K, Ar, and mineral analyses of montmorillonitic mud samples 4233 ft to 16,450 ft deep from a well in the Mississippi Delta area show that, with depth, the apparent K-Ar ages of the bulk samples decrease 100 m.y.; the >2μ fraction is relatively constant in apparent age at approximately 400 m.y.; and the apparent age of the <2μ fraction decreases 40 m.y. with depth. With increasing depth and temperature, K (0.8 percent) appears to be released from the course K-feldspar and mica and to become fixed between the montmorillonite layers. Due to the formation of chloritic layers, fewer illitic layers are formed than is commonly supposed. K-Ar data may be useful in reconstructing the thermal and burial history of shales.

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