Holocene Environmental Changes in the Choshi Peninsula and its Surroundings, Easternmost Kanto, Central Japan
Holocene Environmental Changes in the Choshi Peninsula and its Surroundings, Easternmost Kanto, Central Japan
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日本中部关东最东部铫子半岛及其周边地区全新世环境变化
DOI：
10.4116/jaqua.24.13
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发表时间：
1985
期刊：
影响因子：
--
通讯作者：
H. Moriwaki
中科院分区：
文献类型：
--
作者：
Y. Ota;Y. Matsushima;M. Miyoshi;K. Kashima;Y. Maeda;H. Moriwaki
长实半岛位于关东构造盆地的东部边缘，除爱谷山这一古老的小型基岩山外，其余均由一系列晚更新世海相阶地组成。在半岛东端的Inubo角附近，最后的间冰期阶地高度约为60m，平均抬升速率估计为0.4m/ 1000年。全新世阶地发育在半岛北侧东流的Tone河河谷和长实半岛的Takagami低地（图1）。在这些全新世阶地中进行了挖掘，在相观测、14C测年和生物组合分析的基础上，采集了系统的样品，重建了环境变化。在Tone河沿岸（图2），全新世阶地发育，海相沉积上限为4.0 ~ 5.0m amsl。据估计，这个表面出现的时间早于大约3500年前。至少从绳纹时代晚期的考古遗址来看是这样的。基于开挖和钻孔的地层剖面如图3所示。海拔9-10米的Takagami低地（图4）被厚达5米的陆生泥炭层覆盖。通过相相观察和硅藻分析确定，海相沉积的上限在低地东部边缘为4.8m，西北部略低（图5和图6）。与地表地形的单向倾斜（图4）相比，全新世海相沉积下的基底地形包括两个方向相反的隐埋山谷（图5），在Loc 10附近发现了一个以前的分水岭。向东倾斜的隐谷被称为“古高上湾”。向西倾斜的山谷充满了沙质沉积物，而古高上湾则充满了沉积在浅淹没山谷海湾头的细海相淤泥或粘土。古高上湾的14C年代为8380年和10410年。基底海相沉积为6、520、6、960和5 220 y.b.p.。还有6个时间点来自于上覆泥炭沉积物的底部，时间范围从6600年到3580年。（图5、表1）。上述海相沉积的海侵与冰后（绳纹）海侵有关。古高上湾的出现被认为是在泥炭最低部分沉积之前立即发生的，即大约在5000 - 5500年前。通过对Loc. 2最长岩心的软体动物、介形虫、硅藻和花粉的组合分析，可以重建全新世古高上湾的古生态（图7）。以软体动物组合为例，在岩心下部发现了潮间带的咸淡水或浅海组合。在岩心的中间部分，主要是由几米深的泥质底部常见的物种组成的组合。岩心上部包括岩心底部的浅海组合特征，以及其他具有浅海砂底潮间带环境特征的组合。由软体动物组合推断出的水深和环境变化与硅藻、介形虫和花粉的分析结果一致。不同指标得到的海相沉积上限（图7）具有较好的一致性，相观察为3.3m，软体动物组合为2.3m，介形虫为1.8m，硅藻为3.3m。古地理图如图8所示，描绘了古地理演化的三个阶段。第一阶段为绳纹海侵早期。
The Choshi Peninsula, located at the eastern margin of the Kanto tectonic basin, is composed of a series of late Pleistocene marine terraces except for a small older bedrock hill, Mt. Atago. Near Cape Inubo at the eastern tip of the peninsula, the last interglacial terrace is about 60m high, and the average rate of uplift is estimated to be 0.4m/1, 000 years. Holocene terraces are developed in the valley of the Tone River flowing eastward on the north side of the peninsula, and in the Takagami lowland in the Choshi Peninsula (Fig. 1).Excavations were carried out in these Holocene terraces in order to take systematic samples for reconstruction of environmental changes based on facies observation, 14C dates, and analysis of biological assemblages. Along the Tone River (Fig. 2), the Holocene terrace is well developed, and its upper limit of marine deposits ranges from 4.0 to 5.0m amsl. It is estimated that this surface emerged earlier than ca. 3, 500y.B.P. at the latest, judging from the presence of an archeological site of the late Jomon Period. Stratigraphic sections based upon excavation and bore hole are shown in Fig. 3.Takagami lowland, at an altitude of 9-10m amsl (Fig. 4), is overlain by a terrestrial peat bed up to 5m in thickness. The upper limit of the marine deposits, identified by facies observation and diatom analysis, is 4.8m amsl at the eastern margin of the lowland and is slightly lower northwestward (Figs. 5 and 6). In contrast with the unidirectional slope of the surface topography (Fig. 4), the basal topography beneath the Holocene marine deposits includes two buried valleys sloping in opposite directions (Fig. 5), and a former divide is found near Loc. 10. The eastward-sloping buried valley is called “Paleo Takagami Bay”. While the westward-sloping valley is filled with sandy deposits, Paleo Takagami Bay is filled with fine marine silt or clay deposited in a bay-head of a shallow drowned valley. 14C dates from Paleo Takagami Bay are 8, 380 and 10, 410y.B.P. for the basal marine deposits, 6, 520, 6, 960 and 5, 220y.B.P. for an analyzed marine clay, and six dates from the base of overlying peat deposit ranging from 6, 600 to 3, 580y.B.P. (Fig. 5 and Table 1). The transgression resulting in deposition of the marine sediments mentioned above is correlated with the post-glacial (Jomon) transgression. The emergence of Paleo Takagami Bay is considered to have taken place immediately prior to the deposition of the lowest part of the peat, that is, ca. 5, 000-5, 500y.B.P.Analysis of assemblages of molluscs, ostracods, diatoms and pollen from the longest core of Loc. 2 permits the reconstruction of the paleoecology of Paleo Takagami Bay during the Holocene (Fig. 7). Using molluscan assemblage as an example, a brackish water or shallow-marine assemblage from the intertidal zone is found in the lower part of the core. In the middle part of the core, an assemblage consisting of species common to a muddy bottom deeper than several meters predominates. The upper part of the core includes the shallow-marine assemblage characteristic of the base of the core as well as other assemblages characteristic of a shallow, sandy-bottom intertidal environment. Changes in water depth and environment deduced from the molluscan assemblages are concordant with those estimated from the analysis of diatoms, ostracods, and pollen. The upper limit of marine deposits obtained by different indices (Fig. 7) shows a good agreement, that is 3.3m amsl by facies observation, 2.3m by molluscan assemblages, 1.8m by ostracods, and 3.3m by diatoms.Three stages in the paleogeographic evolution of the Choshi Peninsula are depicted in paleogeographic maps in Fig. 8. Stage I is the early stage of the Jomon transgression.