A NEWLY RECOGNIZED POSITIVE CARBON ISOTOPE EXCURSION IN THE MCGREGOR MEMBER OF THE PLATTEVILLE FORMATION, EASTERN IOWA

The Harris (Louisa County, SE Iowa) and Big Spring 4 (Clayton County, NE Iowa) drill cores were sampled for carbon and oxygen isotopic values of carbonate muds as part of a regional study of the chemostratigraphy of Ordovician Galena and Platteville strata in the Upper Mississippi Valley. Both cores show the marked Chatfieldian positive carbon isotope excursion in the Guttenberg Limestone of the Decorah Formation, with peak d¹³C values reaching between +1 and +2‰ VPDB. Both cores also show a lower positive carbon isotope excursion. This excursion in the McGregor Member of the Platteville Formation is characterized by a gradual rising limb starting from baseline values between -2 and -1.5‰, peaking at values between -0.3 and +0.1‰, followed by an abrupt falling limb that decreases to values between -3.0 and -1.75‰. This is the first report of a positive carbon isotope excursion in the McGregor Member of the Platteville Formation.