| Cambrian through Ordovician rocks in the core of the Ouachita Mountains are comprised of interbedded black shales and sandstones, capped by a banded chert. These units were deposited in deep-water slope and rise environments flanking the North American Platform, and from oldest to youngest are divided into the following formations: Collier Shale, Crystal Mountain Sandstone, Mazarn Shale, Blakely Sandstone, Womble Shale, and Bigfork Chert. This study focuses on refining age determinations for the Middle and Upper Ordovician Blakely Sandstone, Womble Shale, and Bigfork Chert Formations. The recovery of conodonts from thin limestone interbeds within the studied units affords a detailed biostratigraphic subdivision of the sequence. These microfossils permit confident stratigraphic assignment of geographically localized and stratigraphically limited exposures, which have hampered geologic mapping within the region. Additionally, conodonts representing both the North American and the North Atlantic Faunal Provinces have been identified. The faunal differences between the two realms are acute, but many samples from the Ouachitas contain representatives of both provinces. These tie points are extremely important for intercontinental correlation.; Over 15,000 identifiable conodonts, isolated from 84 samples, allow considerable biostratigraphic refinement of the formations studied. Limestone clasts in the Blakely Sandstone contain lower Whiterockian species such as “ Cordylodus” horridus and Histiodella holodentata. The Womble Shale spans the upper Whiterockian and Mohawkian Series and several conodont zones. Cahabagnathus directus occurs in basal portions of the Womble Shale near Caddo Gap, Arkansas, indicating the base of the formation may be as old as the Eoplacognathus suecicus Zone. Conodonts of the succeeding Pygodus serra Zone have been found in the Womble at many localities, whereas assemblages consistent with those of the younger Pygodus anserinus Zone are encountered less frequently. The upper part of the Womble contains an assemblage in the Amorphognathus tvaerensis Zone. The youngest formation examined, the Bigfork Chert, falls chiefly within the Cincinnatian Amorphognathus ordovicicus Zone. |
