Medicine Lodge thrust system, east-central Idaho and Southwest Montana

Abstract

The Medicine Lodge thrust system, exposed in east-central Idaho and southwest Montana, is a major segment of the North America Cordilleran fold and thrust belt on which Precambrian and Paleozoic rocks have been telescoped and transported far east of their depositional area. The fault has overlapped rocks that initially were deposited in differing sedimentary environments on opposite sides of the northwest-trending geanticlinal Lemhi arch. The nature of the arch itself has been obscured by later thrusting, but it can be roughly reconstructed as a major landmass recurrently uplifted from Precambrian Y time at least through the Mesozoic. Although sections of the large landmass have been identified in the past as small islands, the landmass was instead a southern extension of Belt island, and it exerted a major influence on sedimentation patterns. The thrust has obscured recognition of correlative sedimentary rock units. It also appears to have controlled the distribution of volcanic rocks in satellite centers of the main field of Tertiary Challis Volcanics. The thrust system has exerted a major influence on the emplacement of granodiorite-quartz monzonite stocks and related mineral deposits in east-central Idaho. Recognition of the fault as a major influence in intrusive activity and deposition of metallic mineral deposits suggests new possibilities for mineral resource exploration in this region. And consideration of the stratigraphic, paleogeographic, and structural framework of sedimentary rocks beneath the thrust suggests the possibility of petroleum and natural gas resources in the region north of the Snake River Plain.

Geologic map of the central Beaverhead Mountains, Lemhi County, Idaho, and Beaverhead County, Montana

“…The Apple Creek-Gunsight contact mapped in this study closely matches the location of a thrust fault mapped by Staatz (1973) and Ruppel (1978) and of the "Medicine Lodge thrust fault" of Ruppel (1978), Ruppel and Lopez (1984), Ruppel and others (1993). The present mapping indicates that the contact in the central Beaverhead Mountains is a sheared unconformity, but evidence was not found to substantiate the presence of the Medicine Lodge thrust fault.…”

“…Thus, the Mississippian strata as well as the underlying Mesoproterozoic strata in the Goat Mountain domain originated in deeper water settings (farther west) than most other stratigraphic sections in the map area. This structural interpretation is significantly different from published maps and reconnaissance studies because, in one interpretation, the Mesoproterozoic unconformity was depicted as a thrust fault (Staatz, 1973;Ruppel, 1978) and, in another interpretation, this angular unconformity was depicted as a normal fault (called "Goat Mountain fault" in Janecke and others, 2001).…”

“…Additionally, the depositional contact of Paleozoic units over all three Mesoproterozoic units is widely preserved in the Grizzly Hill domain, indicating Precambrian-age tilting of the older rocks before Paleozoic deposition. On previous maps, several thrust and normal faults were interpreted within the Mesoproterozoic section (Staatz, 1973;Ruppel, 1978;and Janecke and others, 2001); additionally, the Mesoproterozoic-Paleozoic unconformity in the Grizzly Hill domain was previously named the Grizzly Hill fault (Janecke and others, 2001). The interpretations herein are significantly different from those previous.…”

“…The present mapping indicates that the contact in the central Beaverhead Mountains is a sheared unconformity, but evidence was not found to substantiate the presence of the Medicine Lodge thrust fault. In the Maiden Peak spur, the Paleoproterozoic-Mesoproterozoic contact ("fault D" of M 'Gonigle, 1993'Gonigle, , 1994 is shown as part of the Medicine Lodge thrust fault (Ruppel, 1978), but this contact is reinterpreted as an unconformity (Tysdal and others, 2005). The Medicine Lodge thrust fault was also shown in the Lemhi Range and northern Beaverhead Mountains by Ruppel (1978) and Ruppel and Lopez (1984), but more recent mapping reveals that a thrust fault of this type does not exist in those areas either (Tysdal, 2002;Tysdal and others, 2005, respectively).…”

“…In the Maiden Peak spur, the Paleoproterozoic-Mesoproterozoic contact ("fault D" of M 'Gonigle, 1993'Gonigle, , 1994 is shown as part of the Medicine Lodge thrust fault (Ruppel, 1978), but this contact is reinterpreted as an unconformity (Tysdal and others, 2005). The Medicine Lodge thrust fault was also shown in the Lemhi Range and northern Beaverhead Mountains by Ruppel (1978) and Ruppel and Lopez (1984), but more recent mapping reveals that a thrust fault of this type does not exist in those areas either (Tysdal, 2002;Tysdal and others, 2005, respectively). Thus, a youngeron-older Medicine Lodge thrust fault has not been substantiated in any of the areas where it was originally depicted, including in the central Beaverhead Mountains.…”