Abstract

The “orange zone” within the Bear Den Member of the Golden Valley Formation (Williston Basin, North Dakota) represents a terrestrial weathering profile formed by intense pedogenesis during an ancient (ca. 55 Ma) global warming event referred to as the Paleocene-Eocene thermal maximum. Distinctive features of the orange zone include: (1) high abundances of kaolinite, (2) a strongly leached character with a bright orange iron-enriched horizon, (3) poor organic carbon preservation, and (4) ferric pans/pisoliths at its basal contact, equivalent to modern lateritic materials. Though conclusive evidence, such as a distinctive Paleocene-Eocene thermal maximum flora and/or definitive carbon isotope excursion, is lacking, the stratigraphic succession of palynofloral datums preserved within the upper part of the Bear Den orange zone is similar to that associated with the later stages of other terrestrial Paleocene-Eocene thermal maximum records from the U.S. Western Interior. Bulk δ13 Corg ratios decrease through the orange zone as well, but the magnitude of this isotopic decrease is less than that of the carbon isotope excursion. Thus, the collective evidence indicates that the early initial stages of the Paleocene-Eocene thermal maximum are either preserved within the barren, condensed interval of the lower orange zone or are missing altogether from the Williston Basin stratigraphy, and that the orange zone likely developed during the later recovery stages of the Paleocene-Eocene thermal maximum. The Williston Basin records generally agree with the tenet that continental weathering intensified during the Paleocene-Eocene thermal maximum. Moreover, these records indicate that the paleohydrology of the basin varied markedly and that sequestration of terrestrial organic carbon was greatly reduced as this transient global warming event unfolded.

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