Conodont Biostratigraphy of Shale Lens Overlying the Bucktown Coal Member of the Dugger Formation (Pennsylvanian, Desmoinesian), Pike County, Indiana

Abstract

We collected conodonts from black and gray shale lenses containing limestone nodules lying immediately above the Bucktown Coal Member of the Dugger Formation (Pennsylvanian, Desmoinesian) in a Solar Sources Pride Creek pit 2 miles south of Petersburg in Pike County, Indiana. The shale lens varied from approximately 0.6 to 0.92 m in thickness and was approximately 190 m wide. Our objectives included describing the fauna, interpreting the paleoenvironment, and continuing the establishment of regional correlations in the Midcontinent. We processed ten samples, nine productive, from four intervals along the pit face and identified 507 elements to species level. Conodont faunas are dominated by Idiognathodus and Adetognathus. Neognathodus, Hindeodus, and Idioprioniodus are rare. Adetognathus is uncommonly abundant, indicating that the depositional environment of the shales was very restricted, low energy, shallow-water, possibly a lagoon or a small embayment. The extremely high juvenile to adult ratio of Idiognathodus suggests that the paleoenvironment was harsh and resulted in a high juvenile mortality rate. The rare Idioprioniodus elements further suggest low-energy, reducing conditions but the fragmentation of the Idioprioniodus and Hindeodus elements may indicate introduction into the depositional site by irregular stormevents. The rarity of Neognathodus shows its lack of tolerance for highly euryhaline conditions. The restricted environmental conditions resulted in a lack of generic and specific diversity, particularly the rarity of Neognathodus, the primary biostratigraphic indicator for the Desmoinesian Series. Thus, there are insufficient data for regional correlations. We interpret the geographically restricted basal black shales overlying the coal to be of flotant marsh origin, representing a localized flooding event. Distributary channels associated with a deltaic system subsequently delivered fresh water that resulted in deposition of brackish water gray shales. The evidence supports a model of localized control on sedimentation rather than sedimentation controlled by glacially influenced eustatic sea
level change.