Paludal Stratigraphy of Truk, Ponape, and Kusaie, Eastern Caroline Islands

摘要

Swampy coastal plains up to one-half a kilometer in width that fringe the high islands of the Eastern Carolines have been cited as evidence for recent emergence, but the stratigraphy and morphology of the tidal swamps are more consistent with a history of shoreline progradation during decelerating submergence. The mangrove, Nipa Phragmites, and taro swamps have accumulated no more than 3 m of peat, and generally only about 2 m. Beneath the peat or muck is either: (a) a level coral bench at approximate low-tide level, (b) coarse, well-sorted, shelly beach sand, sometimes in the form of low beach ridges across the shallowest marshes, or (c) brown, organic-rich mud with concentrations of immature pelecypod shells, deposited in former protected bays or estuaries. Estuarine mud was cored to a depth of 11.6 m in many swamps without finding bottom. Radiocarbon dates from intertidal peat layers that overlie former hillslopes of weathered volcanic rock demonstrate submergence of 6.2 m in the last 6500 years. The rate of submergence decreased abruptly during the final 1.7 m of submergence, which permitted extensive progradation by tidal swamps over former reef flats or into former muddy estuaries. By this interpretation, submergence has averaged only about 0.4 cm per 1000 years since 4100 years B.P., in contrast to the rate of 1.9 m per 1000 years between 4100 and 6500 years B.P. The high basaltic islands of Truk, Ponape, and Kusaie are conveniently spaced among the atolls of the Eastern Caroline Islands, across a total distance of nearly 1300 km. The uniformity of their paludal stratigraphy indicates a common late-Pleistocene and recent history for the entire region of a million sq mi or more, independent of the contradictory evidence about postglacial sea levels that has been reported from various atolls.