Evolution of Holocene Coastal Landforms in the Miyazaki Plain, Southern Japan
Evolution of Holocene Coastal Landforms in the Miyazaki Plain, Southern Japan
复制标题
日本南部宫崎平原全新世海岸地貌的演化
DOI：
10.4116/jaqua.30.59
复制
发表时间：
1991
期刊：
影响因子：
--
通讯作者：
Y. Matsushima
中科院分区：
文献类型：
--
作者：
S. Nagaoka;H. Maemoku;Y. Matsushima
日本九州南部的宫崎平原具有发育良好的全新世阶地。根据梯田的分布和海拔高度，将其分为四组：下田岛I阶地、下田岛II阶地、下田岛III阶地和下田岛IV阶地。本文根据野外露头和取芯样品观察、~（14）C测年、考古遗迹、以及软体动物的古生物学分析。结果表明：（1）5000 y. B. P.以前，海平面上升迅速，至少9000 y. B. P.以来，达到+4 ~+5m（a.s.l.）。约6300 y. B. P.海侵期间，从Kikai破火山口喷发的Akahoya火山灰（K-Ah）下方沉积了厚的桑迪或粉砂质沉积物。沉积物中的Tegillarca等浅海软体动物表明，宫崎平原曾广泛形成潮滩和海湾。Akahoya火山灰被2- 3 m厚的桑迪海洋沉积物覆盖，这可能表明火山灰降落后海平面略有上升。从埋藏阶地的高度和桑迪海相沉积的上限来看，最大海侵时，石崎川周围的海平面上升到现今海平面以上7.0-7.5m。Akahoya火山灰上的桑迪沉积物包括许多软体动物，如Tellinemactra edentura，这是生活在公海下桑迪底部的典型热带物种之一。2）5000 - 3000 y. B. P.海退开始于5000 y. B. P.，海岸线后退时，下田岛I表面出现了许多桑迪脊。桑迪后面的泻湖逐渐变成了沼泽。海平面下降到5- 4米（a.s.l.）后，3）3000 y. B. P.后，随着海退的开始，下田岛II面出现了许多桑迪，海退后海平面相对稳定，下田岛III面形成。海平面在1600年前再次下降，并与现在的海平面相邻，形成了下田岛IV面。下田岛I和II阶地的海拔表明，在全新世期间，宫崎平原已经隆起，向北倾斜。由于历史上没有发生过引起宫崎平原出现的地震，也没有发现能抬升宫崎平原的海底活动断裂，因此，宫崎平原的隆升可能是由于地壳运动引起的。南极洲的抬升可能与正在向南九州俯冲的九州-帕劳海岭的浮力有关。
The Miyazaki Plain in southern Kyushu, Japan, is characterized with well-developed Holocene terraces. On the basis of the distribution and altitude of the terraces, they are subdivided into four groups: Shimotajima I, Shimotajima II, Shimotajima III, and Shimotajima IV terraces.This paper discusses the evolution of terraces, sea level change, associated environmental change and tectonic movements during the Holocene in the study area, based upon the field observation of outcrops and cored samples, 14C datings, archaeological remains, and paleontological analysis of mollusks. The results are as follows:1) Before 5, 000y.B.P.The sea level had risen rapidly since at least 9, 000y.B.P., and reached +4 to +5m (a.s.l.). Thick sandy or silty sediments beneath the Akahoya ash(K-Ah) which erupted from the Kikai caldera about 6, 300y.B.P. were deposited during the transgression. The shallow marine mollusks such as Tegillarca in the deposits indicate that tidal flats and bays were widely formed in the Miyazaki Plain. The Akahoya ash is covered with sandy marine sediments 2-3m in thickness, which might indicate that the sea level rose slightly after the ash fall. At the maximum transgression, the sea level rose to 7.0-7.5m above the present sea level around the Ishizaki River, judging from the height of the buried benches and the upper limits of the sandy marine deposits. The sandy deposits on the Akahoya ash include many mollusks such as Tellinemactra edentura, one of the typical tropical species which lived on the sandy bottom beneath the open sea. Their presence suggests that the bays were buried by sandy material which was carried by the river and the long-shore current and changed into the open sea with a sandy beach.2) 5, 000-3, 000y.B.P.The regression started about 5, 000y.B.P. While the shoreline retreated, many sandy ridges emerged on the Shimotajima I surface. Lagoons behind the sandy ridges gradually changed into swamps. After the sea level lowered to 5-4m (a.s.l.), it became stable or rose slightly, and the Shimotajima II surface was formed.3) After 3, 000y.B.P.Many sandy ridges on the Shimotajima II surfaces emerged as the regression began about 3, 000y.B.P. After the regression, the sea level became relatively stable, and the Shimotajima III surfaces were formed. The sea level was declining again around 1, 600y.B.P., and became adjacent to the present sea level, forming the Shimotajima IV surfaces.The altitudes of the Shimotajima I and II terraces show that the Miyazaki Plain has been uplifted, tilting northward during the Holocene. The uplift might be due to aseismic crustal movement, because there have been no historical earthquakes which caused emergence of the Miyazaki plain, and we can find no submarine active faults which could uplift the Miyazaki plain. The aseismic uplift would be related to the buoyancy of the Kyushu-Palau Ridge which is subducting under southern Kyushu.