CONODONT BIOSTRATIGRAPHY OF THE CHATTANOOGA SHALE, MIDDLE AND UPPER DEVONIAN, SOUTHERN APPALACHIAN BASIN, EASTERN UNITED STATES

Journal of Paleontology, Nov 2007 by Over, D Jeffrey

ABSTRACT-

The Chattanooga Shale of the southern Appalachian Basin contains a diverse conodont fauna of the high Givetian, Frasnian, and Famennian. The predominantly fine-grained strata were deposited in an offshore setting where depositional packages are separated by unconformities. Conodonts allow regional and global correlation of these strata, recognition of the Frasnian-Famennian boundary, and narrow biostratigraphic constraint of two Frasnian ash beds, MN Zone 8 for the Belpre Ash and upper MN Zone 13 for the Center Hill Ash. Three new Frasnian palmatolepid conodonts are described in open nomenclature, and the holotype of Palmatolepis regularis Cooper is reillustrated.

INTRODUCTION

THE CHATTANOOGA Shale is characterized by organic-rich "black" shales that range in color from dark gray and dark brown to black, as well as gray shales, siltstones, sands, and minor carbonates that were for the most part shed into the Appalachian Foreland Basin inboard of the Acadian Orogenic Belt in the Middle Devonian through Early Carboniferous of the southeastern and central United States. The Chattanooga and related shales, including the "Genesee shales," as well as lower members of the Ohio Shale in the northern Appalachian Basin, New Albany and Grassy Creek shales of the Illinois Basin, Antrim Shale and Kettle Point Formation of the Michigan Basin, Long Rapids Formation of the Moose River Basin, and Woodford Shale of the North American southern continental margin in the Late Devonian, are important petroleum source rocks widely distributed in central and eastern North America (Fig. 1; de Witt et al., 1993; Boswell, 1996; Schieber and Lazar, 2004). Conodonts are relatively abundant within the Chattanooga Shale and provide the means for biostratigraphic correlation and the determination of the relative time interval of numerous depositional cycles as well as gaps that are evident from physical stratigraphie studies (de Witt et al., 1993; Ettensohn, 1994; Schieber, 1998). Chattanooga conodonts have been previously described and illustrated by Ulrich and Bassler (1926), Holmes (1928), Huddle (1963, 1968), and systematically collected and described by Hass (1956) who recognized three distinct faunas that correspond to the middle and upper Frasnian, Famennian, and Tournaisian, respectively. Conodonts recovered throughout the outcrop region, in addition to re-evaluation of the collections of W. Hass, allow delineation of global biozones that enable correlation of Chattanooga depositional packages, recognition of distinct local taxa, and biostratigraphic control of ash beds, one of which has yielded an absolute date for the Middle Frasnian (Tucker et al., 1998).

GEOLOGICAL SETTING AND STRATIGRAPHY

The Chattanooga Shale was named by Hayes (1891) for strata exposed on Cameron Hill, Chattanooga, Tennessee. Owing to the poor nature of this exposure (notably the base and top of the formation are not exposed), a reference section at Sligo Bridge on Tennessee Highway 26, DeKalb County, was designated (Hass, 1956; Conant and Swanson, 1961; see for review of earlier works). This exposure is now slumped and overgrown with vegetation and another reference section was suggested several kilometers to the northwest by Kepferle and Roen (1981) at Hurricane Bridge on Tennessee Highway 56 (see locality information in Appendix 1). Here the Chattanooga Shale, 10 m thick, lies on the Upper Ordovician Drakes Formation, and is overlain by the Maury Formation, a phosphate-rich unit that contains Lower Carboniferous Tournaisian conodonts-the upper conodont association of Hass (1956).

The Chattanooga Shale of the southern Appalachian Basin is restricted to Alabama, Georgia, Mississippi, Tennessee, and parts of Kentucky and Virginia, as outlined by de Witt (1981; Fig. 1). "Chattanooga" is also used for Upper Devonian black shales in the subsurface of northern Oklahoma and Kansas; these strata are time equivalent and lithologically similar, but not necessarily contiguous with the Chattanooga Shale of the Appalachian Basin. In central Tennessee the Chattanooga is subdivided into three formal members, in ascending order, the Flynn Creek, Dowelltown, and Gassaway members. The Flynn Creek Member (Schieber and Over, 2005) consists of black shales that were deposited within the Flynn Creek Impact Structure, as well as thin remnants below the Dowelltown or Gassaway members in other locations. The Dowelltown, the lowest widely occurring unit of the Chattanooga, consists of a lower, more resistant black organic-rich unit equivalent to the Rhinestreet Shale of New York State and the Frasnian portion of the Blocher Shale in the Illinois Basin, and an upper, generally recessive gray unit that corresponds to the Angola Shale and Java Shale of New York and the Selmier Shale of the Illinois Basin; the Center Hill Ash is found near the top of the Dowelltown. The Gassaway Member is predominantly black and lighter gray shales subdivided in to three parts: lower, middle, and upper units (see de Witt et al., 1993). In eastern Tennessee, pre-Dowelltown Member shales equivalent to the Blocher/Geneseo have been included with the Dowelltown Member (Dennison and Boucot, 1974; Kepferle and Roen, 1981).