PRELIMINARY GEOMORPHIC ASSESSMENT OF SUBMARINE AND SUBAERIAL-TO-SUBMARINE LANDSLIDES IN BARRY ARM FJORD, ALASKA

Knowledge of submarine landslides is critical for understanding the history of slope failures and related tsunami impacts in rapidly deglaciating fjord environments. The recent discovery of the ~500 million m³ slow moving, subaerial Barry Arm Landslide in northwest Prince William Sound, Alaska, highlights the need to better document volumes and mobilities of landslides in fjords to accurately model potential landslide-generated tsunamis. Here, we present a preliminary geomorphic assessment of existing submarine and subaerial-to-submarine landslides in Barry Arm, using a combination of variable-resolution Digital Elevation Models (DEMs) from multibeam sonar bathymetry (~4 m), IFSAR (5 m), and LiDAR (1 m). To identify evidence of Holocene landslides, we mapped scarps and deposits using a Red Relief Image Map (RRIM) of the available elevation data. We found evidence of four key submarine geomorphic features related to landslides and estimated landslide volumes and mobilities: 1) submarine scarps and deposits from landslides initiated in moraines and other glacial sediments (volumes of ~ 1x10⁴ to 1x10⁶ m³); 2) submarine deposits from subaerially initiated bedrock landslides along fjord margins (volumes of ~1x10⁶ m³); 3) landslides in active, prograding fan deltas along fjord margins (volumes of ~1x10⁴ m³); and 4) substantial (>25%) coverage of the seafloor with often-coalesced turbidite lobes that initiated from river outlets, submarine canyons within moraines, and low-angle post-glacial sedimentary features along the seafloor. Exposure of submarine terrain through Holocene glacial retreat splits the field area into three distinct zones separated by terminal moraines with ages of ~10, 100, and 2,500 years. Progressing from the youngest to the oldest zone, we observed an increase in submarine sediment volumes and numbers of landslides in sediment-mantled slopes. In line with this observation, much of Barry Arm is dominated by shallow submarine sediment failures with high mobilities (height/length (H/L) of ~0.01 – 0.1, in this study). However, we only observed deposits from bedrock landslides in the two youngest zones. As Barry Glacier continues to recede, our work suggests that bedrock landslides with lower mobility (H/L of ~0.5 – 1, in this study) will dominate in newly exposed terrain.