Analytical study of turbidites, Otadai Formation, Boso Peninsula, Japan
Analytical study of turbidites, Otadai Formation, Boso Peninsula, Japan
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日本房总半岛 Otadai 组浊积岩的分析研究
DOI：
10.1111/j.1365-3091.1977.tb01914.x
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发表时间：
1977
期刊：
影响因子：
3.5
通讯作者：
Terumasa Nakajima
中科院分区：
文献类型：
--
作者：
Jiro Hirayama;Terumasa Nakajima
在东京东南方的房总半岛上，借助凝灰岩标志层，已追踪了约38 km的个别浊积岩层。
砂岩的厚度通常比互层泥岩的厚度变化大得多，呈透镜状，横向连续性大，可达30 km或更大。但后者比前者有更大的范围，形成广泛的席与轻微的上坡增厚。大多数砂岩表现出垂直和横向的分级，而泥岩则在上坡逐渐变粗。砂岩具有各种典型的浊积岩沉积结构，而泥岩几乎没有浊积岩。砂岩中含有强烈磨损的浅水动物群，而泥岩中含有保存完好的深水动物群。因此，泥岩大多为半远洋成因，浊流泥岩（如果存在）的数量可以忽略不计。
一般而言，上游侧的单个砂岩层厚度变化要比下游侧大得多。这可能是因为在上坡一侧存在一些小的渠道和相邻的堤坝状建筑物。在较厚的砂岩层底部，通常会发生局部的准同生侵蚀，形成一些较小的河道，两侧有小的堤坝状沉积物。最大河道的下坡侧通常两侧是最大的路堤，与每个砂岩层的最厚部分一致。小河道和堤状沉积物中的砂岩含有大量从下卧地层中剥离出来的泥块，随着距离河道的增加，泥块的大小和数量减少，圆度提高。一般来说，较厚的砂岩在最厚的部分周围显示Bouma abcde序列。在最厚部分的下游侧优先发展Bouma d-分裂和回旋。在横向上，随着层厚的减小，整个沉积层序依次由下而上依次为Bouma bc/c、c/e层序、Bouma bc/c、c/e层序。
所研究的层位的相变可能代表了从一个扇的渠化部分的下坡端延伸到具有光滑表面的外部部分的一段。
On the Boso Peninsula, southeast of Tokyo, individual turbidite layers have been traced over about 38 km with the aid of tuff marker-beds.
Sandstones are generally much more variable in thickness than the interbedded mudstones, showing lenticular shapes with great lateral continuities up to 30 km or more. But the latter have greater extents than the former, forming extensive sheets with a slight upslope thickening. Most sandstones show vertical and lateral grading, while mudstones gradually coarsen upslope. Sandstones have a variety of sedimentary structures typical of turbidites, whereas mudstones are virtually devoid of them throughout. Sandstones contain intensely abraded shallow-water fauna in contrast with mudstones with well-preserved deep-water fauna. Thus, mudstones are mostly of hemipelagic origin, and turbiditic mudstones, if present, are of negligible amounts.
In general, individual sandstone beds are much more variable in thickness on the upstream side than on the downstream side. This is probably because of the existence of some minor channels and the adjacent levee-like embankments on the upslope side. At the base of thicker sandstone beds local penecontemporaneous erosion has usually taken place, giving rise to a few minor channels bordered by small levee-like embankments on both sides. The downslope side of the largest channel is commonly flanked by the greatest embankment that is coincident with the thickest part of each sandstone layer. Sandstones in the minor channels and levee-like embankments contain a lot of mud lumps torn out of the substratum, which decrease in size and amount and improve in roundness with increasing distance from the channels. In general, thicker sandstones show Bouma abcde sequences around the thickest part. Bouma d-division and convolution are developed preferentially on the downstream side of the thickest part. With decreasing bed thickness laterally, the complete depositional sequences arc successively replaced by Bouma bcde, then cde, and de sequences downslope, while by Bouma bc/c and then c/e sequences upslope.
The facies transition of the horizon studied probably represents a section extending from the downslope end of the channelized portion of a suprafan to the outer portion with a smooth surface.