PALEOMAGNETIC ANALYSIS OF THE UPPER CRETACEOUS (CAMPANIAN) CLAGGETT AND JUDITH RIVER FORMATIONS IN NORTH-CENTRAL MONTANA

The Claggett Formation and overlying Judith River Formation are Late Cretaceous (Campanian) in age and are broadly exposed throughout the Western Interior Foreland Basin, with their type areas in the Upper Missouri River Breaks National Monument in north-central Montana. The Claggett Formation consists largely of marine shales while the Judith River Formation consists predominantly of continental strata, although shallow marine lithologies are present at the base (Parkman Sandstone Member) and top (Woodhawk Member) of the unit.

An investigation of the paleomagnetic character of the lower part of the Judith River Formation and the upper part of the Claggett Formation was conducted in order to obtain a more precise chronostratigraphic constraint on these strata. 124 block samples from 30 sampling sites in three overlapping sections were collected from the upper 80 meters of the Claggett Formation and the lower 27 meters of the Judith River Formation. Stratigraphic spacing between sites ranged from .2 meters to 24.5 meters, with an average spacing of 3.6 meters. Samples were prepared and analyzed at the Institute for Rock Magnetism at the University of Minnesota.

Bulk susceptibility and hysteresis studies reveal multiple carriers of magnetic remanence, including goethite, magnetite, and a minor hematite component. Progressive thermal and alternating field demagnetization behavior indicate that magnetite is the primary carrier of the characteristic remanent magnetism. Orthogonal diagrams were used to evaluate demagnetization behavior, principal components analysis was used to determine the orientation of the characteristic magnetic component, and Fisher statistics were used to calculate site and group mean directions. Results indicate that the entire section is of normal polarity, corresponding to chron C33N based on previously published radiometric dates from the Judith River Formation.

This work was funded by a UNA College of Arts and Sciences Research Grant to GAB.