Diagenesis and the K-Ar Dating of Shales and Clay Minerals

Gulf Coast sediments were obtained from an oil well drilled in coastal Louisiana, and their diagenesis was documented by the mineralogic changes in mixed layer illite/smectite. Samples were selected to represent varying degrees of diagenesis and separated into four size ranges for K-Ar analysis; bulk shale samples were also analyzed.

The measured K-Ar ages of the mixed layer illite/smectite become progressively younger with increasing depth of burial because of the diagenetic addition of potassium caused by conversion of smectite layers to illite layers. Detrital illite and feldspar are destroyed to provide potassium for the diagenetic illite layers, and the detrital phases have lowered ages because of the greater loss of radiogenic argon relative to potassium. As a result of both processes, the measured ages of whole shale also decrease with increasing diagenesis, and radiogenic argon is lost from the system.