Comparison of K-Ar Dates of Diagenetic Illite and Magnetic Characteristics in Concretions, Disturbed Belt, Montana

The Disturbed Belt is composed of folded and faulted rocks of Paleozoic and Mesozoic age trending roughly NW-SE located in northwestern Montana. The last episode of deformation occurred during the late Cretaceous (Laramide Orogeny) which deformed Devonian through Cretaceous rocks. Further east, the Disturbed Belt is bounded by gentle folds or arches. The study of diagenesis of Cretaceous rocks can be fit into a well-known context of burial and thermal history. Clay mineral thermometry was established from prior studies of the Disturbed Belt (Hoffman and Hower, 1979). K-Ar ages of diagenetic I-S determined in this study agree with previous studies and our data show an early Tertiary age of illitization. The Disturbed Belt provides an opportunity to test ideas regarding the origin of chemical remanent magnetization (CRM) and the relation of CRMs to the smectite to illite conversion. The Marias River Shale contains numerous bentonites and limestone concretions which contain a CRM in magnetite. In addition, the measurement of hopane biomarker ratios provides an independent measure of the thermal maturity of the Marias River Shale. Our results show a correlation between the degree of structural deformation of Cretaceous rocks in the Disturbed Belt, increased levels of thermal maturity, increased magnetic intensity of the CRM in the concretions, increased amount of illite layers in I-S, and a decrease in intra-sample variation of K-Ar ages of diagenetic I-S. These results support the idea that increased thermal maturity leads to illitization which may have caused the acquisition of a CRM in limestone concretion.