[HQR04-P03] Holocene depositional environment and crustal movement trend at the Omoto Plain, northern Sanriku coast, northeast Japan
Keywords:northern Sanriku coast, Holocene, vertical deformation trend, 2011 Tohoku-oki earthquake
Well-dated Holocene sediment showed environmental changes influenced by sea-level changes; upward growth and landward progradation of barrier/flood tidal delta from 10 to 9.0 ka BP, estuarine environment after decay of barrier/flood tidal delta from 9.0 to 8.0 ka BP, and regressive prodelta ~ delta front and subsequent fluvial environment after 8.0 ka BP. In the upstream site, delta plain sand and silt with upward-finning successions, at uppermost of which abundant tidal flat diatom species is identified, covers prodelta ~ delta front sediments and is dated to 7.4 to 7.2 ka BP.
Uppermost part of delta plain sediments is considered to be deposited under intertidal environment on the basis of abundance of tidal flat diatom species. Thus, height of this horizon (-7.6 m relative to the present sea-level) approximates paleo sea-level at 7.4 to 7.2 ka BP. This level is higher than paleo sea-level in the Tsugaruishi plain at the same timing (-11.5 m relative to the present sea-level; Niwa et al., 2017). In the Tsugaruishi plain, ca. 6 m thick of marsh sediments which covers mid-Holocene intertidal sediments, and aggradational sediment stack of middle to late Holocene delta system are interpreted to reflect millennium-scale relative sea-level rise induced by subsidence trend (Niwa et al., 2017). On the other hand, in the Omoto plain such sedimentary property shown in the Tsugaruishi plain is not observed. This suggests that little upward addition of accommodation space has been produced in the Omoto plain during middle to late Holocene. These features indicate that subsidence trend (, if it exists at all,) is not noticeable in the Omoto plain in contrast with the Tsugaruishi plain.
This difference corresponds to contrast between central to southern Sanriku with large coseisimic subsidence during the 2011 earthquake and northern Sanriku with small coseismic subsidence. Results of this study suggest that style of crustal movement is different within the Sanriku coast across vicinity of Miyako.
Ikeda et al. (2012) Journal of the Geological Society of Japan, 118, 294 – 312 (in Japanese with English abstract).
Niwa et al. (2017) Quaternary international, 456, 1 – 16.