LATE PALEOZOIC REMAGNETIZATION AND ITS CARRIER IN THE TRENTON AND BLACK RIVER CARBONATES FROM THE MICHIGAN BASIN

Paleomagnetic study of subsurface samples of the Upper Ordovician Tren ton and Black River Limestones collected from six industrial wells in the southern part of the Michigan Basin yields a well-clustered charac teristic direction (mean Dec/Inc of 170.6 degrees/-10.9 degrees, alpha (95) = 7.6 degrees) after rotation of random declinations by aligning a present day field overprint with geographic north. The corresponding paleomagnetic pole at 52.4 degrees N 111.0 degrees E is similar td th ose of the remagnetized carbonates in Ontario and New York State, sugg esting that the Michigan formations were also remagnetized in the late Paleozoic. Rock magnetic characterizations suggest that the character istic magnetization is carried by single and pseudosingle domain magne tite grains. Very fine rounded single crystal magnetite grains, believ ed to be products of chemical precipitation, observed by scanning and scanning transmission electron microscope (SEM/STEM) are inferred to b e the carrier of the remagnetization. Paired carbon isotope analyses o f bulk carbonate and anhysteretic susceptibility (X(arm)) of cuttings from two wells were examined to test the hypothesis that oxidation of hydrocarbons may lead to formation of authigenic magnetite. Oil satura ted horizons and overlying strata in a producing well exhibit lower X( arm) intensities than the equivalent horizons in an adjacent non-produ cing well. Little correlation between X(arm) and delta(13)C is observe d in carbonate-rich lithologies due to the masking of the hydrocarbon related cement compositions by the host carbonate. In contrast, carbon ate-poor shale horizons overlying hydrocarbon-rich strata in the produ cing well exhibit strong correlative peaks in X(arm) and delta(13)C. T he overall lower X(arm) intensities in the hydrocarbon-producing well suggest that it is not merely the presence of hydrocarbons which promo tes authigenic magnetite formation. Rather, the magnetite/hydrocarbon relationship more likely records processes associated with hydrocarbon migration and interaction with oxidizing subsurface fluids in areas p eripheral to sites of accumulation.