MICROFOSSILS PRESERVED IN EARLY DIAGENETIC CALCAREOUS CONCRETIONS WITHIN THE OAKLEY AND EXCELLO SHALES OF THE MIDCONTINENT

The Middle Pennsylvanian Series (upper part of the Desmoinesian Stage) Oakley and Excello black, fissile, phosphatic shale facies often have large (up to 2 m wide and 45 cm thick) calcareous concretions along the entire Midcontinent outcrop belt from Oklahoma to Iowa. In some areas of Oklahoma, Kansas and southern Missouri, the concretions coalesce to form an almost continuous limestone layer in the shale, along a single outcrop. The Excello Shale is the sea level high stand deposit of the lower Fort Scott cyclothem (lowermost part of the Marmaton Group), whereas the Oakley Shale is the highstand deposit of the Verdigris cyclothem (lowermost upper part of the Cherokee Group). These types of concretions are rarely found in the highstand deposits of other Midcontinent cyclothems. Several taxa of pelagic fossils are commonly found in the Excello Shale concretions including the well preserved apparatus of the conodont Idiognathodus spp. along with Neognathodus cf. N. roundyi (Gunnell) and Gondolella magna (Stauffer and Plummer), agglutinated foraminifers Scherochorella and Reophax, radiolarians Albaillella, Pseudoalbaillella, Entactinia, Provisocyntra, Polyedroentactinia, Tormentum, and Tetragregnon, fish debris, bivalves, microgastropods (Streptacis?), and sponges. Concretions in the Oakley Shale mainly contain different species of radiolarians and conodonts, including the conodonts species Idiognathodus spp. along with Neognathodus asymmetricus (Stibane) and Gondolella pohli (von Bitter and Merrill). The concretions are dark gray to black in the interior and very organic- and phosphate-rich, with common pyritized zones. Microfossil content and preservation in the concretions is poor to excellent and varies considerably in very short distances within the concretions. Preservation of fossils in the shale surrounding the concretions is poor to non-existent, except for conodonts and fish parts. Radiolarians that are originally biogenic silica (opal-A) are usually preserved as quartz and pyrite, rarely as dolomite and phosphate, and even more rarely as an unknown “organic” material.