神奈川県三浦半島西岸の後氷期における古環境変遷 : ボーリングコア中の貝形虫化石群集とイベント堆積物

抄録

Four sediment cores were collected from Holocene deposits along drowned valleys in Zushi (Site MB 3) and Miura Cities (Sites MB 1, 2 and 5), the Miura Peninsula, Central Japan. One hundred and thirty-two ostracode species were found in 40 samples from three of these cores (Core MB 5 was not studied). Numerical analyses of ostracode fauna in the cores reveal six biotopes : BB (muddy bottom in inner brackish bay, about 2-5m in depth); SB (muddy middle bay, about 5-10m in depth); DB (muddy middle to outer bay, about 10-15m in depth); AS (rocky shore with calcareous algae and/or sandy foreshore with Zostera under the influence of open sea); SS (open sea with a sandy beach); AB (mixture of SB and AS). Vertical changes of depositional environments at the three sites are interpreted as follows : 1) ca. 9, 500 y. B. P. : Site MB 3 was inundated with tidal flats or salt marsh, but other sites were on land; 2) ca. 8, 400-9, 000 y. B. P. : Enclosed inner to shallow middle bay areas shallower than about 10m were developed at Sites MB 2 and MB 3; 3) ca. 7, 700-8, 400 y. B. P. : Enclosed middle bay areas ca. 10m deep were developed at Site MB 1 as a result of a rapid marine transgression, whereas enclosed middle to outer bay areas of 10-20m depth were present at Sites MB 2 and MB 3; 4) ca. 7, 700-6, 400 y. B. P. : As the transgression continued, open sea waters invaded Sites MB 2 and MB 3 but not Site MB 1; 5) ca. 6, 400-3, 000 y. B. P. : With a regression about 6, 400 y. B. P., water depth decreased at all sites. The depositional environment changed into sandy bay coasts at Site MB 1 and into rocky shore with calcareous algae and/or foreshore with Zostera under the influence of open sea at Sites MB 2 and MB 3. Site MB 1 has been terrestrial since about 5, 000 y. B. P. At least seven horizons of reworked deposits (T1 to T7) have been recognized in the study cores. The ostracode fauna in two of these horizons (T1 : ca. 8, 900 y. B. P. and T2 : ca. 7, 600-7, 800 y. B. P.) shows apparent mixtures of open sea and enclosed inner bay elements, but no distinctions have been made between horizons T3 to T7 and their surroundings.