POSITION OF THE MATUYAMA-BRUNHES PALEOMAGNETIC DATUM WITHIN THE PRE-ILLINOIAN ALBURNETT FORMATION, EASTERN IOWA

The Matuyama-Brunhes reversal is a paleomagnetic datum within the pre-Illinoian stratigraphic sequence of the North American midcontinent, and it marks the proposed boundary between early and middle Pleistocene deposits. In eastern Iowa pre-Illinoian Wolf Creek Formation deposits have normal remanent polarity, whereas glacial diamictons of the older Alburnett Formation have reversed polairity. In this study additional magnetic characterization of the Alburnett Formation was accomplished by systematically sampling sediments of various facies, including till, valley-fill alluvium, and valley-slope colluvium.

Two Alburnett Formation tills have reversed detrital remanent magnetization. In contrast, younger locally preserved Alburnett Formation valley-fill and valley-slope sediments inset into the reversely magnetized tills and capped by the Westburg Geosol, have normal detrital remanent magnetization. Therefore, the reversed-to-normal polarity change occurred during the latest phase of Alburnett Formation deposition in eastern Iowa.

All evidence indicates that this intraformational polarity reversal is the Brunhes-Matuyama. Therefore, the Alburnett Formation tills and their correlatives were deposited during a major continental glaciation shortly before 0.78 myr and coinciding with marine oxygen isotope stage 20.