TESTING POTENTIAL TRIGGERING MECHANISMS OF LONG-RUNOUT CATASTROPHIC ROCK AVALANCHE DEPOSITS IN THE NOOKSACK RIVER BASIN, WHATCOM COUNTY, WASHINGTON

Large earthquakes are commonly invoked as potential triggering mechanisms for long-runout catastrophic rock avalanches. We attempted to test this idea by characterizing the morphology and emplacement timings of deposits from four such landslides in the Nooksack River basin, NW Washington: Racehorse Creek (20 x 10⁶ m³), Middle Fork (39 x 10⁶ m³), Maple Falls (15 x 10⁶ m³), and Van Zandt (75 x 10⁶ m³; Malick, 2018). We use two independent dating methods: radiocarbon (¹⁴C) and ¹⁰Be cosmogenic radionuclide exposure (CRN) dating. For ¹⁴C dating, we sampled and dated small twigs embedded in the basal-most layers of sediment cores collected from several bogs formed on the surfaces of the debris fields. For CRN dating, we collected 3-5 rock samples from large, stable boulders occurring on each deposit (n=16).

Unsurprisingly, our results are complex. Our best estimates for emplacement of the main lobes at each site are: Racehorse Creek (~4.5-3.4 kyr BP), Middle Fork (~10.2-9.7 kyr BP and ~4.1-3.2 kyr BP), Maple Falls (~3.8-3.0 kyr BP), and Van Zandt (1330-1285 cal. yr BP; Malick, 2018). Our most well-constrained deposit, Zan Vandt, overlaps with CSZ event T4 (Goldfinger et al., 2012) and provides some evidence for a seismic trigger. However, the timings for the remaining deposits are not as well-constrained and cannot be conclusively correlated with a specific seismic event. However, the timings for we found for both Maple Falls and the younger Middle Fork failure both overlap with the timing of the nearby Boulder Creek (earthquake B; 3190-2980 cal. yr BP; Sherrod et al., 2013).

The complexity of the CRN ages in particular further indicates that: (1) each deposit (except Racehorse Creek) is likely composed of multiple overlapping debris lobes, suggesting that each landslide is prone to repeat failures; and (2) issues with CRN dating (i.e., cosmogenic inheritance) may imply that runout dynamics are more laminar than previously anticipated.

Although our age constraints for the emplacements of some deposits overlap with paleoseismic records from both the Cascadia Subduction Zone (Goldfinger et al., 2012) and the Boulder Creek Fault (Sherrod et al., 2013), the complexities in our CRN dates precludes a more definitive result. However, our CRN results are intriguing and underscore the need for further study of this method for such deposits.