This study contributed baseline information regarding the framework geology, coastal change, and archaeological record along the fjord-indented coastlines of southcentral Alaska. Field data was collected in July 2015 from James Lagoon, a 30 m deep drowned cirque valley adjacent to McCarty Fjord in the Nuka Bay region of Kenai Fjords National Park. A 1790 cal CE terminal moraine deposited by McCarthy Glacier extended across the cirque opening and delimited the lagoon.
Acoustic surveys of the lagoon revealed bedrock overlain by deltaic, lacustrine, and marine facies containing beds that indicate past submarine landslides. Lagoon and marsh sediment cores contained coarse sand horizons interbedded with silt and peat, evidence of repeated low-frequency, high-intensity events. The largest of the event beds in the marine cores occurred near the surface and associates with the 1964 Great Alaska Earthquake (moment magnitude 9.2). Trees, killed during the 1964 earthquake, fringed the coastline and indicate ~2 m of coseismic subsidence.
Radiocarbon ages of five exposed subfossil stumps indicated two pre-1964 subsidence events occurring between 1710 to 1789, and 1078 to 1125 cal CE (median probability). A Sugpiaq culture archaeological site dated to 1565 cal CE existed on the western shore of James Lagoon in the interval between these two events, possibly for proximity to an ice flow harbor seal rookery at McCarty Glacier. Archaeological site elevation above mean high water at James Lagoon and around Nuka Bay inversely related to the extent of cumulative site erosion from coastal submergence during the last two great earthquakes. These combined findings laid the groundwork for future research and provided context to ongoing coastal hazards and their impacts to cultural resources.