Event Title

Potential Triggering Mechanisms of Long-Runout Catastrophic Rock Avalanches in the Nooksack River Basin, Whatcom County, Washington

Research Mentor(s)

Clark, Doug

Description

The Cascade foothills east of Bellingham, WA in Central Whatcom County (Fig. 1) preserve a remarkable concentration of large rock avalanche deposits. These deposits present an excellent opportunity to test ideas related to potential triggering mechanisms (i.e., the causes) of such landslides. Though widely dated, common triggering mechanisms include ground accelerations during earthquakes, increased pore-water pressure from heavy precipitation events, or recent glacial or fluvial debuttressing of hillslopes. I examine each of these potential triggering mechanisms by determining landslide collapse ages through two independent dating methods: radiocarbon dating (14C) and cosmogenic radionuclide exposure dating (CRN). Collapse ages of these deposits are compared to known local or regional paleoseismic events (e.g., Cascadia Subduction Zone or Boulder Creek Fault ruptures; Fig. 2). For 14C dating, organic macrofossils (e.g. twigs, logs, seeds, etc.) were collected from sediment cores (Fig. 3) of bogs (e.g., Fig. 4 – C, E, D, H, I) formed on top of landslide debris fields. For CRN dating, rock samples were collected from the tops of large (> 3 m) boulders in boulder fields of each deposit (e.g., Fig. 4 - A, F, J) and will be analyzed at the University of Vermont - Community Cosmogenic Facility. If these rock avalanches were triggered by earthquakes, their ages should overlap with known paleoseismic events (Fig. 2). Conversely, other triggers should show different temporal distributions: glacial debuttressing should cluster immediately after deglaciation (Fig. 2), whereas precipitation triggering or fluvial debuttressing should display a more stochastic temporal pacing. To date (May 2020) we have collected 6 sediment cores, recovered 11 14C samples, and 9 CRN boulder samples. Unfortunately, the COVID-19 situation has delayed results for this study. We are awaiting the reopening of 14C-AMS facilities before more direct age control can be established.

Document Type

Event

Start Date

May 2020

End Date

May 2020

Department

Geology

Genre/Form

student projects, posters

Type

Image

Rights

Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.

Language

English

Format

application/pdf

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May 18th, 9:00 AM May 22nd, 5:00 PM

Potential Triggering Mechanisms of Long-Runout Catastrophic Rock Avalanches in the Nooksack River Basin, Whatcom County, Washington

The Cascade foothills east of Bellingham, WA in Central Whatcom County (Fig. 1) preserve a remarkable concentration of large rock avalanche deposits. These deposits present an excellent opportunity to test ideas related to potential triggering mechanisms (i.e., the causes) of such landslides. Though widely dated, common triggering mechanisms include ground accelerations during earthquakes, increased pore-water pressure from heavy precipitation events, or recent glacial or fluvial debuttressing of hillslopes. I examine each of these potential triggering mechanisms by determining landslide collapse ages through two independent dating methods: radiocarbon dating (14C) and cosmogenic radionuclide exposure dating (CRN). Collapse ages of these deposits are compared to known local or regional paleoseismic events (e.g., Cascadia Subduction Zone or Boulder Creek Fault ruptures; Fig. 2). For 14C dating, organic macrofossils (e.g. twigs, logs, seeds, etc.) were collected from sediment cores (Fig. 3) of bogs (e.g., Fig. 4 – C, E, D, H, I) formed on top of landslide debris fields. For CRN dating, rock samples were collected from the tops of large (> 3 m) boulders in boulder fields of each deposit (e.g., Fig. 4 - A, F, J) and will be analyzed at the University of Vermont - Community Cosmogenic Facility. If these rock avalanches were triggered by earthquakes, their ages should overlap with known paleoseismic events (Fig. 2). Conversely, other triggers should show different temporal distributions: glacial debuttressing should cluster immediately after deglaciation (Fig. 2), whereas precipitation triggering or fluvial debuttressing should display a more stochastic temporal pacing. To date (May 2020) we have collected 6 sediment cores, recovered 11 14C samples, and 9 CRN boulder samples. Unfortunately, the COVID-19 situation has delayed results for this study. We are awaiting the reopening of 14C-AMS facilities before more direct age control can be established.