The geological age and paleoenvironment of the lower Seogwipo Formation, Cheju Island, Korea

Abstract

The marine deposits of the lower Seogwipo Formation, Cheju Island are very important materials to reconstruct the paleoenvironment and the evolution of the island. Analysis of planktonic foraminifera from fifteen samples of the lower Seogwipo Formation indicates that its geological age is Middle Pleistocene (late N22 Zone, i.e., Globigerina calida calida Subzone, and older than 400 Ka) based on the occurrences of Globigerina calida calida, Beella digitata, Globigerina bermudezi, and Globoquadrina conglomerate in the section. The ratio of planktonic to total foraminiferal fauna decreases from the lower to the upper parts in the section. At the same time, warm water species Globigerinoides ruber and G. trilobus also decrease while the temperate water species Neogloboquadrina dutertrei, Globigerina quinqueloba increase. Quantitative analyses of both the benthic and planktonic foraminifera disclosed the changes of paleoenvironment: A warm middle shelf (or offshore) water might be dominant during the deposition of the lower part and then the paleoenvironment changed to a shallower inner shelf (or nearshore) with a temperate water in the upper part of the section. With the paleoenvironment varying from the offshore to the nearshore, the lower Seogwipo Formation recorded a sedimentary history of regression (including three sub-regressions and two sub-transgressions).