(6)Structure and development of the Holocene sandy gravel spit in the Osaka Plain
Minao Sakurai (Doshisha Univ.), Fujio Masuda (Doshisha Univ.)
Structure and development of the Holocene sandy gravel spit (“Tenma spit”) deposit in the Osaka Plain were firstly clarified using by borehole database and previously reported 14C ages. The deposit within the middle marine clay part of the Holocene sediments was deposited 8000 to 5000 years ago during the transgressive and maximum flooding periods. The deposit is characterized by a long and narrow platform, a progradational upward-coarsening sediment sequence, and signs of emergence at its top. The deposit extends continuously from a lower offshore to an upper landward end. It includes depositional structures generated by storm-waves and marine fossil shells. The deposit is most extensively developed in the area of the paleo-wave cut cliffs and benches on the northwestern margin of the Uemachi Upland. These features strongly support an origin from coastal sand and sandy gravel spit systems. The Tenma spit developed in two stages, bounded by the rapid rise of sea level 8000 to 7000 years ago. This final developed spit was 7-8 km long and less than 100 m wide from Tenma through Nagara to Awaji in 6000 to 5000 years ago. After 5000 years ago during the highstand period, the spit evolved into a wide strand-plain system with long sand or sandy gravel beaches.