Conodont biostratigraphy of the Woodford Shale (late Devonian-early Carboniferous) in the Arbuckle Mountains, south-central Oklahoma.

摘要

The Woodford Shale of south-central Oklahoma was deposited in an offshore, quiet-water oxygen-poor setting on the southern margin of North America. The Frasnian-Famennian (F/F) and Devonian-Carboniferous (D/C) boundaries are recognized, based on conodonts, and lie at thin coarse-grained deposits within otherwise lithologically continuous sections.; The basal Woodford consists of quartzose sand, phosphate and limestone pebble conglomerate, and green-brown shale deposited unconformably over lower Paleozoic carbonates as a south to north transgressive unit during the Frasnian and early Famennian. Black shales and cherts lie directly above the basal beds.; The F/F boundary occurs within the lower 20 m of the Woodford, and is recognized by the extinction of species of Ancryodella and Palmatolepis linguliformis, the occurrence of Ancryognathus ubiquitus and Pa. praetriangularis, which are indicative of the boundary interval, and the appearance of Pa. triangularis. The boundary horizon on the Lawrence Uplift and in the northern Arbuckle Mountains is marked by a thin phosphate and conodont lag deposit. Conodont faunas change from a palmatolepid fauna characteristic of offshore biofacies below the boundary to a more nearshore palmatolepid-polygnathid fauna across the boundary, and back to a palmatolepid biofacies above the boundary. The change in conodont biofacies and coarse-grained beds at the boundary are indicative of higher energy conditions and a fall in sea level associated with the F/F boundary interval.; Phosphatic shales in the upper Woodford yield a conodont succession characterized by three faunas. The lower fauna is characterized by Palmatolepis gracilis ssp., Branmehla inornata, Bispathodus stabilis, and Pseudopolygnathus marburgensis trigonicus, indicative of the Late Devonian Upper expansa Zone to Lower praesulcata Zone. The middle fauna, which transcends the D/C boundary, is characterized by Polygnathodus communis communis and species of Protognathodus. On the Lawrence Uplift the D/C boundary is disconformable, indicated by the absence of Protognathodus kockeli before the first occurrence of Siphonodella sulcata. Light-colored phosphate lamina and beds, indicative of erosion and non-deposition, and a change in biofacies from an offshore palmatolepid-bispathodid fauna to a transitional, more nearshore, palmatolepid-polygnathid-protognathod 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 Pr. kockeli below the appearance of Si. sulcata. 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 is conformably overlain by the "pre-Welden Shale" and its equivalents, or unconformably overlain by the lower Caney Shale in the northern outcrop regions, and unconformably overlain by the Sycamore Formation in the southern Arbuckle Mountains.