DETAILED GEOLOGIC MAPPING IN THE ELLISTON 30’ X 60’ QUADRANGLE, WEST-CENTRAL MONTANA

The Elliston 30’ x 60’ quadrangle is located in west-central Montana at the junction between the NW-trending Lewis and Clark line (LCL) and the NE-trending Great Falls tectonic zone (GFZ). Much of the Elliston quadrangle has been mapped only at a reconnaissance level, leaving many unresolved questions regarding the region’s complex structural, sedimentation and magmatic histories. The Montana Bureau of Mines and Geology has mapped several 7.5’ quadrangles, cooperatively funded by the U.S. Geological Survey STATEMAP program, to resolve structural and stratigraphic problems prior to completion of the Elliston 30’ x 60’ geologic map.

Recent detailed geologic mapping in the western half of the Elliston quadrangle (Avon Valley and adjacent mountains) shows that Mesoproterozoic (Belt Supergroup), Paleozoic, and Mesozoic sedimentary rocks are unconformably overlain by expansive deposits of Tertiary sediment and volcanic rock, both of which were subdivided and mapped in this study. Middle Paleocene(?) and older rocks are locally tilted and deformed by NW-trending folds and thrust faults. These structures are overprinted by a system of NW- and NE-trending faults, most of which are high-angle and exhibit normal offsets with either dextral or sinistral motion. Preliminary kinematic data suggest that these oblique-slip faults evolved synchronously, with linked Bingham strain axes indicating NW-shortening and NE-lengthening.

The dominantly NW-trending structural grain in the Elliston quadrangle is interpreted to be consistent with LCL tectonism. NW-trending folds and thrust faults resulted from sinistral transpression during the Late Cretaceous to middle Paleocene(?), coeval with volcanism and sedimentation. A transtensional fault system then overprinted these shortening structures, initiating NW- and NE-trending oblique-slip faults that exploited pre-existing crustal anisotropies related to older LCL and GFZ structures. Continued detailed geologic mapping and radiometric dating of volcanic intervals will further document the timing and style of deformation along this part of the LCL.