INTEGRATED CONODONT, δ13C, TRACE ELEMENT, AND SEQUENCE STRATIGRAPHIC ANALYSIS OF THE GIVETIAN-FRASNIAN BOUNDARY INTERVAL IN IOWA AND NEVADA, USA

The Givetian-Frasnian Frasnes Event is a second-order mass extinction associated with widespread black shale intervals in western European sites and a positive carbon isotope excursion (falsiovalis excursion). These three markers also characterize Mesozoic OAEs and the Frasnes may represent a Paleozoic OAE. However, the timing, order, impact, and possible causes of the Frasnes/falsiovalis excursion are poorly understood because high resolution integrated datasets are largely not available.

New integrated data from Nevada (conodont-carbon isotope bio-chemostratigraphy, major and trace element chemostratigraphy, and sequence stratigraphy) are the first to use all four techniques to evaluate the Frasnes/falsiovalis in a carbonate section. Data from Iowa are correlated using bio- and sequence stratigraphy. The Givetian-Frasnian boundary interval at both localities is represented by shallow-water biofacies assemblages dominated by species of Icriodus, Polygnathus, and Pandorinellina insita which do not allow for precise placement of the boundary because of the absence of diagnostic species of Ancyrodella. The onset of Devonian T-R Cycle IIb-1 closely precedes the onset of excursion and data from Nevada indicate anoxic shale facies which may represent a late Givetian age Frasnes event black shale horizon below the excursion. In Iowa whole rock carbon isotope data show the onset of the falsiovalis excursion very low in the interval of the Pandorinellina insita Fauna. In both sections, the appearance of deep-water early Frasnian (Frasnian zones 2-4) faunas provides an upper constraint on the excursion. In Nevada and Iowa, the recorded excursion likely begins during the late Givetian based on correlations of the Iowa sections with the Waterways Formation in western Alberta where the falsiovalis excursion is clearly shown to begin within the norrisi Zone interval and reaches peak δ¹³C_carb values within the interval of Frasnian Zone 1.