Abstract The catastrophic Hebgen Lake earthquake of 18 August 1959 (M W 7.3) led many geoscientists to develop new methods to better understand active tectonics in extensional tectonic regimes that address seismic hazards. The Madison Range fault system and adjacent Hebgen Lake–Red Canyon fault system provide an intermountain-active tectonic analog for regional analyses of extensional crustal deformation. The Madison Range fault system comprises fault zones (~100 km in length) that have multiple salients and embayments marked by preexisting structures exposed in the footwall. Quaternary tectonic activity rates differ along the length of the fault system, with less displacement to the north. Within the Hebgen Lake basin, the 1959 earthquake is the latest slip event in the Hebgen Lake–Red Canyon fault system and southern Madison Range fault system. Geomorphic and paleoseismic investigations indicate previous faulting events on both fault systems. Surficial geologic mapping and historic seismicity support a coseismic structural linkage between the Madison Range and Hebgen Lake–Red Canyon fault systems. On this trip, we will look at Quaternary surface ruptures that characterize prehistoric earthquake magnitudes. The one-day field trip begins and ends in Bozeman, and includes an overview of the active tectonics within the Madison Valley and Hebgen Lake basin, southwestern Montana. We will also review geologic evidence, which includes new geologic maps and geomorphic analyses that demonstrate preexisting structural controls on surface rupture patterns along the Madison Range and Hebgen Lake–Red Canyon fault systems.

Abstract On this field trip, participants will get their hands dirty while characterizing soils formed on five different rock types: Archean Gneiss, Flathead Sandstone, Wolsey Shale, Meagher Limestone, and Absaroka Volcanics (a basaltic andesite rock). We first recap prior soil survey efforts across the Gallatin National Forest in southwestern Montana and introduce a state factor approach to understanding soils. For over 50 years, Montana State University faculty have explored parts of this lithosequence, using it as a natural laboratory for thousands of students. We continue this tradition with this field guide, emphasizing how the combination of field and laboratory data can enrich our understanding of soil processes. We will observe and measure striking differences in soils; these differences in physical and chemical properties, from textures and colors to pH and elemental composition, are discussed in the context of quantifying the influence of the underlying rock on soil properties. We use these differences to ask whether heterogeneity in soil properties justifies the inference that soil properties are dominated by the underlying lithology. We conclude that the underlying rock strongly influences soil properties, but in variable ways across this lithosequence. This influence is both direct and indirect: chemical weathering of the rock leads to compositional changes in overlying soil, but rock weathering also leads to coarse fragments in the soil profile, which alters soil hydrology.

Abstract We summarize the geologic settings, generalized geology, and inferred conditions of talc formation for two major deposits in southwestern Montana. Imerys Talc operates the Yellowstone Mine in the Gravelly Range. Barretts Minerals Inc., a subsidiary of Minerals Technologies Incorporated, mines talc from two large deposits—the Regal and the Treasure—in the southern Ruby Range. Talc mineralization in southwestern Montana is associated with hydrothermal alteration of Archean dolomitic marbles along faults in the southern margin of the middle Proterozoic Belt Seaway. Conditions of talc formation appear to have varied across the region and probably range from shallow hot spring systems to connate brine circulation pathways in Belt basin sediments. A road log description of the geology along a loop from Bozeman to Dillon, Montana, to visit both the Yellowstone and Regal talc mines accompanies this paper.

The Scarface thrust of the western Madison Range, Montana, is a 17° west-dipping Late Cretaceous thrust that places Archean gneisses over a complexly folded panel of Phanerozoic sedimentary rocks. The Archean-Cambrian contact on the footwall of the Scarface thrust is nearly vertical, and both bedding in the cover and foliation in the gneisses near the contact were rotated by 38° during folding. Paleozoic rocks up section in the footwall are overturned, with an axial surface that dips less than 10° west. The Scarface thrust is locally folded over lower Paleozoic rocks on the footwall. Folding was produced by post-Scarface thrust movement on a minor east-dipping splay fault that follows bedding in Devonian rocks. Of the two dominant shear fracture and fault sets in the basement (strikes and dips of N52°W, 47°NE; N20°W, 50°SW), the northeast-dipping set is parallel to foliation and reflects a strong influence of foliation on basement deformation. Intergranular fractures nucleated at the tips of biotite grains. Narrow zones of cataclasis containing shredded biotite formed along the intergranular fractures. Advanced stages of deformation were accompanied by formation of thicker zones of wavy, foliated cataclasites defined by dark seams of comminuted biotite, feldspar, and quartz. The recumbent footwall syncline is superimposed on the west limb of a large, more open syncline in Paleozoic and Mesozoic rocks. We are unable to resolve which fold formed first. Faulting sequences are also equivocal. The Scarface thrust may have been emplaced as a shallowly dipping sheet, or it may have been steeper initially and rotated during movement on the structurally lower Beaver Creek thrust.