The chronostratigraphic correlation of the Silurian units in Iowa is complicated by complex carbonate depositional environments and poor biostratigraphic control. In this study, we integrate conodont biostratigraphic data with carbon isotope (δ 13Ccarb) chemostratigraphic data from the Blanding, Hopkinton, Scotch Grove, and LaPorte City formations of Iowa to provide a precise chronostratigraphic framework for regional and global correlation. Conodonts from the LaPorte City Formation of eastern Iowa (East-Central Iowa Basin) in the Garrison Core indicate an early to middle Telychian age for the formation. Conodonts diagnostic of the Pterospathodus eopennatus Superzone, Pterospathodus eopennatus ssp. nov. 2 Zone, and Pterospathodus amorphognathoides angulatus Zone were recovered, allowing for the first direct comparison of the stratigraphic ranges of conodont species from the North American Midcontinent and the Baltic basin. The morphology of Pseudolonchodina fluegeli ssp. n. Männik (2007) co-occurs with Pseudolonchodina fluegeli fluegeli sensu Männik (2007a) in the LaPorte City Formation and are not stratigraphically separated in Iowa as they are in the East Baltic. Wurmiella? polinclinata polinclinata ranges much lower in the East-Central Iowa Basin (Pt. eopennatus ssp. nov. 2 Zone) than the Baltic Basin, so it cannot be used as an index fossil diagnostic of the Pt. am. amorphognathoides Zone in global correlations.
Three carbon isotope excursions were recovered from the Hopkinton and Scotch Grove formations. The late Aeronian (herein renamed “Farmers Creek”) carbon isotope excursion and a heretofore unrecorded carbon isotope excursion were recorded from the Hopkinton Formation in the SS-10 Core (Jones County). The Farmers Creek Excursion can be correlated to Johnson County, where it was recorded in the Hopkinton Formation by McAdams et al. (in prep). The Valgu excursion was recovered from the uppermost Hopkinton Formation through the overlying Buck Creek Quarry Member of the Scotch Grove Formation in the Garrison Core (Benton County). This excursion can be correlated to Johnson County, where it was recorded in the same formations. The integration of conodont biostratigraphic and carbon isotope chemostratigraphic data from the Silurian of Iowa allows for the first regional chronostratigraphic correlations at a resolution finer than stage level. The oxygen and carbon isotope values from the Garrison Core and the evidence for post-diagenetic karsting and fluid movement through the units may provide further evidence that the dolomitization process of the LaPorte City Formation was halted by the influx of meteoric phreatic water.
This study shows the first high-resolution chronostratigraphic correlation of Silurian units in Iowa, and highlights the utility of integrated carbon isotope chemostratigraphy as a tool for chronostratigraphic correlation in strata with poor biostratigraphic control. The conodonts recovered from the LaPorte City Formation shows the utility of the small limestone formations on the northwest flank of the East-Central Iowa Basin for refining global Silurian conodont biostratigraphic zonation.