Palynostratigraphy of John's Nose, a new Cretaceous–Paleogene boundary section in southwestern North Dakota, USA

Abstract

This study documents the terrestrial palynological record at the John's Nose section, a new Cretaceous–Paleogene (K–Pg) boundary site in North Dakota, USA. In addition to Mud Buttes and Pyramid Butte, John's Nose represents the third K–Pg section in southwestern North Dakota that preserves direct evidence of the Chicxulub asteroid impact, allowing for direct comparison over the timing and trends of the palynological record in respect to this event. The palynological analysis of John's Nose section reveals the presence of 68 pollen and spore taxa. Immediately above the boundary clay, a high abundance of fern spores of the genera Cyathidites and Laevigatosporites is recorded (with 59% of the assemblage being represented by Cyathidites). This very distinctive K–Pg ‘fern spike’ event is correlated with the devastation of land plants immediately following the asteroid impact and matches the composition generally reported from other sites in southwestern North Dakota. Palynostratigraphy demonstrates that the placement of the K–Pg boundary based upon the identification of the Last Appearance Datum (LAD) of typical Maastrichtian taxa (K-taxa) may be misleading. The presence of occasional K-taxa up to a few meters above the boundary clay at John's Nose represents an important difference when compared to previous reports. In light of this observation, LADs should be used cautiously as the primary criteria to identify the boundary; some K-taxa may have a short-term presence in the earliest Paleogene, or be reworked. In the John's Nose section, major changes and extinction in the palynological record occur at the geochemical K–Pg boundary, indicating that a catastrophic turnover took place over a short time.

Keywords:

• Cretaceous–Paleogene boundary
• extinction
• fern-spore spike
• floral turnover
• Last Appearance Datum
• palynology

Acknowledgements

This paper is dedicated to the memory of Douglas J. Nichols, a great colleague and friend. The manuscript benefited from comments and discussions with Arthur Sweet and an anonymous reviewer. We would like to thank the Brown and Clark families for allowing access to the study area as well as Kathy and Don Wilkening, Merle Clark, the Pioneer Trails Regional Museum interns and Ohio State University students for the help and assistance with sampling of the section. Processing of samples from John's Nose section was supported by funding from the National Geographic Society to John Hunter, Joseph Hartman and Dean Pearson.