Abstract
Twenty species of calcareous nannofossils belonging to 11 genera are identified from the Seoguipo Formation in Cheju Island, Korea. On the basis of the marker species, the Seoguipo Formation is biostratigraphically assigned to the Pseudoemiliania lacunosa Zone (NN19), which corresponds to the combined zones of Emiliania annula-Emiliania ovata (CN13a-CN14a) of the latest Pliocene and Early Pleistocene. Generally, cold-water species are dominant in the lower part, and warm-water ones in the upper part. This suggests that the paleoceanographic condition of the study area changed from a cooling to a warm phase. The change in floral composition and abundance of specific species allows recognition of four ecostratigraphic zones in the Seoguipo Formation and the migration of an oceanographic frontal boundary. According to nannofossil distribution in the study area, the position of an oceanographic boundary between warmer water and cooler water appeared to have oscillated north-south over the Korea Strait and Cheju Island in response to glacial and interglacial cycles. The geologic time of the interpreted paleoceanographical changes determined by nannofossil biochronology agrees well with the results obtained from the Japan Sea (East Sea) and Japan-Sea side of Japan.
