CATACLASTIC DEFORMATION AND FLUID ACTIVITY ALONG THE ARMSTEAD AND ERMONT THRUST FAULTS, SOUTHWEST MONTANA

Silicified breccia occurs discontinuously along the Armstead and Ermont thrust faults, on the eastern edge of the Cordilleran thrust belt, west of Dillon in southwest Montana. These jasperoids also occur in the hanging wall immediately above the two thrust faults. The clasts in these silicified rocks are primarily chert, with occasional silicified carbonates, cemented by microcrystalline quartz. The jasperoids along the thrust are juxtaposed either directly against the unsilicified, Upper Cretaceous-Tertiary foreland basin deposits of the Beaverhead conglomerate (Armstead area), or in the case of the Ermont thrust (Badger Pass), on the Tertiary volcanic rocks in the footwall.

Previous research of jasperoids in the Mission Canyon Formation near Livingston in southwest Montana identified the jasperoids as silicified karst breccia from the late Mississippian or as solution breccia from the Late Cretaceous-Early Tertiary. Our field mapping along the two thrusts in the Armstead anticline and Badger Pass areas suggests that the jasperoids are fault-related, cataclastic rocks, formed when a fold-and-thrust-belt of Paleozoic-Mesozoic sequence of sedimentary rocks was emplaced on the foreland basin deposits of the Beaverhead conglomerate.

Deformation and silicification along the two thrust faults probably occurred after the Beaverhead conglomerate was deposited and overthrust by the allochthonous stack of thrust sheets. The post-Beaverhead (i.e., post-Late Cretaceous-Early Tertiary) thrusting was probably accompanied with the flow of silica-bearing fluids along the main detachment fault and adjacent splay faults, forming the jasperoids. The absence of silicification in the Beaverhead conglomerate suggests that fluid flow was localized to a narrow zone along the thrust faults. This, and the localization of the cataclastic rocks along the faults indicate that fluid flow was driven by the movement along the main thrust.

The main mechanism of deformation along the thrusts was cataclasis and involved significant amounts of fluid, which enhanced the cataclastic deformation and led to the silicification of the breccia.