IMAGING SUBSURFACE COASTAL MORPHOLOGY USING GROUND-PENETRATING RADAR NEAR TANGINAK SPRING ARCHAEOLOGICAL SITE, SITKALIDAK ISLAND, ALASKA

The middle to late Holocene geomorphic history of the Tanginak Anchorage is important for placing the Tanginak Spring archaeological site in the context of its paleoenvironment. We employed ground-penetrating radar (GPR) near the shoreline of Tanginak Anchorage to map the subsurface morphology of the beach ridge complex. Holocene sands and peats are underlain at depths of several meters by glacial till and complexly deformed metasediments. Profiles both normal and parallel to the modnern and older beach ridges were run at antenna center frequencies of 100, 200, and 400 MHz to determine which best resolved the subsurface morphology. GPR signal depth penetration was best (~6m) in the region between the shoreline and the modern salt marsh. The structures observed in the GPR profiles are indicative of a prograding beach with no evidence of significant discontinuities such as transgressive scarps within the beach ridges. Lack of complexity in internal structure of the beach ridges suggests they prograded relatively rapidly. A rapid progradation scenario is supported by geologic evidence, including tephrachronology, and radiocarbon dating from excavations at the Tanginak Spring archeological site and the marsh/beach ridge complex.