A Stratigraphic and Microfossil Record of Coseismic Land-level Changes and Tsunami Deposits from Old Harbor, Central Kodiak Island, Alaska

• Main Author: Janigian, Greta
• Format: Text
• Language: English
• Published: DigitalCommons@URI 2018
• Subjects: Geology; Kodiak; Alaska
• Online Access: https://digitalcommons.uri.edu/dissertations/AAI10840823

To reconstruct the paleoseismic history of Old Harbor on Kodiak Island, Alaska, we undertook exploratory coring at two coastal sites, Big Creek and Bear Terrace, 4 km and 2 km northeast of Old Harbor, respectively. We chose the longest core from Big Creek for analysis (90 cm, BC.15.02). Six sand to sandy-silt layers deposited within organic silts and peats occur in this core. Radiocarbon dating, a tephra deposit, and radiometric marker (137Cs) analyses were used to estimate ages of sand and silt deposition. 137 Cs results confirmed that the uppermost clastic deposit records the AD 1964 Great Alaskan earthquake and tsunami, the most recent large tsunami to inundate Old Harbor. This clastic layer lies 3 cm above a layer of pumice from the AD 1912 eruption of Mount Katmai, which is located ∼150 km northwest of Old Harbor. We were able to use the characteristics of the AD 1964 tsunami deposit (thick, coarse-grained, normally graded sequences, increase in marine and epipsammic diatoms) as a guide for identifying tsunami deposits in the rest of the core. Lithologic, diatom, grain-size, and statistical analyses pointed out characteristics specific to tsunami deposits, helping us to differentiate between the five deeper clastic deposits in the core. We identified the bottommost clastic deposit as a tsunami deposit from the AD 1788 earthquake. Both the AD 1964 and AD 1788 deposits in our core contained the characteristic features of tsunami deposits, but there was little indication of land level change associated with either deposit, which contrasts with observations and previous studies. Detrended correspondence analysis (DCA) on our diatom assemblages identified the difference between the local tsunami deposits and clastic deposits from other depositional mechanisms. Therefore, we inferred that the other four clastic deposits were most likely deposited by floods, storms, or tele-seismic tsunamis.^