PALEOMAGNETISM AND ROCK MAGNETISM OF REMAGNETIZED CARBONATE ROCKS FROM THE HELENA SALIENT, WESTERN MONTANA

The Helena salient is a curved orogen in southwest Montana, characterized by thin-skinned folding and thrusting. Ages dated from volcanic sills in the area imply that deformation in the region began 77 million years ago during the late Cretaceous (Harlan et al., 1998). One objective of this study is to investigate the nature of curvature associated with this orogen using paleomagnetic techniques. A second objective of this study is to determine whether or not these rocks have been remagnetized, via fluid migration, as has been observed in carbonates elsewhere in fold and thrust belts (Enkin et al., 2000; O’Brien et al., 2007; Stamatakos et al., 1996). During mountain building, regional uplift results in a hydrologic system in which fluids can migrate over large distances, precipitating magnetite minerals in carbonate reservoirs, recording the direction of the earth’s magnetic field at that time (Enkin et al., 2000). Evidence for such an event can be shown by different patterns of magnetization age, relative to deformation within different locations of the thrust stack.

For this study, carbonate rocks of the Mississippian Madison Group (near Townsend, MT), and of the Gastropod limestone member of the Cretaceous Kootenai Fm (near Dillon, MT) were sampled. Results from 18 sites from the Kootenai Fm have well defined, but very weak, magnetizations. A mean direction of D=30, I=68, k=2.3, α95=31 in in-situ coordinates, and D=43, I=70, k=9, α95=13 in tilt-corrected coordinates is obtained for this unit. The fold test indicates magnetization was acquired at between 80-120 % untilting. Hysteresis data fall on the SP-SD admixture lines, consistent with rock magnetic data from other remagnetized carbonate units. Using a late Cretaceous NA reference pole, these data indicate large (70° CW) vertical axis rotations in the Dillon area since late Cretaceous time. Paleomagnetic results from the Madison Grp rocks will be presented for the 29 sites collected in July 2009. These data, when compared with results from the Madison Grp north of the Helena Salient (O’Brien et al., 2007), and with red-beds from the clastic members of the Kootenai Fm (Eldredge and Van der Voo, 1988) will be used to assess the timing of (re)magnetization and sense and amount of vertical axis rotation in the southern portion of the Helena Salient.