ORIGIN OF THE PREGLACIAL WHIPPOORWILL FORMATION, NORTHERN MISSOURI

The Whippoorwill formation in northern Missouri is an enigmatic gleyed diamicton locally preserved beneath the oldest glacial till (Atlanta Fm.) in this area. The Whippoorwill is clearly derived from the local bedrock residuum, but lacks soil structure and horizonization, and its distribution is limited to preglacial topographic lows and sideslopes. Thus, it has been unclear whether the Whippoorwill is a sedimentary deposit or part of a weathering profile.

The Whippoorwill retains a strong, well-defined detrital remnant magnetization characteristic of a sedimentary deposit but not paleosols. The downslope margin (where exposed) appears lobate in form and possible ice-wedge casts are preserved in various exposures. Thus, the Whippoorwill is apparently a saturated mass-flow deposit, most likely from solifluction/gelifluction.

Recently we sampled the Whippoorwill for cosmogenic-isotopes (10Be and 26Al) at two sites to obtain ages (burial dates) of the overlying Atlanta-Formation till. Using the recently corrected decay constant for 10Be, we have obtained consistent dates of ~2.5 Ma. The concentrations of the cosmogenic nuclides are very high, reflecting long-term surface exposure prior to glaciation, but their concentration profiles are essentially constant with depth. If the Whippoorwill were an in-situ residual profile, an accretion deposit, or a mass flow that was exposed for a significant duration prior to burial, these concentrations would decrease exponentially with depth. Therefore, we propose that the Whippoorwill is the preglacial weathering- profile materials that were mobilized and homogenized under periglacial conditions shortly before glaciation by solifluction/gelifluction. Thus, the Whippoorwill could be considered either a sedimentary deposit or a buried Gelisol, the new soil order established for periglacial regions.