COSMOGENIC ISOTOPE BURIAL DATES FOR TILLS AT CONKLIN QUARRY, EASTERN IOWA

Since the 1970s Conklin Quarry near Iowa City, Iowa has afforded the best exposure of multiple pre-Illinoian tills in the midcontinent region. The two oldest tills (reversed polarity) are grouped within the Alburnett Formation. Three younger tills with normal polarity, the Winthrop, Aurora, and Hickory Hills (oldest to youngest), are members of the Wolf Creek Formation. Until recently the only direct age constraint of these five tills was their magnetic polarity; the Alburnett Formation must be >0.78 Ma, while the Wolf Creek tills are presumably younger.

The ratio of the cosmogenic isotopes 26Al and 10Be allow calculation of a paleosol's burial age, the date at which the soil was covered by a younger deposit. Thus, we sampled paleosols preserved atop the two Alburnett tills to measure their burial age, which is the age of the overlying till. The upper Alburnett till was deposited at 0.91 +/- 0.45 Ma, while the overlying Winthrop till was emplaced at 0.73 +/- 0.30 Ma. The broad error limits are due to shallow burial depths and low nuclide concentrations reflecting a short exposure time for the lower Alburnett till and truncation of the solum atop the upper Alburnett till.

Despite the broad error limits, these dates are consistent with the magnetic-polarity sequence and they also support previous correlations with tills in northern Missouri where a nearly identical stratigraphic sequence is present. The ~ 0.73 Ma age of the Winthrop till supports its correlation with the Fulton member of the McCredie Formation in Missouri, which is dated at ~0.78 +/- 0.1 Ma. Similarly, the age of the Alburnett till in Iowa is within error limits of the Moberly till in Missouri, which is dated at ~1.2 +/- 0.1 Ma.