Webster and Hageman—Buenellus chilhoweensis n. sp. (lower Cambrian Chilhowee Group)


and Eriksson, 1989; Walker and Driese, 1991; Smoot and Southworth, 2014). Synrift volcanics of the Catoctin Formation (underlying the Weverton formations in Virginia, Maryland, and Pennsylvania) have radiometric ages of 572±5to 564±9Ma (Aleinikoff et al., 1995), and are therefore late Ediacaran in age. Although speculated upon (King and Ferguson, 1960; Smoot and Southworth, 2014), correlative relationships between the Catoctin and Unicoi basalts have not been established. Compressed carbonaceous tubes within the middle Unicoi


Formation are similar to problematic fossils found in Ediacaran assemblages elsewhere (Hageman and Miller, 2016). Trace fossils suggest that the Ediacaran-Cambrian boundary lies within the upper portion of the Unicoi Formation (Walker and Driese, 1991; Hageman and Miller, 2016). The middle Chilhowee Group, a 200–800m thick succes-


sion of sand, silt, and shale, is mapped as the laterally equivalent Nichols Shale and Hampton Shale in Tennessee and southern Virginia (Fig. 2.1). The Harpers Formation of northern Virginia, Maryland, and Pennsylvania has usually been considered to be a northern lateral equivalent of the middle Chilhowee Group (e.g., King, 1949; King and Ferguson, 1960; Cudzil and Driese, 1987; Walker and Driese, 1991), but has recently been proposed to correlate to the younger Murray Shale (Smoot and Southworth, 2014; see also Bloomer and Werner, 1955). (Smoot and Southworth [2014] instead suggested that the Loudon Forma- tion of Maryland and Pennsylvania is age-equivalent to the Nichols and Hampton shales [Fig. 2.1].) The contact between the lower and middle Chilhowee Group appears to be con- formable (Mack, 1980). The middle Chilhowee Group repre- sents a marine transgression in a prodeltaic to low-energy mud shelf setting that was episodically affected by storms (Walker and Driese, 1991). A thick black mudstone interval within the lower part of the Nichols Shale of Tennessee was deposited during the time of maximum flooding; the rest of the Nichols Shale represents a highstand systems tract (Mack, 1980; Simpson and Eriksson, 1990; Tull et al., 2010). Trace fossil assemblages from the middle Chilhowee Group indicate that the Cambrian Substrate (Agronomic) Revolution had initiated (Hageman and Miller, 2016). However, searches for body fossils have met with little or no success (Laurence and Palmer, 1963; Neuman and Nelson, 1965; Appendix): only a single, fragmentary, conical shelly fossil of uncertain affinity has been reported (Simpson and Sundberg, 1987), and the biogenicity of even that specimen has been questioned (Hageman and Miller, 2016). The upper Chilhowee Group is a siliciclastic succession that accumulated on a passive margin. Eustatic sea level control


445


on sedimentation is evident in the form of two transgressive sequences (Tull et al., 2010; Smoot and Southworth, 2014; Hageman and Miller, 2016). In southern and eastern Tennessee, the first of these transgressive sequences is represented by the Nebo Quartzite and overlying Murray Shale (Fig. 2). The Nebo Quartzite contains abundant Skolithos burrows (King, 1949; King and Ferguson, 1960; Neuman and Nelson, 1965; Appendix), but nothing of highly refined biostratigraphic utility. The contact between the Nebo Quartzite and the Murray Shale is transitional, with some interbedding of lithologies (Whisonant, 1974). Laurence and Palmer (1963, p. C53) noted that at Murray Gap on Chilhowee Mountain (see below and Appendix) the Murray Shale is 107m (350 ft) thick and consists of three units of roughly equal thickness: “a lower unit consisting of bluish- gray noncalcareous shale with scattered quartz grains and


muscovite flakes up to about 1mmacross and occasional biotite flakes and glauconite grains; a middle unit which is principally a dark-gray muscovite-bearing fine siltstone and which, when weathered, yields buff chips similar to the weathered shale of the


bottom unit; and an upper unit consisting of siltstone, shale, and fine-grained sandstone with many glauconitic layers.” Rb-Sr dating of glauconite grains within the Murray Shale indicates an age of 539±30Ma (Walker and Driese, 1991 [recalibrating the work of Hurley et al., 1960]; see Holmes,1959 and Cowie, 1964 for earlier dating efforts). Fossils from the Murray Shale are the main focus of this paper and are discussed in following sections. The interval of maximum transgression is located within the Murray Shale. The Murray Shale is in sharp but conformable contact


with the overlying Hesse Quartzite (Whisonant, 1974); this transition represents a return to shallow-water wave and tidally influenced conditions. The Hesse Quartzite is a quartz sandstone that contains Skolithos burrows (Neuman and Nelson, 1965; Hageman and Miller, 2016). The second transgressive sequence is represented by the Hesse Quartzite and overlying Helenmode Formation (a quartz siltstone and sandstone with interbedded shale) (Fig. 2). In northeastern Tennessee these same four successive


lithostratigraphic units (Nebo Quartzite, Murray Shale, Hesse Quartzite, and Helenmode) are recognized as members within the Erwin Formation (e.g., King and Ferguson, 1960; Walker and Driese, 1991; Fig. 2). North of central Virginia, the fine- grained sediments of the Murray Shale have typically been considered to be absent and the facies of the Nebo and Hesse quartzites have been interpreted to merge and thicken, so that the entire upper Chilhowee Group is represented primarily by quartz sandstone mapped as the Antietam Formation (e.g., King, 1949; King and Ferguson, 1960; Cudzil and Driese, 1987;


Figure 2. Lithostratigraphic correlations for southern and central Appalachians. Central Appalachians includes central and northern Virginia (approximately north of Roanoke), Maryland, and Pennsylvania. (1) Chilhowee Group plus immediately subjacent and superjacent units. Traditional interpretation of correlation for central Appalachians follows most workers (e.g., King, 1949; King and Ferguson, 1960; Mack, 1980; Cudzil and Driese, 1987; Walker and Driese, 1991; Walker et al., 1994); alternative hypothesis from Smoot and Southworth (2014; gray shaded region indicates marked unconformity). Vertical scale arbitrary and non-uniform. Ediacaran–Cambrian boundary is likely in upper one-third of Unicoi Formation (Walker and Driese, 1991; Hageman and Miller, 2016), but age of base of Chilhowee Group in Tennessee is poorly constrained. (2) Working hypothesis of correlation and approximate ages of lithostratigraphic units of the upper Chilhowee Group. Circles indicate stratigraphic intervals within the upper Chilhowee Group that have yielded trilobites. Age assignment of Murray Shale in eastern Tennessee based on discovery of Buenellus chilhoweensis n. sp. (black circle), as described in present study. White circles indicate trilobite occurrences in uppermost Chilhowee Group of central Appalachians (see text). Laurentian series and stage subdivisions of Cambrian follow Palmer (1998); Begadean and Waucoban series together represent the traditional “lower Cambrian” of this paleocontinent. Age in millions of years before present (Ma) and potential placement of global Cambrian Stage 3-Stage 4 boundary taken from provisional Cambrian global correlation charts presented by Peng et al. (2012). Abbreviations: Fm., Formation; Mb., Member; Qzt., Quartzite.


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