PRELIMINARY REPORT OF A SHEINWOODIAN CONODONT FAUNA FROM THE NORTHWESTERN MARGIN OF THE MICHIGAN BASIN SUBSURFACE

The Miller Brothers-Weinert Core #2–6 (Sec. 6, T18N, R17W, Mason County, Michigan) lies on the northwest margin of the Michigan Basin and spans the lower 7 ft. of the “Brown Niagaran”, all (139 ft.) of the “Gray Niagaran” and 62 ft. of the “Clinton”. Reconnaissance sampling of the core for conodont recovery was done at irregular intervals on the order of 3 to 10 ft. A continuous 40 ft. section (depth 4540–4580 ft.) of the “Gray Niagaran” interval that yielded relatively large numbers of conodonts was re-sampled. The conodont fauna found in the “Clinton” consists of rare Panderodus sp. elements (other fauna: sponge spicules and crinoid ossicles). “Brown Niagaran” conodonts include sparse numbers of Panderodus sp. and Pseudooneotodus sp. (other fauna: sponge spicules, crinoid ossicles, orbiculid brachiopods). All of the “Gray Niagaran” interval is characterized by Panderous sp., Dapsilodus obliquicostatus, Decoriconus fragilis, Walliserodus sp., Pseudooneotodus bicornis and P. beckmanni (other fauna: at least four species of arenaceous foraminiferida, orbiculid brachiopods, crinoid ossicles and occasional sponge spicules). The continuously sampled section, in addition, yielded Kockelella walliseri, K. ranuliformis and Ozarkodina sagitta rhenana. K. walliseri appears in a single sample 15 feet above the first appearance of K. ranuliformis, which co-occurs with O. s. rhenana. Only ambiguous kockelellid elements appear in between. This section of the core may, consequently, represent the middle Sheinwoodian boundary between the Lower K. walliseri and underlying O. s. rhenana Zones (Jeppsson et al. 2006). It also suggests correlation of this section of the core with surface exposures of the lowermost Racine and uppermost Waukesha Dolomites of southeastern Wisconsin that have yielded, respectively, K. walliseri and K. ranuliformis.