Palaeomagnetism of the Moberly formation, northern Missouri, confirms a regional magnetic datum within the pre‐Illinoian glacial sequence of the midcontinental USA

Abstract

The Moberly formation of northern Missouri, USA includes glacigenic sediment deposited during the second major pre‐Illinoian glaciation and is overlain by three younger normal‐polarity tills. The Moberly (mostly till) preserves two different magnetic remanences. A detrital remanent magnetization has reversed polarity with shallow (‐30 to ‐40) inclinations. The shallow inclination is regionally persistent and spans different facies, including those not prone to large inclination error. A secondary overprint of normal polarity is associated with a thin oxidized zone and weakly developed paleosol in the upper portion of the till. This chemical remanent magnetization is distinguished by high coercivities typical of authigenic ferromagnetic minerals and by scattered remanence vectors upon stepwise demagnetization. The secondary normal remanence was likely acquired during a brief interglacial period between deposition of the Moberly formation and the next glaciation. The short interglacial and the shallow inclinations indicate that the glaciation which deposited the Moberly occurred shortly before a major polarity change from reverse to normal, probably the Brunhes‐Matuyama reversal.