Upper Katian (Upper Ordovician) trans-Atlantic delta C-13 chemostratigraphy: the geochronological equivalence of the ELKHORN and PAROVEJA excursions and its implications

摘要

Since 2010 when the North American ELKHORN and Baltoscandic PAROVEJA isotope excursions were first described and named, their mutual age relations have remained uncertain, if not controversial. This was at least partly due to the incompleteness of the ELKHORN excursion in its reference section in western Ohio. The unexpected discovery of an apparently complete ELKHORN excursion in a drill core from St Marys in western Ohio has led to the conclusion that in terms of stratigraphical position and delta C-13 curve correspondence, the ELKHORN and PAROVEJA excursions are so similar that they apparently represent the same isotopic curve perturbation. The ELKHORN/PAROVEJA excursion occurs in the D. pacificus Graptolite Zone and uppermost A. ordovicicus Conodont Zone in the uppermost Katian Stage (Stage Slice Ka4 of Bergstrom et al. Lethaia 42, 97-197, 2009). Because the designation PAROVEJA was published two months before that of ELKHORN, it has priority as excursion designation. This excursion is particularly well represented in the carbonate successions in the Great Basin of western United States. Chemostratigraphy and biostratigraphy in that region show that the Richmondian transgression was contemporaneous with the beginning of the middle Katian WHITEWATER/MOE excursion. The onset of the Richmondian transgression has long been controversial but now available evidence suggests that it is of essentially the same age across large regions of the southern, western and central United States.