TERRACES AND PALEO SEA-LEVEL ESTIMATES AT MARINE ISOTOPE STAGE 3 IN THE LOWER ISUMI RIVER BASIN, BOSO PENINSULA, CENTRAL JAPAN
Marine terraces formed during the relatively low sea-level epoch after 125 ka can be found out on land in the lower Isumi River basin, Boso Peninsula, Central Japan, because uplift rate has been high during the Quaternary in this area. A geomorphologic and geologic investigation into the terraces of this area identified three terraces, named the Taitozaki, Nagasaka, and Yoshifu in descending order, as terraces formed in the estuary environment at the marine isotope stage 3 (MIS 3).
Four characteristics in particular indicate that the Yoshifu Terrace was formed in the estuary environment revealed by a transgression at about 30 ka: (1) the projected profile of the terrace surface along the lower reaches of the Isumi river is flat and horizontal; (2) part of the terrace deposit bury the drowned valley; (3) the terrace surface is covered by tephric soil deposit including Aira-Tn Tephra (24~25 ka), but not by Hakone-Tokyo Tephra (52 ka); and (4) the 14C ages of the terrace deposit are 31~34 ka. Moreover, the result of pollen analysis for the terrace deposit does not conflict with the environment that had been described about 30 ka in previous works.
Mean uplift rates calculated using the Shimosueyoshi Terrace (MIS 5e, 125 ka) and the Holocene Terrace (MIS 1, 6 ka) in this study area are about 2.0~2.2 m/ka, which are nearly equal to the rates at the Huon Peninsula, Papua New Guinea, where there are several coral terraces formed at MIS 3. Under the assumptions that uplift rate of 2.1 m/ka has been constant and that the Yoshifu, Nagasaka, and Taitozaki Terraces were emerged at 28~29 ka, 37~40 ka, and 44 ka, namely, the times at which the marine oxygen isotopic curve developed by Shackleton (1987) shows peaks of transgressions; their paleo sea-level estimates are -29~-31 m, -28~-34 m, and -22 m, respectively. These values do not agree with recent estimates deduced from raised coral terraces at the Huon Peninsula (e.g. Chappell et al., 1996).