Characteristics of the late Quaternary tephra layers in the East/Japan Sea and their new occurrences in western Ulleung Basin sediments

The lithologic characteristics, stratigraphic relationships, and areal distribution of late Quaternary tephra layers have been determined based on four piston cores, recovered from the western Ulleung Basin of East/Japan Sea. The results show that, using morphological and major element compositional data, volcanic glass shards dispersed in tephra layers are identical to those of fallout deposits of the Ulleung–Oki (ca. 9.3 ka), Aira–Tanzawa (ca. 22 ka), and Ulleung–Yamato (ca. 33 ka). The lapilli tephra layers (Ulleung–Oki and Ulleung–Yamato) originating from Ulleung Island consist predominantly of pumice-type glass shards associated with minor amounts of alkali-feldspar, biotite, and plagioclase. On the other hand, the ash layers (including the Aira–Tanzawa ash) derived from the Japanese islands are mainly composed of bubble-wall and/or plane-type glasses that contain higher SiO2 and lower Al2O3 than the lapilli tephra layers. The occurrence of tephra layers in the western Ulleung Basin and their stratigraphic correlation extend the distribution of pumice-type glass shards about 50–100 km west of the previously known fallout zone.