Holocene Paleoshoreline Record in Tonga: Geomorphic Features and Archaeological Implications

Abstract

Coordinated geomorphic and archaeological observations indicate that ancient (c. 3000 years old) archaeological sites in Tonga typically lie inland from present coasts on paleoshorelines associated with a regional mid-Holocene highstand of sea level. Shorelines in Tonga include both seacliffs, which dominate windward coasts, and coral sand beaches, many of which fringe accretionary sand flats on leeward coasts. Seacliffs are characteristically notched at high-tide level by solution and bioerosion. Emergent paleoshoreline notches of mid-Holocene and last-interglacial ages record higher local stands of relative sea level on many Tongan islands. Other indicators of local mid-Holocene sea levels include emergent microatolls, paleobeachrock exposures, beach-ridge berm crests, and fossil beach placers of black sand derived from tephra deposits. Paleoshoreline indicators on Tongatapu and 'Eua, and in the Nomuka and Hahake subgroups of Ha'apai, show that mid-Holocene sea level stood 2.0-2.6 m higher than present sea level, with tectonic changes 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 Holocene rates of c. 0.5 mm/yr, enough to counteract the post-mid-Holocene fall in local relative sea level. Elevations and locations of ancient archaeological sites are generally compatible with independent geomorphic evidence for stability or subsidence of individual islands. Parts of Tongatapu were evidently inundated in 1853 by the temporary runup of a local tsunami associated with an earthquake generated by volcanic activity along the nearby volcanic arc.