HR: 1340h
AN: T53B-1427    [Abstracts]
TI: Fluid Migration in and Across Fault Zones With Footwall Shales and Hanging Wall Carbonates (Sawtooth Range, NW Montana)
AU: * Sabato Ceraldi, T
EM: sabatot@geo.tamu.edu
AF: Texas A & M University, Dep. Geology and Geophysics, Texas A & M University, College Station, TX 77843-3115 United States
AU: Kirschner, D L
EM: dkischn@eas.slu.edu
AF: Saint Louis University, 3507 Laclede Ave., Earth and Atmospheric Sciences, Saint Louis, MO 63103 United States
AB: Faults can either conduct or trap fluids depending on many parameters including lithology, deformation history, temperature/pressure conditions, fluid pressure, and fluid chemistry. This research is focused on syntectonic fluid migration and entrapment in five early Tertiary thrust faults of the Sawtooth Range, NW Montana. West-dipping thrusts transported Mississippian carbonates over Lower Cretaceous shales and sandstones. We integrated fieldwork and laboratory analyses. The fieldwork involved geologic mapping, collecting structural data, and collecting samples of host rocks, fault rocks, veins, and slickenfibers. Laboratory work involved petrographic analyses of thin-sections to document the mineralogy, microstructures, and veining history, and stable isotope analyses of carbonates to document fluid-rock interactions and source(s) of fluid. The principal strike directions for mesofaults and fractures in the carbonate hanging walls are NNW-SSE and ENE-WSW. The NNW-SSE oriented mesofaults record reverse movements; the ENE-WSW mesofaults primarily record strike-slip movements. Both sets of faults developed during thrusting. The stable isotope analyses of the host rock far from the thrusts and major fractures have values of δ¹⁸O between 27‰ and 32‰. The δ¹³C values are between 1‰ and 2.5‰. These values are typical of Mississippian marine carbonates. Within a decimeter to meter of the thrust fault surfaces, the isotopic values of the host rocks are much lower. Slickensides in the mesofaults have lower δ¹³C and δ¹⁸O values than the host rocks. Veins and the latest fractures have the lowest δ¹³C (-8‰) and δ¹⁸O (15‰). In the carbonate hanging walls, pervasive fluid flow was limited to a decimeter- to meter-wide zone adjacent to each thrust. Fluid also flowed from the footwall to the hanging wall through secondary fracture networks that developed during deformation. The isotope data are consistent with the fluid being primarily meteoric in origin with variable amounts of organic-derived carbon.
DE: 8010 Fractures and faults
DE: 8108 Continental tectonics: compressional
SC: Tectonophysics [T]
MN: Fall Meeting 2005