Carbon Isotope Stratigraphy Reveals More Gaps than Record in the Ordovician-Silurian Boundary Succession of the Arbuckle Mountains, Oklahoma, USA

This abstract serves to update on our ongoing studies on the Ordovician-Silurian boundary interval in different palaeocontinents. The Hirnantian Stage oolites are a focus for the research. These are widely distributed in southern and western North America, Baltoscandia, and South China, and we identify them as a time-specific facies and the combined effects of the latest Ordovician glacial eustasy and the Late Ordovician Mass Extinction (LOME). The enclosing strata are of high priority as well to create context and to understand longer-term global change in this time interval and the post-LOME transition into the Silurian. As part of this research, we studied the carbon isotope chemostratigraphy of the latest Ordovician (Hirnantian) Keel Oolite and the Silurian Cochrane, Clarita, and lowermost Henryhouse formations (all part of the Hunton Group) in the Arbuckle Mountains of Oklahoma (USA), and combined the new data with a detailed evaluation of the published conodont record in the studied sections. The first highly time-resolved δ13Ccarb analyses of the Keel Oolite at the type locality in the Holnam quarry, at the Highway 77 road-cut, and in the Sunray DX-Davis 1-5 drill core, reveal only fragments of the Hirnantian Isotope Carbon Excursion (HICE). The δ13Ccarb data from the three sections suggest that only the plateau-peak interval of the HICE is preserved, whereas brachiopods of the same strata belong in the Edgewood-Cathay Fauna, which normally appears later, in the time interval corresponding to the falling limb of HICE. This possible contradiction makes any detailed chemostratigraphic correlation of the local Hirnantian difficult. At the Highway 77 road-cut, a well-known and classic locality, our δ13Ccarb data extends to nine metres into the Silurian succession immediately overlying the Keel Oolite. The discontinuous character of the δ13Ccarb data corroborates previous interpretations of an extremely discontinuous stratigraphy in this interval, with several unconformities, and more stratigraphic gaps than previously recorded. Despite this situation, we can infer or identify the overall position for preserved parts of several Silurian δ13C excursions; including the Late Aeronian Carbon Isotope Excursion (LACIE), the Valgu Carbon Isotope Excursion (Valgu CIE), Early Sheinwoodian Carbon Isotope Excursion (ESCIE), the Mid Homerian Carbon Isotope Excursion (MHCIE), and the Mid Ludfordian Carbon Isotope Excursion (MLCIE). Our geochemical data thus reinforce a modest stratigraphic completeness of the Ordovician-Silurian boundary succession in the Arbuckle Mountains.