OCCURRENCE OF ABUNDANT WELL-PRESERVED PENNSYLVANIAN RADIOLARIANS IN THE EXCELLO SHALE (BASAL MARMATON GROUP, UPPER DESMOINESIAN) OF SOUTH-CENTRAL IOWA

Well-preserved radiolarians have been found in the black shale facies of the Excello Shale Member (Mouse Creek Formation, Basal Marmaton Group) at several locations in south-central Iowa. The Excello Shale represents late transgressive, highstand condensed interval, and early regressive deposits of the upper Desmoinesian lower Fort Scott cyclothem. Radiolarians occur in extrapolated estimated abundances of tens to hundreds of millions per kilogram of sample. Most are found in a 5-6 cm-thick zone near the middle of the black shale facies, associated with the conodonts Gondolella sp. 2 of Swade 1985, Neognathodus, Idioprioniodus, Idiognathodus delicatus, I. acutus, and I. sp. 3 of Swade 1985, along with sponge spicules and fish remains. The radiolarians range from 60 to 500 microns in diameter and are preserved as microcrystalline quartz that replaced the original opal-A of the test. The component quartz crystals are usually less than 2 microns across, and preserve original porous test structure, spines, interior medullary shells, and cross bars. The best preserved specimens were incorporated into early diagenetic, organic-rich, calcilutitic carbonate concretions that protected the radiolarians before the structure of the test could be significantly altered. The Excello radiolarian fauna is relatively diverse and is dominated by spumellarians (entactinids, actinommids, cenodiscids, spongodiscids) and albaillellids (pseudoalbaillellids, parafollicucullidids). This fauna fits well within the Pseudoalbaillella zone established by Holdsworth and Jones for late Morrowan to Leonardian time, which includes the Desmoinesian Stage.