AN EXAMINATION OF SEVERAL PALEOSOL PROFILES ALONG THE MISSISSIPPIAN/PENNSYLVANIAN BOUNDARY IN EAST-CENTRAL KENTUCKY

The systemic boundary separating Mississippian and Pennsylvanian rocks in East-Central Kentucky has been the subject of controversy for many years. This study provides evidence in support of the idea that the boundary separating these two subsystems experienced prolonged pedogenic alteration. Evidence for this includes: an irregular erosional surface between the two subsystems, the presence of rooting structures within the upper portions of this paleosol, the development of soil peds, the possible presence of soil horizons within this interval, as well as a number of other pedogenic features.

Spot samples were collected at four locations in the area of investigation. Sampling began at the top of each paleosol and continued down through each profile until unaltered rock was encountered. Each sample was examined/described in the lab and later analyzed for its grain-size distribution and clay mineral content. In order to verify that we were at the Mississippian-Pennsylvanian contact, we examined the rocks immediately above and below the paleosol for their microfossil content. Some of the results we have to date are as follows: 1) the clay mineral kaolinite shows a relative increase in abundance up profile; 2) there is a tendency for the crystallinity of illite to decrease up profile; 3) as the percent of kaolinite increases it naturally forces a decrease in the relative abundance of illite and smectite. This increase in kaolinite at the expense of illite/smectite is probably due to the chemical weathering or hydrolysis of illite to kaolinite according to the reaction below:

2KAl₃Si₃O₁₀(HO)₂ (Illite) + 2H₂CO₃ + 3H₂O → 3Al₂Si₂O₅(HO)₄ (Kaolinite) + 2K⁺ + 2HCO₃^-

4) one textural trend observed in these profiles is a increase in the weight percent of quartz sand in the upper portions of several of these ancient profiles. This may be due to the residual concentration of this highly stable mineral phase in this zone of intense weathering. We believe that these soils formed during the lower Pennsylvanian when east-central Kentucky was situated in an equatorial climatic region, which is characterized by relatively high amounts of precipitation and warm tropical temperatures.