CONODONT BIOSTRATIGRAPHY AND STABLE ISOTOPE CHEMOSTRATIGRAPHY OF THE BROWNSPORT FORMATION (LUDFORDIAN, LUDLOW, SILURIAN), WEST-CENTRAL TENNESSEE

The Upper Silurian Brownsport Formation of western Tennessee is well known for its abundant and diverse shelly fauna, but aspects of its internal stratigraphy remain uncertain and accurate age determinations of its members do not exist. Using conodont biostratigraphy and carbon isotope chemostratigraphy, we can resolve some of these problems, but the results raise new questions about the depositional history of the Brownsport Formation. The basal Beech River Member comprises green-gray to brown calcareous shales and argillaceous skeletal wackestone that may grade upward in some sections into an echinoderm packstone to grainstone facies assigned to the Bob Member. A diverse conodont fauna of the Ludfordian Polygnathoides siluricus Zone ranges through the Beech River Member. This fauna includes common Panderodus unicostatus and P. recurvatus, less common elements of Ozarkodina confluens, Oulodus siluricus, and Po. siluricus, and rare other species. Conodont elements are rare in more argillaceous strata and common in packstones and grainstones. In the western sections, darker gray argillaceous carbonate mudstones, packstones and shales (not typical of the Bob Member) overlie the Beech River section, within which occur the mid-Ludfordian Isotope Excursion and Lau Oceanic Event, which extend through 5 m of section and reaches values of δ¹³C greater than +6‰. In eastern sections, however, the mid-Ludfordian Excursion (δ¹³C +5‰) appears near the base of, and ranges through a 4- to 5-m section of coarse-grained echinoderm grainstones that overlie Beech River lithofacies. The Po. siluricus fauna disappears (start of the Lau Event) as the values of δ¹³C start to rise from a short interval of negative values in both areas and a conodont fauna with Wurmiella excavata, Dapsilodus, Decoriconus, and rare O. snajdri is present. In both areas, strata that comprise the mid-Ludfordian Excursion and the overlying interbedded calcareous shale and packstone and grainstone of the Lobelville Member, yield only a few isolated elements of W. excavata, Dapsilodus or Pseudooneotodus. Only a few undiagnostic coniform elements have been recovered across the contact between the Lobelville Member and the low-insoluble-residue wackestone to packstone that forms the base of the overlying Decatur Formation.