A Paleomagnetic Estimate of the Age and Thermal History of the Kentland, Indiana Cryptoexplosion Structure

Abstract

The Kentland, Indiana, cryptoexplosion structure consists of a centrally uplifted zone of steeply-dipping fault blocks surrounded by a ring depression and a low-amplitude structural high with a radius of 12 km. The age of the deformation is constrained stratigraphically to the interval between Mississippian and Quaternary. Paleomagnetic samples were collected from seven carbonate formations and one siltstone unit. Of these only the Middle Ordovician Quimby's Mill Limestone contains a stable magnetization with consistent directions. Stepwise thermal and alternating-field demagnetization shows the presence of a northwesterly, moderately steep characteristic remanence with a median destructive field of 10 to 30 mT and maximum unblocking temperatures of 300°C. The pole position calculated from the mean direction falls near Late Cretaceous paleomagnetic poles. Dispersion increases when a bedding correction is applied, indicating post-deformational magnetization. Because the magnetization fails the fold test it is not a shock remanence. It seems likely, however, that its origin is related to the deformational event, which can thus be tentatively dated as Late Cretaceous. Acquisition of isothermal remanence indicates that magnetite is the only significant remanence-carrier in these rocks. Magnetic blocking theory suggests that the magnetization was acquired at temperatures of 150°C or less, over a timescale of approximately 1000 years. A thousand-year cooling time scale requires either continuous heating or a blanketing overburden of about 300 m, thus providing an estimate of the depth of subsequent erosion of the Kentland structure.