FIELD RECONNAISSANCE OF GEOLOGY AND ROCK MASS QUALITY ASSOCIATED WITH LANDSLIDES AT BARRY ARM FJORD, PRINCE WILLIAM SOUND, ALASKA

Multiple bedrock landslides at Barry Arm fjord, Alaska pose a threat to the public because of their potential to generate ocean waves that could impact towns and marine activities in Prince William Sound. The largest landslide on the west side of Barry Arm drew international attention in 2020 following a study describing its movement in response to retreat of the tidewater Barry Glacier. In June 2021, we evaluated geology and rock mass quality in Barry Arm. On both sides of the fjord, we observed metamorphosed argillites and siltstones of the Cretaceous Chugach flysch. Structurally, we saw a prominent, near vertical fracture set, with strikes of N10°W to N30°W that were subparallel with the ~N30°E trend of the fjord, and a shallow dipping (20-30°SE dip) fracture set oriented approximately normal to flysch bedding. We saw differences in bedding orientation and rock mass quality on the east and west sides of the fjord. On the east side, bedding strike ranged from N50°E to N70°E, with dips of 55 to 80°NW, and rock mass quality was fair-to-poor (GSI=35-45, Q=0.12-1.1, and RQD=20-43%). On the west side, flysch bedding was more deformed, either from tectonic stress, soft-sediment deformation, and/or landslide movement. Bedding at rock outcrops adjacent to the largest landslide had strikes from N10°E to N30°E and dips from 55 to 65°NW, with localized tectonic folding or convolute bedding. Exposed bedding at the subaerial toe of the largest landslide had strikes and dips of N40°E, 20°NW and N55°E, 45°SE, indicating probable rotation and tilting of the outcrops by landslide movement. Rock mass quality on the west side was wide ranging due to the presence of both thin and massive beds (GSI=27-40, Q=0.02-1.33, and RQD=10-80%), but was generally lower than the east side due to larger numbers of open discontinuities (bedding, fractures, veins) with smooth surfaces. These observations suggest that: the degree of rock deformation and bedrock landslide susceptibility are positively correlated, landslides on the west side of Barry Arm may be sliding on low angle fracture surfaces dipping to the SE, and the basal-slip surface of the largest landslide likely daylights underwater. Additional work in Prince William Sound during July 2021 will expand the size of the study area and place Barry Arm into a regional bedrock landslide susceptibility context.