Conodonts and the Devonian–Carboniferous boundary in the upper Woodford Shale, Arbuckle Mountains, south-central Oklahoma

Abstract

The Woodford Shale of south-central Oklahoma was deposited in an offshore, quiet-water, oxygen-poor setting on the southern margin of North America in assocation with other dark organic-rich shales of the Upper Devonian–Lower Carboniferous black-shale facies. The basal Woodford was deposited unconformably over lower Paleozoic carbonate strata as a south-to-north transgressive unit during the Frasnian and early Famennian. Black shales and cherts lie directly above the basal beds. Phosphatic shales in the upper Woodford contain a conodont succession characterized by three distinct environmentally controlled faunas. The lower fauna is characterized by Palmatolepis gracilis ssp., Branmehla inornata, Bispathodus stabilis , and Pseudopolygnathus marburgensis trigonicus , indicative of the Late Devonian Lower expansa Zone to Upper praesulcata Zone. The middle fauna, which spans the Devonian–Carboniferous (D/C) boundary, is characterized by Polygnathus communis communis and species of Protognathodus . On the Lawrence uplift the D/C boundary is disconformable, as indicated by the absence of Protognathodus kockeli before the first occurrence of Siphonodella sulcata . Light-colored phosphate laminae and beds, indicative of erosion and nondeposition, and a change in biofacies from an offshore palmatolepid–bispathodid fauna to a more nearshore, palmatolepid–polygnathid–protognathodid fauna indicate higher energy conditions and a lowering of sea level associated with the boundary interval. In the eastern Arbuckle Mountains the D/C boundary is apparently conformable, marked by a green shale interval containing a Protognathodus fauna. Species of Siphonodella , indicative of an offshore setting, characterize the third and youngest fauna. The Early Carboniferous sulcata , Lower duplicata , and Upper duplicata Zones are recognized in the upper Woodford. The Woodford Shale is conformably overlain by the “pre-Welden Shale’ and its equivalents, or unconformably overlain by the lower Caney Shale (Osagean?–Meramecian) in the northern outcrop regions and the Sycamore Formation (late Osagean?–Meramecian) in the southern Arbuckle Mountains.