Holocene paleoshoreline record in Tonga: Geomorphic features and archaeological implications

Coordinated geomorphic and archaeological observations indicate that ancien t (c. 3000 years old) archaeological sites in Tonga typically lie inland fr om present coasts on paleoshorelines associated with a regional mid-Holocen e highstand of sea level. Shorelines in Tonga include both seacliffs, which dominate windward coasts, and coral sand beaches, many of which fringe acc retionary sand flats on leeward coasts. Seacliffs are characteristically no tched at high-tide level by solution and bioerosion. Emergent paleoshorelin e notches of mid-Holocene and last-interglacial ages record higher local st ands of relative sea level on many Tongan islands. Other indicators of loca l mid-Holocene sea levels include emergent microatolls, paleobeachrock expo sures, beach-ridge berm crests, and fossil beach placers of black sand deri ved from tephra deposits. Paleoshoreline indicators on Tongatapu and 'Eua, and in the Nomuka and Hahake subgroups of Ha'apai, show that mid-Holocene s ea level stood 2.0-2.6 m higher than present sea level, with tectonic chang es in island elevations negligible since the last interglacial. By contrast , the Vava'u Group and the Kotu subgroup of Ha'apai have subsided at mean H olocene rates of c. 0.5 mm/yr, enough to counteract the post-mid-Holocene f all in local relative sea level. Elevations and locations of ancient archae ological sites are generally compatible with independent geomorphic evidenc e for stability or subsidence of individual islands. Parts of Tongatapu wer e evidently inundated in 1853 by the temporary runup of a local tsunami ass ociated with an earthquake generated by volcanic activity along the nearby volcanic are.