K-AR AGES OF ILLITE CLAYS OF THE MEADE PEAK MEMBER OF THE PHOSPHORIA FORMATION, WESTERN PHOSPHATE FIELD, SOUTHEASTERN IDAHO

The Meade Peak Phosphatic Shale Member of the Phosphoria Formation in the Western Phosphate Field, southeastern Idaho, is composed of carbonaceous mudstone and siltstone with beds of phosphorite and dark bioclastic limestone. An objective of this study has been to understand the timing of diagenetic illite formation in the Meade Peak and whether the Neogene passage of hydrothermal fluids had a significant role in forming diagenetic illite in these shales. Illite-smectite (I-S) is the predominant clay mineral within the shale samples. Smectite, apatite, and kaolinite were also found in some samples. A distinct second generation of diagenetic illite is present as thin, <1 mm diameter illite “rosettes” in one particular sample of the Meade Peak. The percentage of illite layers in I-S is >90% for all samples, and the I-S exhibits a Kalkberg-type stacking order (IISI). The diagenetic conditions inferred from the stacking order and percentage of illite layers in I-S are consistent with published vitrinite reflectance data of the Meade Peak. The K-Ar apparent ages of I-S range from 186 Ma to 292 Ma. A decrease in K-Ar age of I-S with decreasing particle size is observed in clay sub-micron fractions and is consistent with an interpretation that the clay fraction contains a mixture of detrital illite (or I-S) and diagenetic I-S. The measured K-Ar ages of I-S also decrease with increasing stratigraphic distance above the base of the Meade Peak. The K-Ar age of the diagenetic illite rosettes is about 185 Ma. These results indicate that diagenetic illite was formed in the Meade Peak shales during the Jurassic Period, almost certainly in response to progressive burial over a prolonged duration . Additional heating in response to thrust sheet emplacement during the Cretaceous Period may be responsible for the decrease in K-Ar age upward within the member.