CONODONT BIOSTRATIGRAPHY OF LOWER CHEROKEE GROUP (LOWER DESMOINESIAN, MIDDLE PENNSYLVANIAN) IN MIDCONTINENT BASIN

Recent intensive study of long cores from Kansas to Iowa updates previous outcrop work on the conodont biostratigraphy of the Midcontinent lower Cherokee Group in Iowa and Oklahoma. Idiognathodus amplificus is the most abundant and cosmopolitan conodont species in this interval. The lowest Cherokee cyclothem in the Arkoma Basin of eastern Oklahoma, the McCurtain, currently defines the base of the Desmoinesian Stage in that more complete succession, carrying I. amplificus, I. cf. praeobliquus, and Neognathodus caudatus. However, samples from lower beds in a long core in northeastern Kansas yielded I. amplificus and I. cf. praeobliquus, along with the genus Idiognathoides, which elsewhere is confined to the Atokan. Therefore, it seems that only N. caudatus is currently a valid marker for the base of the Desmoinesian, even though previous workers have suggested I. amplificus, I. obliquus, and I. praeobliquus as potential markers for the Atokan-Desmoinesian boundary. The first occurrence of I. obliquus is above the McCurtain, in the intermediate Sam Creek cyclothem in eastern Oklahoma. It occurs also in the overlying major Doneley cyclothem, which extends beyond the Sam Creek northward into Iowa, where I. obliquus was first identified in the Midcontinent. Above the Doneley, the major Inola cyclothem contains abundant I. podolskensis, and forms definitely identified as I. podolskensis indicate a first occurrence in the Sam Creek cyclothem in Oklahoma and Doneley cyclothem in Iowa, extending its range downward. The Inola cyclothem also contains the first occurrence of Neognathodus asymmetricus in the Midcontinent, and marks the last occurrence of I. podolskensis, I. obliquus, and I. amplificus there. The next higher intermediate Wainwright cyclothem in Oklahoma yields Idiognathodus morphotypes that are more similar to those of the overlying middle Cherokee, which is dominated by forms resembling I. robustus, I. crassadens, I. attenuatus, I. rectus, and I. iowaensis. Because I. praeobliquus, I. obliquus, and I. podolskensis were named from eastern Europe, where they first appear upward in the same order as in the Cherokee, the lower Cherokee can be correlated roughly with the upper Kashirian and Podolskian substages in the middle Moscovian Stage of Russia and Ukraine.