Director: Steven D. Sheriff

The Lower Cretaceous (Aptian) Kootenai Formation was sampled in two localities of western Montana for a paleomagnetic investigation of a possible rotation of the Purcell Anticlinorium during Laramide deformation in Montana. Eight sites from the Ford Creek locality and three sites from the Marias Pass locality were sampled and subjected to standard paleomagnetic techniques in an attempt to test the rotational model. One site from the Marias Pass locality was rejected as it had a site-mean direction that is coincident with the secondary, present-day, direction of the other two. The two remaining sites of the Marias Pass locality yielded a structurally corrected, characteristic, direction that is seemingly rotated counterclockwise 45+/-16^o, with no latitudinal translation. However, based on only two reliable sites, the statistics from this locality are not very robust. Thus results from the Marias Pass locality were not used to test the extent of the rotational model. Six of the eight sites sampled in the Ford Creek locality passed statistically based arbitrary rejection criteria and were compared with Aptian (108-115 Ma) reference poles in the literature. The sites from this locality have been rotated clockwise 23+/-9^o, with little latitudinal translation. While the results of this locality support previously devised rotational models and are statistically coincident with paleomagnetic results from the Purcell Anticlinorium, it can not be conclusively determined that the Purcell Anticlinorium rotated as a coherent unit clockwise through 23+/-9^o. The extent of the rotation can not be determined based on one reliable locality. While a regional rotation is one possible explanation, another is that the locality has experienced a small scale vertical axis rotation.