ORIGIN OF THE MAGNETIZATION IN CRETACEOUS SEDIMENTS, DISTURBED BELT, MONTANA

The objective of this study is to determine the origin of a magnetization which occurs in Cretaceous sedimentary rocks in the disturbed belt, Montana. Samples were collected from the sandstones in the Blackleaf Formation, carbonate concretions in the Marias River Shale Formation, and a mudstone in the Two Medicine Formation. The sites were collected from beds which have different strikes and dips in order to perform a regional paleomagnetic tilt test. The samples contain a modern viscous magnetization with a northerly and down direction that is removed at low demagnetization temperatures. At higher temperatures, a characteristic remanent magnetization (ChRM) with declination of 178.6°, inclination of -64.6° (grouping (k) = 115.5 and cone of 95% confidence (a95) = 6.3°) is removed. A regional tilt test based on the Watson and Enkin method produced a pretilting component at 97.5% ± 5.5% untilting, indicating that the magnetization was acquired prior to the deformation. The pole position plots close to the Tertiary part of the polar wander path. The ChRM in these sediments is interpreted as a chemical remanent magnetization (CRM). The chemical mechanism of remagnetization is still under investigation.