USING 1:24,000 SCALE MAPPING TO SOLVE REGIONAL GEOLOGIC PROBLEMS IN SOUTHWESTERN MONTANA

Montana Bureau of Mines and Geology’s STATEMAP program mapped two key 7 ½’ quadrangles in 2003 to help resolve questions on regional Proterozoic stratigraphy and the extent of early Tertiary extension associated with the recently recognized Anaconda metamorphic core complex. The Dickie Hills quadrangle, located south of the core complex in the West Pioneer Mountains contains plutonic, sedimentary, and high-grade metamorphic rocks similar to hanging wall and footwall rocks in the core complex. New mapping shows that the high-grade, dynamically metamorphosed rocks grade into unmetamorphosed sedimentary and plutonic rocks that are structurally continuous with those of the hanging wall of the core complex. The detachment zone is therefore postulated to swing along the southeast flank of the Pintlar Range northwest of the Pioneer Mountains. The major structural features in the quadrangle include the Cretaceous Johnson thrust fault that is deformed into tight, northwest-trending folds, and northwest-trending sinistral (?) faults that cut the thrust and probably developed in a transtensional setting related to the Anaconda core complex. The thick quartzite section in the hanging wall of the Johnson thrust includes a thick Cambrian unit as well as previously mapped Proterozoic Belt units.

The Kelly Lake quadrangle, located in the central Pintlar Range along the southwest extension of the Anaconda core complex, illustrates the complex structural history of this region. At least five episodes of deformation are recorded that produced thrusts and detachments, a bedding-parallel mylonitic foliation, at least two subsequent fold generations, and younger high-angle faults. The high-angle faults separate the hanging wall and footwall of the northeast-striking Cretaceous Georgetown thrust and are of probable Tertiary age. Original thrust-fault relationships are not preserved. Tectonically attenuated Proterozoic Belt Supergroup strata in the footwall appear to be spatially associated with the bedding-parallel foliation. This structural thinning of Belt strata may help resolve previous difficulties in explaining abrupt thickness and facies changes in the Pintlar Range.