Paleomagnetism of Upper Cretaceous volcanic rocks of southwestern Montana

Paleomagnetic studies of folded and flat-lying volcanic rocks in southwestern Montana indicate that most of the natural remanent magnetization (NRM) is primary and that secondary magnetization, where found, has been produced mainly by lightning. NRM directions of most rock units are grouped in either a normal or reversed sense after partial demagnetization, although directions of a few units of probable mudflow origin are highly dispersed. No evidence of self-reversed magnetization has been found; consequently, a field polarity sequence has been tentatively established in connection with Late Cretaceous and early Tertiary orogenic events. All folded rocks investigated belong to the Upper Cretaceous Elkhorn Mountains Volcanics; stratigraphic correlation between widely separated, structurally dissimilar outcrops of petrologically similar rocks is supported by NRM directions and polarities. Mean NRM directions of structurally restored, age-equivalent sites do not differ significantly from one another, which suggests that fold axes are nearly horizontal; however, they differ significantly from directions of nearby flat-lying volcanic rocks of probable Tertiary age. The virtual geomagnetic pole calculated for the Elkhorn Mountains Volcanics (69°N, 171°W) agrees generally with Late Cretaceous poles previously calculated by others for North America, Madagascar, and the U.S.S.R., but it differs significantly from Early Cretaceous poles of Japan and Australia.