28 Geologic Mapping in the West (Posters)
This poster session highlights geologic mapping in the western Cordillera and Rocky Mountains. We encourage submissions that explore bedrock and surficial geology, geophysics, hydrology, hazards, and more. Both professional and student submissions are welcome.
Tuesday, 19 May 2026
8:00 AM - 5:30 PM
Alvarado D/E
Geologic mapping of mid-Cretaceous porphyritic intrusions and Laramide frontal structure on and near Line Creek Plateau, Beartooth Mountains, Montana-Wyoming
Paper number: 28-10
Program: Technical Sessions
Time: 8:15 AM – 8:15 AM
Authors
Abstract
Geologic mapping of ca. 104–92 Ma trachyte to trachydacite porphyritic intrusions is being conducted on the Line Creek Plateau in the southeast Beartooth Mountains. Incidental mapping of the intrusions at the southern and western portion of this plateau occurred at ~1:27,000 scale in the 1950’s, however northern portions were only ever mapped in reconnaissance at ≤1:100,000 scale. New 1:24,000 scale mapping focused on the intrusions is providing critical updates and new insights. Intrusions display at least three mappable units designated by phenocryst assemblage; these include Kpp (plagioclase), Kpkp (plagioclase+K-feldspar), and Kpq (plagioclase+quartz). Crosscutting relations and zircon U-Pb geochronology portray a complex intrusive history that is not sequential by map unit type. Upper-crustal emplacement into Archean basement occurred as dikes, sills, and possible localized sub-km scale stocks and/or possible conduits. Some locations display emplacement concordant with metamorphic foliation while elsewhere intrusions do not appear to follow structural fabrics. One location exhibits possible stretched quartz phenocrysts at its contact that may imply structural deformation during emplacement (ca. 92 Ma). Existing geologic mapping provides conflicting interpretations between the intrusions and range-front faults that uplifted the Beartooth Mountains, showing either faults that crosscut intrusions or faults that are crosscut by the intrusions. Mapping thus far shows that faults crosscut porphyry, providing a robust constraint on timing of large-magntidue range-front faulting. Mapping further reveals possible structural partitioning of the Beartooth front into forethrusts and lateral ramps which have not been previously recognized. Therefore, this work provides a new look at Cretaceous–Paleogene structural development of this classic Laramide arch. Additional implications include detailing mid-Cretaceous intraforeland magmatism, and examination of bedrock substrate and soil development as it relates to climate-stressed sub-Arctic disjunct plant populations.
Geological Society of America Abstracts with Programs. Vol. 58, No. 4
doi: 10.1130/abs/2026RM-410582
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