対馬北端に分布する海老島石灰岩層の形成環境と年代の考察

概要

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We re-examined the age and depositional environment of the Ebishima Limestone in the northernmost Tsushima Islands, which had previously been interpreted as Pliocene shallow-marine deposits. Limestone that overlies Miocene siliciclastics consists mainly of well-sorted marine bioclasts and calcite cement. The occurrence of high-angle planar cross-stratification and fossil land snails indicates deposition in a coastal sand-dune environment. The cement was meteoric, as indicated by the low values of carbon and oxygen isotopes (-5.7 and -4.5 per mil PDB, respectively) and the absence of sulfides. Radiocarbon analyses of bulk inorganic carbon yield an age of 41,497±574 cal. BP., which is possibly influenced by the meteoric calcite cement. Therefore, the carbonate dunes were deposited prior to 40 ka. Considering the environment of subsidence at the Tsushima Islands, the coastal dunes represented by the Ebishima Limestone were probably deposited during a period of high sea level in the Middle-Late Pleistocene.