FLUID MIGRATION IN AND ACROSS FAULT ZONES WITH FOOTWALL SHALES AND HANGING WALL CARBONATES IN SAWTOOTH RANGE OF THE ROCKY MOUNTAINS, W MONTANA

The study of fluid-rock interactions is needed to understand better the mechanics of faulting, seismogenesis, and formation of hydrocarbon deposits. Faults can conduct or trap fluids depending on the rock types, deformation history, temperature/pressure conditions, fluid pressure, and fluid chemistry. This research is focused on the identification of the most important parameters controlling fluid-rock interactions and fluid migration in and across limestone-shale-hosted thrust faults.

We are focusing our investigation on thrust faults in the foreland fold-thrust belt of the Sawtooth Range, west-central Montana. The north-south trending belt is considered to be one of the best exposed examples of a fold-thrust belt in North America. The incisions of the Sun River canyon and others east-west trending valleys provide good cross-sectional exposures of the belt. West-dipping thrusts transported the Mississippian carbonates of the hanging walls over Lower Cretaceous shales and sandstones of the footwalls.

We are using a multidisciplinary approach that integrates fieldwork and laboratory analyses. The fieldwork involves 1) detailed geologic and structural mapping 2) collecting structural data, and 3) collecting samples of host rocks, fault rocks, veins, and slickenfibers for isotopic analyses. Laboratory work involves: 1) analyzing samples and thin-sections by petrographic, cathodeluminescence, and SEM/microprobe techniques to document the mineralogy, microstructures, cementation events, and 2) analyzing carbonate, phyllosilicates, and organic matter for their stable isotope values (C, O, H).

During an earlier field season, V. O’Brien and D. Kirschner focused their work in the Sun River canyon and collected more then 100 samples across two thrust faults. The initial results of the stable isotope analyses are consistent with the hypothesis that the cores of fault zones acted as pervasive seals. Fluid flowed from footwall to hanging wall only through transient secondary fracture networks that developed during deformation. Our study, which builds on the result of the earlier work, will allow us to understand better the deformation history of the area and will furnish constraints on fluid-rock interaction and fluid movement during development of the fold-thrust belt.