PALEOMAGNETISM OF THE CRETACEOUS KOOTENAI FORMATION, MCCARTHY MOUNTAIN FOLD AND THRUST BELT, SW MONTANA

The role of vertical axis rotation to produce fold and thrust belts with arcuate map-patterns can provide useful constrains on the origin of these curved orogenic belts. The Laramide fold and thrust belts in SW and central Montana are classic areas to study these deformation features. Prior paleomagnetic study of the Helena and McCarthy Mountain Salients (Eldredge and Van der Voo, 1988) focused on red-beds of the clastic members of the lower Cretaceous Kootenai Formation, finding that these rocks retain a prefolding magnetization carried by hematite, that the pattern of block rotation in these fold and thrust belts is heterogeneous, and controlled by local buttressing effects rather than coherent bending of the fold and thrust belts. This study focuses on additional areas within the McCarthy Mountain salient to help better constrain the pattern and spatial extent of vertical axis rotation, sampling two areas near Dillon, MT (Argenta and Block Mountain). In these areas some of the better exposures of the Kootenai Fm are of the carbonate lithologies- this study concentrated sampling on the informally named Gastropod Limestone member that occurs at the top of the formation. Oriented block samples were collected by students as part of their field mapping course, and some of the lab work presented here was also conducted by students in a follow-up paleomagnetism course. The block samples were drilled to produce standard size paleomagnetic core samples, which were then measured and step-wise demagnetized using either alternating fields or thermal methods. Results from the Argenta location are reported here. The remanence of these carbonates is very weak (< 0.5 mA/m² in general), but the majority of samples have well-defined magnetizations. The characteristic remanence was the second-removed component, and was isolated between 30 and 160 mT during a.f. demagnetization, or between 350 and 520 C during thermal demagnetization. The second-removed component passes the paleomagnetic fold test, with best clustering of remanence at 100% untilting- the tilt-corrected mean is D = 23, I = 68, α₉₅ = 10, k = 31. Using a North American reference pole for early Cretaceous time, Dx = 332, Ix = 69- resulting in a relatively large CW rotation of 51 +/- 20 degrees. Additional results from samples collected this summer at Block Mountain will be presented.