PROTOSALVINIA DAWSON AND ASSOCIATED CONODONTS OF THE UPPER TRACHYTERA ZONE, FAMENNIAN, UPPER DEVONIAN, IN THE EASTERN UNITED STATES

Journal of Paleontology, Jan 2009 by Over, D Jeffrey, Lazar, Remus, Baird, Gordon C, Schieber, Juergen, Ettensohn, Frank R

In the Clegg Creek Member of me New Albany Shale Palmatolepis glabra angusta occurs in great abundance above the first occurrence of Protosalvinia-beanag shale and a diverse conodont fauna characteristic of the Lower and Upper trachytera zones. Two drill cores from the Clegg Creek Member of the New Albany Shale in Indiana yielded Protosalvina and conodonts. In the 1-3 Kavanaugh core (locality D2; Table 1), me lowest occurrence of Protosalvina is at or above a conodont-rich horizon that contains Palmatolepis perlobata sigmoidea, P. perlobata grossi, P. glabra distorta, and possibly Polylophodonta confluens. A conodont-rich horizon containing a similar fauna, likely at the same disconformity, is found in outcrops in northern Kentucky (localities A and B, Table 1). The fauna here contain taxa that are typical of the Upper marginifera Zone (e.g., Palmatolepis marginifera utahensis, as well as those that range through this zone, like P. perlobata grossi, and P. rugosa rugosa; Figure 6). The occurrence of Palmatolepis glabra distorta and P. falcata above this widespread disconformity in the Walton 1-12 core (locality C2; Figure 7), as well as the even higher occurrence of P. glabra angusta, in which the entire element assemblage is preserved (locality C3), indicates that the disconformity and Protosalvinia are no higher than the Upper trachytera Zone.

Four other reports have suggested the relative conodont-based position of the Protosalvinia Interval. Ettensohn et al. (1988) placed the Protosalvinia Interval in the Lower expansa Zone based on one sample from the New Albany Shale in Kentucky (Table 1, locality B), using the conodont Polygnathus experplexus Sandberg and Ziegler, 1979 as the key taxon, a conodont which has a reported range from the base of the Lower expansa Zone into the Middle expansa Zone. The sample also includes Palmatolepis glabra distorta, P. falcata, P. perlobata grossi, Polygnathus brevilaminus Branson and Mehl, 1934a, Polygnathus pennatulus Ulrich and Bassler, 1926, P. semicostatus, and Polylophodonta confluens, in which the occurrence of P. glabera distorta and other taxa indicate a lower biostratigraphic position or a reworked fauna. Considering the synonymy off. experplexus with P. perplexus (Metzger, 1989) the range of P. perplexus is from within the Lower trachytera Zone into the Upper expansa Zone; Dreesen and Dusar (1974) indicated an even lower first occurrence of P. perplexus, in which specimens from Belgium first occur in the Lower marginifera Zone. Thus, the report interpreted as within the Lower expansa Zone by Ettensohn et al. (1988) based on P. experplexus is questionable; other taxa suggest the Upper trachytera Zone. The fauna at the disconformity associated with the first occurrence of Protosalvinia from locality A suggests that the horizon is correlative with the disconformity at localities B, Cl, and D2 (Table 1; Appendix 1).

The zonal position for Protosalvinia recovered 2 m above the base of the Bakken Shale in northwest North Dakota from NDGS well no. 4340, as reported by Holland et al. (1987), is problematic because the occurrence of conodonts from the lower Bakken in other cores relative to the occurrence of Protosalvinia is unclear. Lower Bakken conodonts described by Hayes (1985) and Holland et al. (1987) from drill core include Bispathodus jugosus Branson and Mehl, 1934a, Polygnathus experplexus, and P. granulosus Branson and Mehl, 1934a, that are interpreted as Lower expansa Zone. Yet, the Trident Member of the Three Forks Formation in Montana, that is correlated to the Three Forks Formation that underlies the Bakken, contains a conodont fauna of the Upper velifer Zone (= Upper trachytera Zone; Sandberg and Poole, 1977; Holland et al., 1987). Thus the base of the Bakken Shale and position of Protosalvinia are not well constrained. Disconformities in the Upper Devonian black shales are often cryptic, and stratigraphie packets of similar lithologies can have irregular distributions (e.g., Fuentes et al., 2002; Schieber and Over, 2005). The occurrence of Protosalvinia in the Lower Bakken is likely no higher than the Lower expansa Zone, but it may be as low as the Upper trachytera Zone.