Paleomagnetic Data from the Chugwater Group and Lower Jurassic Nugget Sandstone, Gros Ventre Range, Northwest Wyoming

The Gros Ventre Range in northwest Wyoming is a Laramide style, Precambrian-cored basement uplift defined by a series of west-northwest trending high-angle reverse faults on its southwest margin. The northeast flank of the range contains a typically gently dipping sequence of Paleozoic to lower Tertiary strata. Southwest of the range, east-verging structures of the Sevier fold-thrust belt come within a few kilometers of the southwest-verging Laramide style, basement involved reverse faults. We have collected paleomagnetic data from hematite-cemented siltstones and fine sandstones of the Triassic Chugwater Group (Lower Triassic(?) Red Peak Formation, Alcova Limestone, Crow Mountain Sandstone, Popo Agie Formation, and Jelm Formation) and hematite-cemented fine to medium sandstones of the Lower Jurassic Nugget Sandstone to determine if the orientation of the Gros Ventre Range has been affected by vertical axis rotation during Cretaceous to early Tertiary contraction of this part of the Cordillera, possibly as a result of differential shortening along basement involved Laramide-style structures. Locally along the northeast flank of the range these strata are deformed large-amplitude overturned to upright folds with west-nortwest trending fold axes, affording rigorous fold tests. We have collected appropriate strata from five general localities along the Gros Ventre River valley, along the northeast margin of the range. NRM directions from much of the collection (over 600 samples) and preliminary thermal demagnetization results are interpreted to indicate that the Gros Ventre Range (assuming that the Phanerozoic section is not detached) has not experienced appreciable vertical axis rotation.