NEW K-AR AGES FROM MIDDLE ORDOVICIAN K-BENTONITES DATE LATE PALEOZOIC BRINE MOVEMENTS IN NORTHWESTERN OHIO

New radiometric age dates and data on clay mineralogy for mixed layer illite-smectites are reported here that document Late Paleozoic brine movements in the Findlay Arch region of northwestern Ohio. These data establish the extent and timing of illitization of Middle Ordovician K-bentonites, and likely date movements of hydrocarbons and brine deposited MVT mineralization in the region. Subsurface samples of Millbrig K-bentonites were obtained for four sites from the core repository of the Ohio Division of Geological Survey. A Williams County sample came from a depth of 763 m, a Wyandot County sample from a depth of 411 m, and two samples from Marion County from depths of 462 and 472 m. The <1 micron clay fraction was utilized to exclude detrital illite, and analyzed by XRD and conventional K-Ar dating techniques. The Williams County site is located on the west flank of the Findlay Arch and east flank of the Kankakee Arch in the Michigan Basin, while the Wyandot and Marion County sites lie on the east flank of the Findlay Arch in the Appalachian Basin. The illite content of mixed-layer illite-smectite is >90% for the Williams County core, the highest of the four sites, and the ordering type is R 3. This relationship is likely related to movement of potassium-rich brines from Silurian evaporites of the Michigan Basin. For the Wyandot and Marion County cores, the illite content of mixed-layer illite-smectite is 73-75%, and the ordering type is R 1/R 3. The K-Ar value of 322±4 Ma from Williams County compares well with the data of Elliott and Aronson (1993) from northeastern Indiana. The age data from Wyandot County, 302±4 Ma, and Marion County, 278±4 Ma and 267±4 Ma, correspond to the Alleghanian orogeny. These data document brine movements in the Findlay Arch region of Late Mississippian-Early Pennsylvanian age from the Michigan Basin and Late Pennsylvanian-Early Permian age from the Appalachian Basin.