OROGENIC REMAGNETIZATIONS, SAWTOOTH MOUNTAINS, MONTANA

The purpose of this investigation is to determine the origin of the orogenic remagnetizations in Mississippian carbonate rocks from the Sawtooth Mountains in northwestern Montana. The Allan Mountain Limestone and the Castle Reef Dolomite of the Madison Group were sampled in several east-west trending transects across the thrust faults. Samples were collected from tilted outcrops in the major thrust sheets and from several folds (Teton Anticline and a small anticline associated with a thrust fault). Preliminary paleomagnetic data and rock magnetic results indicate that the Madison Group contains a widespread chemical remanent magnetization (CRM) with southerly declinations and steep negative (up) inclinations that resides in magnetite. The two fold tests suggest that the CRM was acquired prior to folding or during an early stage of folding (early synfolding). Some sites contain an apparent late Paleozoic CRM with southerly declinations and shallow inclinations that also resides in magnetite. The Early Tertiary CRM is similar to a CRM reported by others from equivalent units in the southern Canadian Cordillera. To date, a remagnetization trend perependicular to the mountain front as reported from the southern Canadian Cordillera has not been found in the study area. Preliminary geochemical studies (⁸⁷Sr/⁸⁶Sr) indicate that both the limestones and dolostones that contain the CRMs were altered by radiogenic fluids. In addition, degraded hydrocarbons are also present, particularly in the dolostones. The Tertiary CRM is interpreted to be related to alteration by fluids, and there are two likely remagnetization mechanisms: hydrocarbons which migrated into the unit and externally derived radiogenic fluids. Further geochemical and paleomagnetic studies will investigate the origin of both CRMs and diagenetic history of the rocks.