Chemostratigraphic correlation of four fossil-bearing sections in southwestern North Dakota

Abstract

A-1.5‰ to -2‰ carbon isotope shift in surface ocean dissolved inorganic carbon has been reported stratigraphically above the Cretaceous-Tertiary (K-T) boundary in the global stratotype section at El Kef, Tunisia, and in many other marine sections worldwide. Because a change in the δ 13 C value of paleoatmospheric CO 2 would have accompanied the marine surface carbon isotope shift, the K-T carbon isotope anomaly can be used to diagnose and correlate the boundary in terrestrial sections as well. We use this shift and other secular variation in the carbon isotope signature from the Hell Creek and Fort Union Formations to correlate four sections in Slope County, North Dakota. At Pyramid Butte, the K-T boundary is marked by a-2.8‰ carbon isotope shift approximately 10 cm above the iridium-bearing impact clay. This shift is larger in magnitude than might be expected by plant vital effects or taphonomic variation. The Pyramid Butte carbon isotope shift allowed correlation to the Bobcat Butte (shift of-2.1‰), Terry's Fort Union Dinosaur (shift of 1-1‰ to -1.6‰), and the New Facet Boundary (shift of-2.3‰) sections, where an iridium enrichment was not preserved. An interval of carbon isotope fluctuation marks the transition from HCII to HCIII megafloral zones in the Bobcat Butte section, suggesting that some instability in the physical environment was correlated with the observed vegetation change. These data demonstrate that carbon isotope chemostratigraphy can provide an additional line of evidence that is largely independent from taxonomic extinction for correlating fossil-bearing terrestrial sections. Chemostratigraphic interpretations may also recognize incomplete or condensed sections, which will aid interpretation of the fossils found in them.