CONODONT BIOSTRATIGRAPHY AND CARBON STABLE ISOTOPE CHEMOSTRATIGRAPHY OF THE LLANDOVERY (SILURIAN) BLANDING, HOPKINTON, SCOTCH GROVE, AND LAPORTE CITY FORMATIONS OF IOWA (NW ILLINOIS BASIN)

The carbonate-dominated Silurian strata in Iowa span the Llandovery through Wenlock series, and were deposited in a sag that linked the northwest corner of the Illinois Basin with the North Kansas Basin. Global and regional chronostratigraphic correlations of many of the formations and their constituent members remain poorly constrained due to complex lateral facies variation in these carbonate depositional environments and the paucity of biostratigraphic data. New carbon isotope chemostratigraphic (δ¹³C_carb) data will be integrated with existing and additional conodont biostratigraphic data to enable more precise regional and global chronostratigraphic correlations of the members and formations. Samples of 3-6 kg were collected at ~1 m intervals for conodont processing, and samples for carbon isotope analysis were collected at 10 cm intervals from outcrops of the Scotch Grove and LaPorte City in Delware County. Samples for carbon isotope analysis were collected from the Blanding, Hopkinton, Scotch Grove, and LaPorte City formations from two Iowa Geological Survey cores. No biostratigraphically useful conodonts were found from the Scotch Grove outcrop. Conodonts diagnostic of the Pterospathodus eopennatus Super Zone and Pt. amorphognathoides angulatus Zone were recorded from the LaPorte City outcrop. The conodont Ozarkodina polinclinata polinclinata occurs with Pt. a. angulatus Zone conodonts, which is significantly lower than its first occurrence in the Pt. a. amorphognathoides Zone in the Baltic.