Masaaki Tateishi

Sediment facies and Late Holocene progradation of the Mekong River Delta in Bentre Province, southern Vietnam: an example of evolution from a tide-dominated to a tide- and wave-dominated delta

Abstract The Mekong River Delta, southern Vietnam, is a typical mixed tide and wave energy delta with a wide delta plain formed during the last 6 ka and is one of the largest deltas in the world. Three cores were taken from Bentre Province in the lower delta plain, with the objective to describe the sediment facies and to clarify the changes from tide-dominated to tide-wave-dominated delta of the Mekong River Delta during the Late Holocene. Three cores (BT1, BT2, and BT3) were obtained in 1997. Holocene sediments in BT1 and BT3, with thickness ranging from 10 to 20 m, were mainly composed of deltaic sediments unconformably overlying the Late Pleistocene sediments. The BT2 core, however, consisted of transgressive estuarine sediments covered by deltaic sediments infilling the paleo-Mekong River incised valley, which is more than 70 m deep and was formed during the last glacial period. The deltaic sediment facies and succession in the BT2 and BT3 cores taken from the outer delta plain, with beach ridges, were different from those of the BT1 core taken from the inland delta plain. The BT2 and BT3 cores provide a good example of tide- and wave-dominated delta sediments and were characterized by a coarsening-upward delta front facies covered by a fining-upward subtidal to intertidal flat facies, followed by a coarsening-upward foreshore/dune facies. These changes from the inland delta plain to the beach ridge delta plain were accompanied by changes in progradation rates and subaqueous delta topography caused by increased wave influence.

Tectonics and volcanism in the Sado-Pohang Belt from 20 to 14 Ma and opening of the Yamato Basin of the Japan Sea

Abstract Tertiary volcanism around the Japan Sea is closely related to the opening of the Japan Sea. Paleogene to early Middle Miocene formations in the Pohang-Yangnam area of the Korean Peninsula mainly consist of volcanic rocks that are stratigraphically and petrographically similar to similar age rocks on the Japan Sea side of the Japanese Islands, especially those of Sado Island. We propose that all these early Cenozoic rocks formed part of a single tectonic-volcanic belt called Sado-Pohang Belt. It is inferred that this belt was located on the south side of the Yamato Ridge in Paleogene time. After the Japan Basin opened, the Yamato Basin opened during Early Miocene to the early Middle Miocene time and part of the Sado-Pohang Belt migrated southward. The tectonics of the Sado-Pohang Belt is considered to support the hypothesis of the two-stage opening of the Japan Sea.

Geologic fault model based on the high-resolution seismic reflection profile and aftershock distribution associated with the 2004 Mid-Niigata Prefecture earthquake (M6.8), central Japan

The Mid-Niigata Prefecture earthquake in 2004 (MJMA 6.8) generated surface ruptures along the eastern rim of the Uonuma Hills. To elucidate the structural linkage between the surface ruptures and the source fault at depth, the high-resolution seismic reflection profile across the surface ruptures and nearby active faults, and the data of aftershock distribution are examined. The 5.2-km-long, high-resolution, depth-converted seismic section reveals an emergent thrust beneath the surface ruptures. A two-dimensional model of the fault geometry has been constructed based on the aftershock distribution and the shallow reflection profile. The development of the main geologic structure are well explained by forward modeling using a balanced cross-section method. In detail, the fault system generated the main shock dips at a steep angle (60°) below 5 km depth and more shallowly (30°) near the surface.