OZARKS RESIDUUM - A POTENTIAL ARCHIVE OF CLIMATE DATA

Clay-rich carbonate “residuum” may be the most widespread Quaternary sediment in the Ozarks. This material traditionally was thought to form by concentration of accessory clay minerals as carbonates dissolved from the top down, but this model is not credible. First, the clay reaches a thickness >30 m in Missouri, which would require dissolution of more than a kilometer of carbonate that lacks any detrital intervals. Moreover, a residual lag of many Missouri carbonates would be mostly chert. But chert is dispersed within the clay at approximately the same concentration as the underlying rock formation. Instead, the red clay and other minerals form authigenically via chemical reactions linked to dissolution. In short, the clay is largely a chemical sediment which inverts the usual age relationship; younger material is at the bottom, while older is at the top.

Paleomagnetic datums are a potential means of dating the clay and measuring its rate of formation. At one site, an interval with a predominant reversed magnetic remanence extends upward from 1.5 – 4.7 m above the top of rock, while the overlying 2.1 m has a normal remanence. The interval below 1.5 m presumably contains the most recent (normal) polarity chron, but this interval was not sampled due to a thick accumulation of displaced material. The overlying segments should represent the Matuyama Reversed and the Gauss Normal Chrons, respectively. Using the 4.7 m datum and the 2.6 Ma age of the Gauss-Matuyama boundary gives an average formation rate of 1.8 m/Ma. More detailed analysis of core material could provide additional datums and the age of other authigenic minerals that may preserve a climate signal.