KATIAN (UPPER ORDOVICIAN) GLOBAL δ13C CHEMOSTRATIGRAPHY: IMPLICATIONS OF THE DISCOVERY OF A COMPLETE ELKHORN EXCURSION IN ITS TYPE AREA IN WESTERN OHIO

In 2010, a distinctive δ¹³C excursion, named the Paroveja excursion, was recognized in the Katian of Estonia. In the same year, a δ¹³C excursion, referred to as the Elkhorn excursion, was described from the Elkhorn Formation in western Ohio. Whereas the Paroveja was based on a complete positive excursion interval, the Elkhorn excursion was represented by only its beginning portion, the rising limb of the positive excursion, most of the excursion interval being cut out by the significant unconformity that separates Ordovician and Silurian strata in Ohio. Some subsequent studies have suggested that the Elkhorn and Paroveja probably are the same excursion but other investigations have presented them as temporally separate excursions. The profound differences between the Katian faunas in the North American Midcontinent and those in Estonia make it impossible to clarify the excursion relations by biostratigraphy. The chemostratigraphy in a recently investigated drill core from St. Marys in western Ohio unexpectedly showed a complete Elkhorn excursion recorded in the top part of the Richmondian Stage. The St. Marys Elkhorn δ¹³C curve is so similar to that of the Paroveja excursion in its typical development in the Estonian Jurmala R-1 drill core that we conclude that the Elkhorn and Paroveja are the same excursion. This excursion has been identified in quite a few Baltoscandian and North American successions, but has not yet been recorded elsewhere in the world. In the standard graptolite zone succession, it is pre-Normalograptus extraordinarius Zone and probably post-Dicellograptus ornatus Zone in age. It occurs in the upper part of the Aphelognathus divergens Zone in the North American Midcontinent conodont zonation.