SEDIMENT SORTING AND DEPOSITION IN A LANDSLIDE GENERATED TSUNAMI, 1967 GREWINGK, ALASKA

Rare and disastrous flows of water such as tsunamis, flash floods, glacial lake outburst floods, and storm surges can leave behind a sedimentary record. Interpreting these deposits is challenging because the dynamics of such flows are typically poorly known; they are nonsteady, non-uniform, and multiphase, and their large scale makes easy experimental reproduction impractical. We present analysis of deposits from the 1967 Grewingk Lake tsunami, where a landslide falling into Grewingk Lake and adjacent Grewingk Glacier initiated a tsunami that overtopped the glacial outwash plain between the lake and ocean. Image analyses comparing airphotos from 1964 with those in 1967, 1969, 1970 and 1979 combined with a field survey in 1988 by Wiles and Calkin showed removal of vegetation on the outwash plain, over an area 2 km wide extending 6 km to Kachemak Bay, water-transported ice blocks and forest debris, and evidence of water scouring.

Field excavations in 2021 and 2022 in the Grewingk outwash plain revealed thick (1-2 m) deposits of silt to boulders, with recurring systematic grainsize and sorting characteristics. Although initially difficult to distinguish from outwash plain deposits, we identified the tsunami deposits’ lower contact based on the presence of a sharp soil contact at sites vegetated in the 1964 airphoto. Distinctive patterns of deposition across excavations include simultaneous coarsening of the coarse fraction and fining of the fine fraction—deposits are mixed sand and pebbles at the base, grading into a relatively well-sorted cobble-boulder gravel with interstitial sand and silt. Broken vegetation (branches, sticks, etc) were preferentially found near the base. Most locations were capped by well-sorted open-framework boulders, often imbricated. When compared to similar deposits from the 2015 Taan landslide-generated tsunami, these recurring systematic characteristics provide new insight into sedimentary processes in these violent flows, potentially helping interpret a wide range of tsunami deposits.