Pumice layers in marine terraces: Implications for tectonic uplift rates on the east and northeast coasts of Taiwan over the last hundreds of years

Two very rare pumice-rich layers were recently found at two sites in Taiwan, more precisely in the lowermost marine terrace of the Sanhsientai area on the southeast coast, and in that of the Yenliao area, on the northeast coast. The pumice-rich layer in the former is about 15 cm in thickness and is composed of 60-70% gray to dark gray, subangular to angular pumice fragments with barnacle shell fragments. The layer in the latter, on the other hand, is about 10 cm in thickness and is composed of 40-50% light gray to gray, subrounded to subangular pumice fragments with a few carbonized wood fragments. Despite their different occurrences, both pumice fragments very closely resemble each other in terms of major and trace as well as isotopic compositions and, therefore, may be considered to have come from the same source, the Luzon or Philippine arcs. ¹⁴C-dating gives the layers an equal age ranging from around 420 to 402 cal. yr BP. Based on the age and altitude of the marine terraces in the Sanhsientai and Yenliao areas, however, the uplift rates are 10.9 and 5.4 mm/yr, respectively. The results strongly indicate that, firstly, the uplift rates may have accelerated in the last thousands of years during the Holocene in the Sanhsientai-Chengkung area and that, secondly, the northern coastal area of Taiwan likely suffered from a spasmodic uplift in the very latest Holocene.