolinite in the BDm is purely detrital while others have argued for in situ pedogenesis and kaolinitization of preexisting material. We contend that while some of the kaolinite is indeed detrital, evidence of in situ kaolinitization is preserved. Specifically, the presence of iron-oxide filled, polygonal fractures within the kaolinitic “orange zone” suggests that the kaolinite in this interval was indeed converted from preexisting shrink-swell clays (i.e. smectite). Warm, well-drained conditions (i.e. subtropical to tropical) are necessary for rapid kaolinitization. Further evidence includes the presence of iron-enriched bands, sometimes in the form of a competent “pan”, indicating intense leaching within this zone. As such, our evidence indicates that the BDm represents a paleoweathering surface that occurred at the Paleocene-Eocene boundary. When placed within the context of early Cenozoic climate change, the Paleocene Eocene Thermal Maximum (PETM) provides the necessary climatic perturbation to explain the features exhibited by the BDm. We therefore propose that this interval is a terrestrial record of intense weathering associated with the increased warmth and accelerated hydrologic cycle associated with the PETM.