PALEONTOLOGY, PETROLOGY AND PETROGRAPHY OF EARLY DIAGENETIC CALCAREOUS CONCRETIONS IN THE OAKLEY SHALE MEMBER OF THE VERDIGRIS FORMATION AT SAYLORVILLE DAM EMERGENCY SPILLWAY, POLK COUNTY, IOWA

Several major overflow events, since 1983, at the Saylorville Dam Emergency Spillway have exposed the middle part (including the Verdigris cyclothem) of the Middle Pennsylvanian (Desmoinesian Stage) Cherokee Group. The Verdigris cyclothem, from bottom to top, consists of strata from the top of the paleosol below the underlying Whitebreast Coal at the top of the Floris Formation, an unnamed lenticular limestone, the Oakley Shale, the Ardmore Limestone, and shales and mudstones to the top of the paleosol below the overlying Wheeler Coal in the Swede Hollow Formation. A rarely seen concretionary limestone (lime-wackestone to packstone) between the Whitebreast Coal and Oakley Shale contains abundant carbonized-wood, dolomite and pyrite replaced gastropods, clams and pyrite. This unnamed lenticular limestone represents early transgressive deposits of the Verdigris cyclothem. On outcrop, the nodules in this limestone can easily be confused with nodules occurring within the Oakley Shale. The overlying Oakley Shale is a black, fissile, non-sandy, phosphatic shale about 60 cm thick, which represents deposition during sea level high-stand, during maximum transgression. Early diagenetic, organic-rich (lime-mudstone to lime-wackestone) concretions, up to 45 cm across, occur at several randomly scattered horizons within the Oakley Shale, and contain abundant phosphorite lenses and laminae, and pyrite. These concretions also contain conodonts (Gondolella pohli, Neognathodus spp., Idiognathodus spp., and Diplognathodus sp. cf. D. coloradoensis), fish teeth and scales, abundant spumellarian and pseudolalbaillellid radiolarians and sponge spicules. Most of the originally silica (opal-A) radiolarians and sponges in the calcareous concretions are poorly preserved as casts of dolomite with some being excellently preserved as pyrite casts. Radiolarians and sponges in phosphorite nodules are preserved as pyrite and phosphate casts. No radiolarians or sponges were recognized in the black shale.