PALEO-HYDROSTRATIGRAPHY OF THE PALEOZOIC – MESOZOIC STRATIGRAPHIC SECTION IN THE SAWTOOTH RANGE, MONTANA

This project determines the paleo-hydrostratigraphy of Paleozoic and Mesozoic sedimentary rock layers within the Sawtooth Range of the 'disturbed belt' in west-central Montana. The structure is that of a thrust-duplex with ~10 stacked imbricate thrust sheets. Mineralized fractures were sampled from the Devonian Jefferson Fm. through the Cretaceous Blackleaf Fm., with most samples from the Mississippian Allen Mountain and Castle Reef Fms.

Fractures and fault zone breccias were filled and or cemented with calcite. Stable isotopic values of δ¹⁸O and δ¹³C from calcite and host rock samples were analyzed to constrain the history of how fluids migrated through the rock layers before and after large-scale deformational events.

Host Rock samples from throughout the Sawtooth Range have isotopic values ranging from: -0.75 to 2.71 ‰ for δ¹³C and -8.00 to -2.88 ‰ for δ¹⁸O. These values are significantly different from Phanerozoic seawater (Veizer et al., 1999), indicating post-burial alteration of the host rock. The overall range of isotopic values for the vein calcite ranges from -23.44 to -2.05 ‰ for δ¹³C and -20.89 to -4.95 ‰ for δ¹⁸O. Most bed parallel veins are different from the host rock with a 10 to15 ‰ depletion in δ¹⁸O and 2 to 5 ‰ depletion in δ¹³C. In contrast, most bed-normal veins have isotopic values similar to the host rock. Three fault zone samples have severely depleted δ¹⁸O values -15 to -12 ‰ and δ¹³C values -23 to -15 ‰ for both the host rock and the vein material indicating significant fluid alteration.

Fluid inclusion microthermometry of two-phase aqueous inclusion shows a progressive increase in fluid homogenization (Th) temperature and salinity with stratigraphic depth. The Cretaceous Two Medicine and Blackleaf Fms. have inclusions that homogenize at less than 115° with 0 % salinity, while the Castle Reef Fm. has Th values of 135° to 150° and salinities of 1 to 15 wt. % NaCl equiv. the stratigraphically lower Allen Mountain Fm. has Th values of 160° to 170° and salinities of 11 to 13 wt. % NaCl equiv.

In summary, isotopic data indicates that the bed-parallel fractures and the fault zones were significant fluid pathways, while the bed-normal fractures showed little fluid connectivity. The fluid inclusion data indicates that the fluid hydrostratigraphy was not disturbed by the thrusting.