Structure, Burial History, and Petroleum Potential of Frontal Thrust Belt and Adjacent Foreland, Southwest Montana

Proterozoic to Phanerozoic Tectonism in Southwestern Montana Basement Ranges Constrained by Low Temperature Thermochronometric Data

“…this fault system is ∼7 km (Figure 1, Perry, 1986;Perry et al, 1983). The maximum reheating temperatures for the northern and southern Madison ranges are in agreement with other thermochronologic estimates from Carrapa et al (2019).…”

“…In the vicinity of our sample locations, the Paleozoic Era is represented by Cambrian through Permian sedimentary units of variable thickness with a prominent unconformity that places Devonian on Cambrian rocks (i.e., Ordovician and Silurian units are absent in southwestern Montana, Lonn et al, 2000;Skipp & Janecke, 2004;Sloss, 1950). Previously measured conodont alteration indices (Perry et al, 1983) and isopachs (Perry, 1986) show that maximum Phanerozoic burial depths were significantly greater in the Snowcrest trough, which encompasses our Gravelly and Ruby ranges samples, as compared to locations southeast of the Snowcrest-Greenhorn thrust system (Figure 1). These observations are considered in subsequent sections that focus on the interpretations for our thermal history models.…”

“…The timing and driver of Cretaceous and Paleogene Laramide deformation in southwestern Montana is also of interest as the traditional delineation of Laramide-style and Sevier-style faulting into discrete episodes is challenged in this region by several lines of evidence. Previous studies suggested that basement-involved deformation in southwestern Montana overlapped spatially and temporally with Sevier-style deformation (≥84 Ma¸DeCelles, 1986;McDowell, 1997;Nichols et al, 1985;Perry et al, 1983;Schmidt & Hendrix, 1981;Tysdal et al, 1986) and predated shortening accommodated in the type-section Laramide-style structures in Wyoming (<ca. 70 Ma, Harlan et al, 1988;Kellogg & Harlan, 2007;Peyton & Carrapa, 2013).…”

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In contrast to much of the western North American Cordillera, where crystalline basement rocks remain buried beneath a veneer of Phanerozoic sediment, southwestern Montana preserves widespread exposures of Archean and Proterozoic basement rocks that record an extensive Proterozoic through Phanerozoic tectonic history. Though these crystalline basement rocks, termed the Montana metasedimentary province (e.g., Mogk et al, 1992) in our study area (Figure 1), were uplifted and exhumed during the late Phanerozoic Laramide orogeny (Carrapa et al, 2019;Perry et al, 1983), they experienced prolonged histories of burial, metamorphism, and exhumation in Archean and Paleoproterozoic time that extend well beyond the most recent Cretaceous-Paleogene phase of deformation (

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Pre‐Thrusting Stratigraphic Control on the Transition From a Thin‐Skinned to Thick‐Skinned Structural Style: An Example From the Double‐Decker Idaho‐Montana Fold‐Thrust Belt

“…The observed tabular, thick, and sometimes curved calcite twins (Figure 7d) are characteristic of Type II and Type III twins (Burkhard, 1993;Ferrill et al, 2004), suggesting a rough temperature range of 200°C-250°C. This is in agreement with an independent maximum temperature estimate of 190°C-300°C (Dembicki, 2016) obtained from a conodont color alteration index of "4" for a sample of Mississippian rocks (Perry et al, 1983) collected along Railroad Canyon ∼5 km north of the study area. Assuming a 20°C surface temperature and geothermal gradient of 25-30°C/km, temperatures of 200°C-250°C suggest a depth of deformation of 6-9 km.…”