Correlation of the Silurian rocks of southeastern and northeastern Wisconsin using conodonts and conodont strontium isotope ratios and proposed conodont and ostracode biofacies models for the environs of the middle Aeronian Michigan Basin.

摘要

The Engadine Formation of northeastern Wisconsin and the Brandon Bridge Formation of southeastern Wisconsin each contain conodonts indicative of the Pterospathodus celloni Zone (Telychian/earliest Wenlock). The Burnt Bluff Group of northeastern Wisconsin contains conodonts indicative of the upper Icriodella descreta-Icriodella deflecta Zone while the undifferentiated Burnt Bluff Formation of southeastern Wisconsin contains conodonts indicative of the upper Distomodus kentuckyensis Zone and the lower Aspelundia fluegeli Zone. These zones indicate an early to middle Aeronian age for the Burnt Bluff of each area. 87Sr/86Sr ratios derived from conodont samples from the forgoing units support these correlations when plotted on the Silurian 87Sr/86Sr curve of Amzy et al. (1999). Twenty-eight conodont species representing thirteen genera are described, including Harrisodus halquisti new genus new species, Ozarkodina new species A and Ozarkodina excavata waldhuetteri new subspecies. Twenty-four ostracode species representing seventeen genera are described including the four new species: Paraechmina watkinsi, Steusloffina clarki, Thlipsuroides lundini and Ulrichia turnerae. The Burnt Bluff Group of northeastern Wisconsin, the Michigan Upper Peninsula and the northern Michigan Basin subsurface, characterized by laminated carbonate mudstone containing macrofauna representative of BA's 1–3, was deposited in a shallow water environment. The non-panderodid conodont faunas of these areas, dominated by Distomodus sp. cf. Distomodus kentuckyensis, Kockelella manitoulinensis, Kockelella species A s.f. (Over and Chatterton) and Ozarkodina excavata waldhuetteri, new subspecies, represent a nearshore conodont biofacies. The undifferentiated Burnt Bluff Formation of southeastern Wisconsin, characterized by bioturbated mudstone containing macrofauna representative of BA's 4 and 5, was deposited in quiet water below normal storm wave base. The non-panderodid conodont fauna of this area, dominated by Aspelundia fluegeli, Walliserodus sancticlairi, and Ozarkodina new species A, represent an offshore conodont biofacies. The distribution of Burnt Bluff ostracodes parallels that of the conodonts. A nearshore ostracode fauna (dominated by leperditiid species) is associated with the nearshore conodont fauna and an offshore ostracode fauna (dominated by smooth-shelled podocopes, Pachydomella wolfei, Steusloffina clarki new species and Paraechmina watkinsi new species) is associated with the offshore conodont fauna. These ostracode faunas represent, respectively, the Eifelian and Thuringian ecotypes of Becker (1975).