Inconsistency - Inconsistency

The relationship in which two strata overlap one above the other with an erosion surface in between is called an unconformity, and the erosion surface is called an unconformity surface. A relationship without an erosion surface is called a conformity. However, when the erosion period is very short compared to the way the upper and lower strata were deposited, such as inside riverbed deposits or small erosion surfaces at the base of turbidity sandstone layers, it is not called an unconformity. When the strike and dip of the strata above and below the unconformity surface are the same, it is called a parallel unconformity, and when they are different, it is called an inclined unconformity. When strata are deposited over a massive igneous body, the base is also called an inclined unconformity or unconformity. The base of the upper layer is often of a different rock type from the lower layer, and is deposited at the unconformity surface, cutting the bedding of the lower layer. This allows the unconformity surface to be found. The basal layer is often made of conglomerate or sandstone, and is called basal conglomerate or basal sandstone. However, when seafloor erosion occurs in a mudstone depositional area, both the upper and lower layers are unstratified mudstone, and the existence of the unconformity becomes apparent only due to the absence of fossil zones such as planktonic foraminifera. Unconformity erosion surfaces can be formed on the seafloor, but are usually formed by land erosion.

The unconformity formed by erosion on land in the middle of overlapping marine strata was formed when the sea area turned into land, i.e., during the regression period. The regression phenomenon occurs due to global sea level fall and continental and regional crustal movement. A parallel unconformity indicates that even if the regression was caused by crustal movement, the ground could not have tilted in that area. On the other hand, an oblique unconformity indicates that the ground tilted due to the tilt or folding of the land mass. In the past, oblique unconformities were thought to have been caused by short-term mountain building, no matter how small the difference in the inclination of the upper and lower strata, and were considered to be important events in geological history. This is because it was thought that mountain building occurs when the crust is compressed, and that the compression causes the strata to fold, and at the same time the sea area rises and becomes land, i.e., regression occurs. It was also thought that the period in which the unconformity was formed must be short. However, some tilt unconformities were formed due to continental-wide regressions that occurred during periods of continuous crustal movement throughout the entire depositional period of the upper and lower strata, and in some cases the erosion period lasted much longer than expected. There are also many localized tilt unconformities. The significance of tilt unconformities in terms of crustal movement can only be determined after detailed study of their distribution and the stratigraphy and age of the upper and lower strata. Depending on whether there is a difference in the way the geological structures of the two strata are formed, they are called structural unconformities or structural conformities. Parallel unconformities are usually structural conformities.

[Toshio Kimura]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

二つの地層が侵食面を挟んで上下に重なる関係を不整合といい、その侵食面を不整合面という。侵食面がない関係が整合である。ただし、河床堆積(たいせき)物内部や乱泥流砂岩層基底の小侵食面のように、上下層の堆積の仕方と比べて侵食期が非常に短いときには不整合とはいわない。不整合面の上と下との地層の走向傾斜が等しいものが平行不整合、異なるものが傾斜不整合である。塊状火成岩体を覆って地層が堆積しているときも、その基底は傾斜不整合あるいは無整合とよばれる。上位層の基底部は、しばしば下位層と岩質を異にし、不整合面において下位層の層理を切って堆積している。それによって不整合面をみいだすことができる。基底層は礫(れき)岩や砂岩からなることが多く、基底礫岩、基底砂岩とよばれる。しかし、泥岩堆積区で海底侵食がおこった場合には、上下層とも無層理の泥岩であって、浮遊性有孔虫などの化石帯欠如によってその存在が初めて明らかになる不整合もある。不整合の侵食面は海底でできることもあるが、普通は陸上侵食でできる。

　上下に重なる海成層の中ほどに、陸上での侵食によってできた不整合は、海域が陸域に変わったとき、すなわち海退期にできたものである。海退現象は汎(はん)世界的海面低下、大陸的また地域的地殻変動によっておこる。平行不整合は、地殻変動による海退に起因する場合でも、その区域に地盤の傾きはできなかったことを示す。一方、傾斜不整合は、地塊の傾動または褶曲(しゅうきょく)による地盤の傾きができたことを示す。かつて傾斜不整合は、上下層の傾斜の差がいかに小さくても、短期間の造山運動によって生じたと考えられ、地史のうえでの重要事件とされた。造山運動は地殻が圧縮されておこり、その圧縮によって地層が褶曲すると同時に海域が上昇して陸域となる、すなわち海退がおこると考えられたからである。また、不整合形成期は短期間であるはずであると想像されたからである。しかし、上下層の堆積期全体を通じての継続的な地殻変動期中におこった、大陸的広域の海退に伴ってできた傾斜不整合もあり、侵食期が予想されたよりもはるかに長期であった例もある。また、局所的傾斜不整合が多い。傾斜不整合が地殻変動に対してもつ意味は、それの分布や上下層の層序と時代についての詳細な研究をまって初めて明らかになる。二つの地層の地質構造の形成のされ方に差があるかないかによって、構造的不整合、構造的整合という。平行不整合は普通、構造的整合である。

［木村敏雄］

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