Carbon isotope chemostratigraphy of the Late Ordovician (Hirnantian) Keel Oolite and the Silurian Cochrane, Clarita, and lowermost Henryhouse formations in the Arbuckle Mountains, Oklahoma, USA

 

Authors: Calner M, Wu R, Yan G, Lehnert O, Stanley TM

Published in: Bulletin of Geosciences, volume 101, issue 1; pages: 173 - 189; Received 19 October 2025; Accepted in revised form 28 February 2026; Online 3 May 2026

Keywords: carbon isotope stratigraphy, conodont biostratigraphy, Hirnantian, oolite, Noixodontus fauna, Edgewood-Cathay brachiopod fauna,

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Abstract

Marine carbonate oolites are a key feature of the Hirnantian Stage (445.2–443.8 Ma) and their formation may be the combined result of latest Ordovician glacial eustasy and the associated mass extinction event. In this paper, we provide the first highly time-resolved carbon isotope data (δ13Ccarb) from the Keel Oolite in the Arbuckle Mountains of southern Oklahoma, aiming to clarify its temporal relationship with the overall Hirnantian Isotope Carbon Excursion (HICE) and refine the chronostratigraphy of the oolites in the southwestern United States. However, analyses of the Keel Oolite at the type locality in Holnam quarry, at the classical Highway 77 road-cut, and in the Sunray DX-Davis 1-5 drill core, reveal only a fragmentary carbon isotope record, making any detailed chemostratigraphic correlation of the local Hirnantian difficult. The oolite yields overall consistent δ 13C trend lines and weak to moderate co-variance (R2 ranging from 0.04-0.49) between carbon and oxygen, indicating no to moderate diagenetic alteration, still they display a ca. 1-2‰ offset of the baseline between localities. As there is no firm evidence for any steep increase or decrease in the δ 13C values, it is suggested that the oolites correspond to the peak and plateau interval of the HICE. Brachiopods of the same strata belong in the Edgewood-Cathay Fauna, which normally appears in the time interval corresponding to the falling limb of HICE. At the Highway 77 road-cut, our carbon isotope stratigraphy reaches nine metres into the Silurian succession immediately overlying the oolite, corroborating previous interpretations of an extremely discontinuous stratigraphy with prominent unconformities. Based on the combined information from conodont faunas and our δ 13Ccarb data, however, we infer or identify the overall position for preserved parts of Silurian δ 13C excursions in the section, 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.