Holocene Terraces Along the Tsailiao River, Western Foothills, Southwest Taiwan: Lithofacies, Chronology, Fossil Assemblages, and Neotectonic Implications

Understanding the geohistory of fossil-bearing strata is essential for paleontological research. The Tsailiao area in the Tsochen District of Tainan City is one of the most important terrestrial fossil sites for Quaternary mammalian paleontology in Taiwan because it has yielded the most abundant and diverse mammalian fossils. However, the stratigraphy and depositional processes of the fossil-bearing terrace deposits in the area have not been well understood because previous research is insufficient for the precise distribution of the terraces and lithological and chronological descriptions of their representative deposits. Therefore, we studied terrace deposits, focusing on their distribution and the fossils they contain, from geomorphological, stratigraphic, chronological, and sedimentological perspectives. The terraces in the study area were grouped into three different heights, T1, T2, and T3, in descending order. Meandering rivers formed the T1, T2, and T3 terraces during the early, middle-late, and late Holocene, respectively. Each terrace deposit contained a basal gravel bed that yielded diverse fossils. However, most invertebrate fossils are inconsistent with a freshwater river environment and show signs of abrasion. Thus, they are considered derived fossils from underlying basement rocks. The mammalian fossils, including those of extinct species, were also found in the terrace deposits. Since the ages of the fossils do not correspond to those of the deposits, these fossils are likewise considered derived fossils. The results of our study contribute to the understanding of the terraces in the area. Our study of the terraces is expected to provide additional knowledge for geomorphological and tectonic studies in and around the area.