ROCK MAGNETIC CHARACTERIZATION OF FOLDED LOWER CARBONIFEROUS CARBONATES, NW MONTANA AND SW ALBERTA: IMPLICATIONS FOR OROGENIC REMAGNETIZATIONS

Rock magnetic analysis of Carboniferous carbonates in the Sawtooth Range in NW Montana and the Livingstone Range in SW Alberta was performed to compliment preliminary paleomagnetic research. The primary goal of the research is to test for a link between folding styles in thin-skinned thrust belts and the acquisition and characteristics of orogenic remagnetizations. A recent study in the Sawtooths found that Carboniferous carbonates in fault propagation folds (FPF) contain a syntilting characteristic remanent magnetization (ChRM) that resides in magnetite, whereas, a fault bend fold (FBF) and tilted rocks in thrust sheets contain a similar pretilting ChRM. Preliminary results from a complex FPF along Oldman River in Alberta reveal a ChRM that has southerly declinations and steep up inclinations or an antipodal direction. A tilt test indicates that the ChRM is syn- to pretilting. Several sites in Alberta also contain an intermediate temperature, late syntilting component which is similar to a component reported by previous workers in the Canadian Rockies that is considered to be thermoviscous origin and related to exhumation.

Preliminary high-field rock magnetic data from folds in Montana and Alberta show that saturation is reached before 0.3T and the majority of the samples have wasp-waisted hysteresis loops. This suggests that the samples contain a mixture of magnetite grain sizes. On a log plot of Mrs/Ms versus Hcr/Hc, the data has a power law distribution similar to that reported by previous workers. Interestingly, the samples from a FBF have higher values of Mrs/Ms than samples from the FPFs although it is not clear that the differences are significant with the preliminary data set. The ChRM and the strongest rock magnetic signal is most common in dark gray carbonates that are hydrocarbon reservoirs in the subsurface, suggesting the possibility that there may be a relation between the ChRM and hydrocarbon migration.