Articulate brachiopods from the Richmondian of Tennessee

The Richmondian (Upper Ordovician) strata in the Central Basin of Tennessee yield an abundant and diverse fauna of brachiopods. The section consists in ascending and conformable order of the "Arnheim" and Fernvale Formations. The following species are described: Austinella kankakensis (McChesney), Diceromyonia tersa Wang, Glyptorthis insculpta maquoketensis Ladd, Hebertella occidentalis (Hall), Onniella meeki (Miller), Paucicrura quadrata (Wang), Pionodema alberstadti n. sp., Plaesiomys bellistriatus Wang, P. proavita (Winchell and Schuchert), Platystrophia acutilirata (Conrad), P. ponderosa Foerste, Rafinesquina sp., Retrorsirostra carleyi (Hall), Strophomena planumbona (Hall), Tetraphalerella neglecta (James), Thaerodonta recedens (Sardeson), and Zygospira modesta (Hall). Faunal comparisons confirm correlations with the Cape Limestone of southeastern Missouri, the Maquoketa Formation of Iowa, the upper part of the Viola Formation of Oklahoma, the Aleman and Cutter Formations in West Texas, and the standard Richmond section. The fauna in Tennessee connects the provincial Richmond in the Ohio Valley with the western faunas. INTRODUCTION AND GEOLOGIC SETTING A WELL-PRESERVED and abundant brachiopod fauna occurs in Richmondian strat in the Central Basin of Tennes ee. The rhynchonellacean brachiopods were described in an earlier paper (Howe, 1969). Most of the specimens are silicified. These have been freed from the carbonate matrix by using dilute hydrochloric acid. Howe (1969) reviewed the stratigraphy of the Richmondian strata in Tennessee and described the key Richmondian section at Bakers Station, locality 1, Davidson County, about 19 km north of Nashville. Most of the specimens studied for this paper were taken from this section and 18 other widely scattered localities in the Central Basin. An index map and geographic locations of these localities are given in Howe (1969). The "Arnheim" Formation is limited to the northwest flank of the Nashville Dome, except for a single outcrop at Clifton, locality 19 (Wilson, 1949). It ranges from about 3 to 6 m in thickness and typically consists of micritic limestone containing abundant skeletal material, finely comminuted bioclastics, intraclasts, and siltand clay-sized particles. The overlying Fernvale Formation crops out in a discontinuous belt along the north, west, and south sides of the Central Basin. It ranges in thickness from less than 6 m to about 18 m. To the north, the Fernvale is light olive gray in color. In outcrops south of Clifton, brownish shades predominate, ranging from light pale brown at Mansford Bridge (loc. 15) to a moderately yellowish brown at Clifton (loc. 19). In contrast to the Arnheim Formation, calcarenites (biosparites) characterize the Fernvale Formation. These rocks consist largely of variably colored echinodermal allochems set in a matrix of sparry calcite. Current sorting and cross-bedding are common, indicating deposition in a shallow-water, high-energy environment. A thin interbedded shale and limestone facies, called the "Mannie Shale" by Foerste (1903), can be recognized at the top of the Fernvale at most localities south of Columbia and as far north as locality 14 in Williamson County. It is replaced further north by characteristic biosparites. North of Nashville, highly fossiliferous calcarenites grade into, and are interbedded with, barren, laminated, and sugary-textured calcareous siltstones and shales indicating periodic changes to conditions hostile to marine life and deposition in a low-energy environment.