『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 δ13Corg 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.
Keywords: Paleocene-Eocene thermal maximum; Williston Basin; kaolinite;
Golden Valley Formation』
Introduction
Regional lithostratigraphy and chronostratigraphy
Sentinel Butte Member of the Fort Union Formation
Bear Den Member of the Golden Valley Formation
Camels Butte Member of the Golden Valley Formation
Study sections
Farmers Butte
Murray Ranch
Methods
Clay mineralogy
Bulk elemental content of samples from Farmers Butte
Processing of palynological samples from Murray Ranch
Carbon isotopic analysis of bulk organic carbon (bulk δ13Corg ratios)
Results
Clay mineralogy at Farmers Butte
Bulk elemental content at Farmers Butte
Palynostratigraphy of the study section at Murray Ranch
Bulk organic carbon isotope (δ13Corg)
record at Murray Ranch
Discussion
Implications of clay mineral shift in the orange zone
Biostratigraphic evidence relating the orange zone to the Paleocene-Eocene
thermal maximum
Is the carbon isotope excursion masked by poor organic carbon
preservation?
Summary
Acknowledgments
References cited