Key beds
The key beds (I seem to smell it key bed, marker bed) are the characteristic layers that are used to compare the generation of the stratum, and to identify it. The sedimentation layer is the object layer, and it is the requirement mainly to be the stratum where I deposited in the same times.
The generation generation of the stratum between two points that it was often found horizontally not to continue because the stratum caught the erosion action, and left it the stratum vs. compare it, and it is the materials which the key beds are important to when judge a continuity.
Table of contents
Example of key beds
The stratum which the pumice and tufa layer which the origin can identify or a specific fossil or element produce is used for the key beds mainly. The stratum which distribution is wide, and there is a continuity, and rock-like character is characteristic, and is distinguished from other rocks easily. Other than pumice, tufa, there are a stone coal bed, a chart, limestone.
Pumice, tufa layer
The volcanic ashes by the eruption of the volcano pour into the outskirts or the wide area. Because an ingredient changes every eruption as for this ejectamenta (e.g., pumice and tufa), with the thing produced by the same volcano, I can judge the eruption times. I identify the same volcanic ejecta using this property, and the comparison in the times is enabled even if sedimentation places were different because this is the same time.
There are Aira Caldera and Mount Aso, Oyama, Mount Asama, Mount Fuji, Lake Shikotsu caldera, Mount Tarumae, wide area tephra by the eruption of Mount Usu which the representative key beds in Japan cause great eruption in the past, and brought volcanic ashes widely in Hokkaido.
Volcanic ashes were blown up to the stratosphere, and, in 1991, they said on jet stream at the time of Pinatubo volcanic eruption of Philippines one lap of earth in several days, and I was scattered by the whole earth [1].
such as a fossil and creature trace, mudstone, sandstone, limestone containing carbide
About the fossil, the history of the evolution including the ammonite having a slight fossil and a slightly large fusuline and a large shell can perform the judgment that is approximately generous by analyzing a characteristic about the kind becoming clear every generation. An available fossil is called an index fossil for such a purpose.
Stratum including the specific element
A layer containing iridium which hardly exists on the earth does 挟在 in terminal stratum for the Mesozoic. As for this, a meteorite collides on the earth, and it is thought that it occurred because the slight ingredient spread widely. In addition, I break into the atmosphere, and a big meteorite becomes the tektite and is sprayed on area of several hundred kilometers [1].
For the example which a human being participated in, it is that radioactivity increased by a nuclear test after 1950 [1].
Footnote
References
- Written by Noriyuki Ikeya, Hiroshi Kitazato "evolution ー) Tokyo University publication society 2004 ISBN 4-13-062711-2 of the geobiology ー earth and the life"
Allied item
Outside link
- Association of key beds plain geological feature all sorts of subject - China geological survey business
This article is taken from the Japanese Wikipedia Key beds
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