THE UPPERMOST FAMENNIAN HANGENBERG EXCURSION IN NORTH AMERICA AND THE SEARCH FOR THE BASE OF THE CARBONIFEROUS SYSTEM

The identification of the conodont Siphonodella sulcata, whose FAD was intended to be coincident with the base Carboniferous GSSP, at several levels below the ‘golden spike’ at the stratotype section at La Serre, France, has prompted a restudy of the base of the Carboniferous System. The Latest Famennian carbon isotope (δ¹³C_carb) excursion known as the Hangenberg Isotope Excursion is among the largest excursions during the Paleozoic with values reaching >+5‰, is a useful tool for chronostratigraphic correlation of latest Devonian strata, and is particularly important with respect to the current restudy of the correlation of the base of the Carboniferous System. Whereas the onset of the Hangenberg Isotope Excursion began within the Middle Siphonodella praesulcata conodont zone, peak isotope values (and the majority of the excursion) are limited to the final conodont zone of the Devonian, the Upper Siphonodella praesulcata conodont zone. As a result, the Hangenberg Isotope Excursion can serve as a useful indicator of the presence (or absence) of latest Devonian strata in any given section.

Isotopically, the latest Famennian Hangenberg Excursion was among the last excursions to be recognized globally, and remains as one of the intervals of least recovery among Paleozoic positive excursions. In the overwhelmingly majority of localities globally, the Upper S. praesulcata Zone (the terminal Devonian zone) is on the scale of centimeters to a few meters at best. This is the case in many classical sections, and consequently, the Hangenberg Excursion is poorly represented in many of these sections. Peak values >+5‰ are rarely preserved, most likely due to truncation or extreme condensation of section at this level, and the Hangenberg Excursion rarely exceeds +3‰. As the pertinent subcommissions address the future of the base of the Carboniferous System, we would like to suggest that several localities in North America preserve more than 10 meters of the Upper S. praesulcata Zone, and the presence of strata missing in other localities is confirmed by the preservation of a more complete Hangenberg Excursion in these sections. Sections in Iowa, Missouri, and Nevada, all contain strata that should be considered closely by the working group to restudy the base of the Carboniferous System and should serve as important areas of potential future research.