PETROLOGY, PETROGRAPHY, CONODONT BIOSTRATIGRAPHY AND CORRELATION OF AN UNKNOWN PENNSYLVANIAN CYCLOTHEM FROM AN OUTCROP IN SOUTH-CENTRAL IOWA

An outcrop of lenticular limestone, exposed by road construction, about 9 km southwest of Knoxville, Iowa, in southeastern Lucas County was discovered in 2001 during field work for the Iowa STATEMAP Project. Its stratigraphy position was believed to be somewhere in the Floris Formation, Cherokee Group, Moscovian (Desmoinesian) Stage, Middle Pennsylvanian Series, but its exact stratigraphic horizon was unknown. The exposed outcrop, from bottom to top, consists of limonitic light-gray mudstone below a 65 cm thick coal, overlain by 65 cm of limestone. The limestone is a very argillaceous skeletal wackestone to packstone with an abundant fauna of clams (mainly nuculids), gastropods including Glabrocingulum, Palaeozygopleura, Shansiella and Bellerophon, ostracodes, brachiopods Desmoinesia muricata, Mesolobus mesolobus, Composita ovata, Crurithyris planoconvexa and Linoproductus.sp., along with serpulopsid, fusulinid, tetrataxid and other small foraminifers. Other fossils include abundant carbonized wood fragments and macrospores, crinoid plates, fish teeth, echinoids, orthoconic nautiloid cephalopods, scolecodonts and conodonts. Glassy microspherules of undetermined origin were also found in the upper limestone. The limestone is overlain by several centimeters of blocky gray-green mudstone. A relatively abundant (140 P₁ elements/kg) conodont fauna was found near the top of the limestone. The conodonts include morphotypes of Idiognathodus, Hindeodus, Adetognathus and Neognathodus that are compatible with forms found in a lenticular limestone above the Carruthers Coal bed. This limestone, in Iowa, was named the Elliot Ford Limestone by J.P. Pope in 2012, and is the transgressive limestone of the third cyclothem in the Floris Formation (upper Tiawah of Oklahoma) below the major Verdigris cyclothem. We are also using topographic data in conjunction with nearby known outcrops to tentatively correlate this outcrop to the Elliot Ford Limestone Member of the Floris Formation.