PETROLOGY, PETROGRAPHY AND CONODONT BIOSTRATIGRAPHY OF THE OLMITZ LIMESTONE, BELINDA SHALE AND 1000 ACRE COAL AT THEIR TYPE SECTION IN SOUTH-CENTRAL IOWA

The recently named Olmitz Limestone, Belinda Shale, and Thousand Acre Coal bed were recognized in Iowa by J.P. Pope in 2012. These strata at their type section, are exposed in a road-cut in northeast Lucas County, approximately 21 km northeast of Chariton, Iowa. They are part of the Floris Formation (Cherokee Group, Desmoinesian Stage, Middle Pennsylvanian Series) and comprise an intermediate cyclothem known as the Lower Tiawah cyclothem (in Oklahoma) that occurs between the major Inola and Elliot Ford (Upper Tiawah) cylothems in Iowa. The lower sequence boundary is marked by an exposure surface below the 1.5 cm thick Thousand Acre Coal, which is overlain by 2.5 cm of light gray clay mudstone, together comprising 4 cm of transgressive deposits. Thinness and lack of an abundant and diverse open-marine fauna may indicate a rapid rise in sea level. Maximum transgression and still-stand occur in a 5 cm thick dark-gray mudstone with abundant conodonts (4500-5000 P₁ elements/kg) including Idiognathodus, Diplognathodus, Idioprioniodus, and Neognathodus. This condensed interval grades upward into 13 cm of green-gray mudstone with moderate conodont abundances (300 P₁ elements/kg). Regressive deposits in the upper 25 cm of the Belinda Shale, consist of argillaceous skeletal wackestone nodules in a greenish gray mudstone, containing an abundance of ostracodes, productid brachiopods, crinoids, bryozoans, gastropods, and clams. The topmost marine portion of the cyclothem is represented by the 23 cm thick Olmitz Limestone. The Olmitz Limestone grades upward from an iron-rich, argillaceous, skeletal wackestone to an ostracode-rich skeletal packstone to an argillaceous, skeletal lime mudstone, and contains an abundant open marine fauna including crinoids, clams, fusulinid foraminifers, gastropods, bryozoans, productid brachiopods, sponges, and ostracodes. The upper sequence boundary lies at the top of a blocky mudstone about 2.4 m above the top of the Olmitz Limestone.