堆積シーケンスからみた三保半島及び半島沖大陸棚の形成

抄録

The formation and depositional environments of the sedimentary sequences on the continental shelf off the Miho Peninsula, Central Japan, are studied in detail on the basis of the geomorphology, surface sediment distribution, and acoustic stratigraphy. Sequential analysis based on seismic reflection records defines an acoustic stratigraphy for the region. Acoustic units are A0, A1, A2 and B in descending order. Units A0, A1 and A2 are considered to be parasequences, and unit B acoustic basement. The boundaries of these units are carefully examined and found to be classified as interpeted to be marine flooding surface (mf), maximum flooding surface (mfs), and major erosional surface (sequence boundary; SB). Studies on internal sedimentary structure of each acoustic unit and sediment distribution pattern shown by the isopach maps clearly indicate major changes in sedimentary environment through these units. A series of isopach map indicates northward and also landward (westward) migrations of depositional centers of the shelf sediments. It is found that these changes in depositional environments are closely related with the sea-level fluctuations during last 18,000 years. Correlation between the acoustic stratigraphy on the shelf and boring cores located on the peninsula provides not only lithological confirmation and environmental constraints on the acoustic units but also time scale for the reconstruction of sedimentary / erosional history of the area. Sea-level analysis on the basis of the published sea-level fluctuation curves for the Tokai and Kanto districts shows intimate relation between acoustic stratigraphy and sea-level fluctuations. Reflectors L, K, J, I demarcate Units, B, A2, A1, A0 on the continental shelf off the Moho Peninsula. Reflector L is correlated with 18,000 yBP, Reflector K with 10,000-9,000 yBP, and Reflector I with 7,500 yBP. These correlations do not conflict with the dating from boring core data. Three stages are proposed in terms of formation of acoustic units and submarine spit platforms on the shelf. By the time of about 10,000-9,000 yBP., The unit B was formed and has been a widespread foundation of submarine spit platforms. About 7,500 yBP., the first and second submarine spit platforms were constructed by accumulation of the A1 and A2 units. After 6,000 yBP., the third submarine spit and the Miho Peninsula were formed by accumulation of the unit A0. Migrations or shifts of the acoustic units are found to be closely related with sea-level rise.