MIDDLE ORDOVICIAN CONODONTS FROM THE ARKOMA BASIN, EASTERN OKLAHOMA

The Arkoma Basin in eastern Oklahoma contains a subsurface record of Middle Ordovician marine sedimentation representing the upper Sauk Sequence. As the Sauk Sea receded from Laurentia, isolated to semi-isolated basins developed. Those basins were fertile sites for conodont evolution and yield both early and late representatives of important Midcontinent conodont lineages.

Well samples from Middle Ordovician intervals at five sites in the Arkoma Basin were processed for conodonts. Samples from several wells produced collections of “Plectodina” joachimensis, Phragmodus flexuosus, Erismodus sp., Leptochirognathus sp., among others, and are similar to those derived from outcrops of the McLish Formation (Simpson Group) of southern Oklahoma (Arbuckle Mountains) and the middle Tyner Formation of northeastern Oklahoma. Samples from one well contained Neomultioistodus compressus, Drepanoistodus angulensis “Scandodus” sinuous, Oistodus multicorrugatus, Paraprioniodus costatus, and Histiodella sinuosa. This fauna is similar to that found in exposures of the Oil Creek Formation (Simpson Group) in southern Oklahoma and Burgen and lower Tyner Formations in northeastern Oklahoma.The Arkoma Basin conodont faunas reflect a striking turnover that coincides with the final phase of Sauk regression. During this phase, Midcontinent faunas dominated by species of Neomultioistodus, “Scandodus”, Histiodella, and Oistodus coexist with and then are replaced by members of Phragmodus, “Plectodina”, and Erismodus.