The Glenwood - Platteville Disconformity

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Proceedings of the Iowa Academy of Science, Vol: 72, Issue: 1, Page: 305-311

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Guldenzopf, E. Charles
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Evidence for a disconformity between the Glenwood Member of the St. Peter Formation and the Pecatonica Member of the Platteville Formation is advanced in this report. Physical lines of evidence are the presence of floating St. Peter sand grains and the presence of phosphate grains and pebbles in the basal Pecatonica at several localities in Iowa, Wisconsin and Illinois. Faunal evidence for a disconformity consists of the distribution of four conodont genera within these two units. Chirognathus spp. is found in the Glenwood and the basal Pecatonica, but no higher, while the genera Polyplacognathus, Belodina and Eobelodina first appear in the Pecatonica and are not found in the Glenwood. The presence of a "mixed fauna" at the base of the Pecatonica is taken as evidence of an erosion surface in which Glenwood conodonts have been reworked and redeposited along with Pecatonica forms.