18 Surficial geology and neotectonics in the West: Celebrating and advancing the legacy of Kirk Bryan through field-based geomorphology studies

This session invites and examines recent field-based geomorphology studies in the West. Contributions regarding soils geomorphology, neotectonics, pediments, fluvial terraces, and landscape development are invited.

Tuesday, 19 May 2026
8:00 AM - 12:00 PM
Alvarado A

Geologic Controls on Deep-Seated Landslides along the Ninemile Fault, Missoula Valley, Western Montana Rocky Mountains

Paper number: 18-4
Program: Technical Sessions
Time: 9:10 AM – 9:30 AM

Authors

Abstract

A belt of deep-seated Holocene landslides along the Ninemile fault in the Missoula Valley may have formed co-seismically during large earthquakes. The Ninemile fault is a major southwest-dipping oblique extensional fault of the Lewis and Clark Line in the Montana Rocky Mountains that has been active since Paleogene time. The landslide belt is continuous on both the hanging wall and footwall of a 30-km-long fault segment adjacent to the urban corridor of the Missoula metropolitan area. Many residential neighborhoods are built on the landslides because they offer lower slopes than the undisturbed mountainsides. These juxtapositions potentially pose significant risk to rapidly growing communities.

The landslide-prone Eocene-Miocene Renova Formation mudstone and the overlying Miocene-Pliocene Sixmile Creek Formation gravel filled a deep graben on the down-thrown, hanging-wall side of the fault in the Missoula Valley. These landslides seemingly fluidized the Renova mudstone and collapsed the overlying Sixmile Creek boulder gravel into massive deep-seated earth flows. Some individual landslides cover more than 3 km2 with long runouts into the valley floors. Proterozoic Belt Supergroup quartzite bedrock exhumed in the footwall of the Ninemile fault are the source of various generations of deep-seated rotational slides and shallow landslides in colluvium that either displace or bury the fault scarp, respectively.

Pleistocene and Holocene sediments document the timing of landslide activity relative to fault movement. There appear to have been several generations of landslide occurrence, based on relative geomorphic signatures within the landslide belt along the Ninemile fault. These could record the timing of co-seismic landslides triggered by earthquakes sourced from the Ninemile fault. The landslides displace Glacial Lake Missoula shorelines and postdate deposition of glacial erratic boulders, which indicate landslide activity is younger than latest Pleistocene-early Holocene in age.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410545

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