123-3 Lacustrine paleoseismology of the Bitterroot fault, Lake Como, Montana.

Session: Quaternary Research to Characterize Environmental and Geological Hazards

Presenting Author:

Colin Chupik

Authors:

Chupik, Colin¹, Anderson, Lesleigh², Wadman, Heidi³, Varyu, David⁴, Lamont, Ellen⁵, Laabs, Ben⁶, Gavillot, Yann⁷, Salazar, Raymond⁸, Gabel, Vanessa⁹, Benoit, Vincent¹⁰, Slawson, Jacob¹¹

(1) Bureau of Reclamation, Denver, Colorado, USA, (2) United States Geological Survey, Denver, Colorado, USA, (3) US Army Corps of Engineers, Detroit, Michigan, USA, (4) Bureau of Reclamation, Denver, Colorado, USA, (5) Bureau of Reclamation, Denver, Colorado, USA, (6) Bureau of Reclamation, Denver, Colorado, USA, (7) Montana Bureau of Mines and Geology, Butte, Montana, USA, (8) Montana Bureau of Mines and Geology, Butte, Montana, USA, (9) Bureau of Reclamation, Denver, Coloraod, USA, (10) Bureau of Reclamation, Denver, Colorado, USA, (11) Bureau of Reclamation, Denver, Colorado, USA,

Abstract:

Many reservoirs across the Intermountain West were once natural lakes that preserve evidence of past geologic and environmental phenomena. Analyzing the geologic record of lakes— through sediment coring and sub-bottom geophysics—can better inform the understanding of natural processes that affect the management of critical water infrastructure, supporting the safe and reliable delivery of water resources.

During 20^th-century settlement at Lake Como, Montana, local irrigators increased the storage capacity of the natural lake basin enclosed by terminal glacial moraines dating to approximately 15.4 ka.  A sub-bottom profile survey was conducted to identify the location of the Quaternary-active Bitterroot fault within the lake basin. The survey shows a thick package of post-glacial lake sedimentation preserved within the bounds of the original lake that is disrupted and displaced below a ubiquitous seismic reflector, which we hypothesize represents past shaking and faulting by one or two surface-rupturing earthquakes on the Bitterroot fault. To test our hypothesis, vibracores were collected from Lake Como with the goal of assessing earthquake history and surface rupture location within the lake. Several cores are greater than 4 meters in length and have the potential to provide detailed sedimentary records of the watershed’s post-glacial hydrologic and environmental history.

Preliminary observations of the longest cores reveal a basal grey clay/silt rhythmite unit, consistent with glacial origins, grading upward into an organic rich gyttja, with the exception of two distinct tephra layers. The tephras are provisionally identified as Glacier Peak G (13.6 ka) and the Mazama ash (7.6 ka) based on  stratigraphic position and regional context. Work is ongoing to correlate the tephra layers with the seismic reflection profiles using whole core magnetic susceptibility profiles and sediment compositional analyses.     

The nearly complete section of late Quaternary stratigraphy preserved at Lake Como can be used for many applications critical to the management and safety of regional water infrastructure. As the primary goal of this study, techniques in lacustrine paleoseismology can better inform ground shaking and fault displacement hazard assessments for dams. Additional benefits include the ability to assess long-term and post-wildfire sedimentation rates in reservoir basins, and changes in regional hydrometeorological conditions that affect the delivery of water to water users.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10652

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