Oligocene-Miocene sediments in the Taiwan region were accumulated in the NE passive-continental margin of the South China Sea. We utilized around ~200 boreholes and reflection seismic data to study the Oligo-Miocene sequence stratigraphic framework in the Taiwan region. Major sequence boundaries are used to map out various sediment isopach maps, enabling us to decipher dominant tectonic events during the course of passive-margin evolution.

The Oligocene-Miocene succession is divided into 16 sequences on the basis of well-log correlation. These 16 sequences can be grouped into four (A, B, C, D) sequence sets. Isopach maps of sequence sets B and C (~21–12.5 Ma) show that they blanket the west Taiwan basins with relatively uniform thickness deposited during uniform and slow basement subsidence. Sequence sets A (~30–21 Ma) and D (~12.5–6.5 Ma), however, thickened into fault-bounded troughs, recording two extensional events that were especially active in the outer margin (i.e., the Tainan Basin). There is a general correlation between the Taiwan Miocene sequences and the glacioeustasy during the interval ~21–6.5 Ma. Therefore, eustasy is the dominant control on the Taiwan Miocene stratigraphic development. The deposition of the sequence set A during ~30–21 Ma and possibly the sequence set D in the outer margin during the late Miocene, however, appears to have been strongly modulated by extensional tectonics and local sedimentary factors (e.g., rates of basin subsidence, sediment supply and basin physiography, etc.).