Geochemical characterization of bentonite beds in the Two Medicine Formation (Campanian, Montana), including a new 40Ar/ 39Ar age

Abstract

Terrestrial deposits of the Upper Cretaceous (Campanian) Two Medicine Formation of northwestern Montana preserve multiple bentonite beds (n ≥ 19) that reflect recurrent pyroclastic events in the Western Interior Basin. Major and trace element concentrations were determined on 27 samples derived from four bentonites using X-ray fluorescence spectroscopy. This study evaluates the potential for geochemically distinguishing three of these bentonite beds using a stepwise discriminant analysis of trace element concentrations. Seven elements were found sufficient to establish 100% classification in the group matrix. The elements (in order of decreasing contribution to the canonical discriminant functions) are Zr, Sc, V, Cr, U, Ga, and Th. The validity of these results is strongly supported by cross-validation methods that correctly assigned 100% of randomly-selected bentonite samples left out of the stepwise analysis to their correct bed. These findings indicate geochemical discrimination is a viable tool for correlation within the formation and suggests its application to more distant coeval strata. We also report here a new 40Ar/ 39Ar age of 77.52 ± 0.19 Ma for one of the analyzed bentonite beds. This new radioisotopic age affords insights into the timing of regional eruptive events, and further constrains the age of the Two Medicine Formation and its renowned fossil resources. Finally, the inferred magmatic composition of the original ash (based on trace element compositions) from the two older bentonites beds suggest a source in the Elkhorn Mountain Volcanics whereas the younger bentonites may have been sourced from the Adel Mountain Volcanics.