COASTAL SUBMERGENCE AT UKISHIMA-GA-HARA ADJACENT TO THE SURUGA TROUGH (EASTERN NANKAI TROUGH), CENTRAL JAPAN

Five episodic submergence events during the past 3500 years were recognized at Ukishima-ga-hara lowland, northern coast of Suruga Trough (eastern Nankai Trough). Coastal submergence in this region were originally reported by Osamu Fujiwara and his colleagues (e.g., Fujiwara et al., 2016 in Quat Int) based mainly on changes in lithostratigraphy from peaty layer to overlying light-colored mud and results of microfossil analyses (pollens and diatoms for 57 and 13 samples, respectively). They attributed the submergence events to coseismic deformation associated with earthquakes in Fujikawa-kako fault zone or Suruga Trough. Here I reevaluated micropaleontology of this area using ~200 samples from 15 cores to reconstruct full history of coastal submergence during the last few thousand years. The submergence events were shown not by lithostratigraphy but clearly by changes in fossil diatom assemblages. For example, at about 2.3 m and 3.1 m below the ground surface, while aerophilic diatoms (such as Diadesmis contenta and Diploneis elliptica) dominate underlying peaty layer, freshwater and brackish planktonic taxa (Aulacoseira and Thalassiosira), that like standing water environments, abound in overlying layer. As many as five such changes in diatom assemblages were found in a 8-m core and radiocarbon ages showed ~3500 yr BP at the bottom.