Paper No. 6
Presentation Time: 2:45 PM
ROLE OF THE HANGENBERG EXCURSION AND MAGNETIC SUSCEPTIBLITY STRATIGRAPHY IN SELECTION OF A NEW DEVONIAN-CARBONIFEROUS SYSTEM BOUNDARY
The First Appearance Datum of the conodont Siphonodella sulcata has previously defined the position of the basal Carboniferous GSSP at the stratotype section at La Serre, France. It has recently been recovered at several levels below the ‘golden spike’ at the GSSP and other locations. Consequently other positions and sections for a new GSSP are under consideration. The known records of the Latest Famennian Hangenberg Isotope Excursion (HGE) in North American sections reach peak δ13Ccarb values >+6‰, The HGE is a useful tool for recognition and chronostratigraphic correlation of latest Devonian strata, and is particularly important with respect to the current restudy and redefinition of the base of the Carboniferous System. The onset of the HGE began within the Middle Siphonodella praesulcata Zone and achieves peak isotope values in excess of 6‰ within the interval of the Upper Siphonodella praesulcata conodont zone. In central North America, the onset of the global HGS is directly coincident with major changes in bulk magnetic susceptibility (MS), and both the δ13Ccarb and MS stratigraphies display similar structures and trends. As a result, both the δ13Ccarb (HGE) and MS stratigraphies can serve as a useful indicators of the presence (or absence) of latest Devonian strata in any given section, and proximity to a potential boundary that will be redefined at a new GSSP by the Devonian-Carboniferous (D-C) Boundary working group.
At most localities globally, the interval of the Middle-Upper S. praesulcata zones (latest Devonian zones) is centimeters to a few meters in thickness, and the HGE is poorly represented with peak values rarely exceeding +3‰, with values ranging >+5‰ rarely preserved. The poor expression of the HGE in those sections is likely due to erosional truncation or condensation in the HGE intervals. As demonstrated here, several localities in North America preserve more than 10 meters of strata spanning the Middle-Upper S. praesulcata zones, and the presence of strata missing in other localities is confirmed by the preservation of more complete records of the Hangenberg Excursion in North American sections. The expanded records in Iowa, Missouri, and Nevada may serve as key reference sections or candidates for a new D-C boundary GSSP.