Strata (English spelling)

Japanese: 地層 - ちそう（英語表記）strata

A layered rock body is called a stratum. Most strata are made of sedimentary rock. However, lava (volcanic rock) and pyroclastic rock, which are closely related to volcanic activity, are also called strata. In addition, sedimentary rocks such as coral reef limestone that form nodular rock bodies are also called strata because they are distributed in layers over a wider area.

[Toshio Kimura and Akihiro Murata]

Formation process of bedding

Strata are formed when material is deposited on the earth's surface, whether on the seafloor or on land, or when it flows across the surface. The debris and chemicals that make up sedimentary rocks, including pyroclastic rocks, are deposited parallel to the depositional surface that exists at the time, i.e., the earth's surface, forming bedding (layers). The earth's surface has some unevenness, including microscopic ones. Deposition does not occur where the unevenness is steep and large. Whether or not material will deposit on a slope depends on the angle of repose of the sediment, that is, the maximum angle of inclination at which it can deposit without sliding. The angle of repose tends to be larger for coarse-grained sediments and smaller for sediments that contain more water. Therefore, the angle of repose of mud layers on the seafloor is small, and even on slopes with an inclination of less than a few degrees, they will lose stability and cause landslides or collapses.

On the other hand, coarse-grained breccia on land can create stable slopes with a considerable inclination. Also, on land where there is no water, volcanic ash often stratifies in accordance with the undulations of the earth's surface. In other words, the strata can have a considerable inclination from the beginning of deposition. However, on the bottom of water, which is the main depositional site of sedimentary rocks, deposition tends to occur in depressions, which tend to be gradually filled in and the horizontal areas tend to expand. In this way, horizontal layers are deposited on the bottom of water with a large expanse. This is called the "law of primary horizontal deposition." When horizontal layers are deposited one on top of the other in this way, the layer on top is younger than the layer below. This is called the "law of superposition of strata" (or the law of superposition), and is the criterion for determining the age of two adjacent strata.

[Toshio Kimura and Akihiro Murata]

Single layer and foliation

A layer of similar rock type that is sandwiched between clear bedding planes is called a laminae. Finer layers within a laminae are called foliations. This term is usually used when the layers are thinner than 1 cm. Foliations are parallel to the laminae that encase them, but may cross them at an angle to form cross-lamination. Strata may also be lenticular, rather than laminae. Lenticular layers become thinner around their edges until they disappear. A layer that disappears in only one direction is called a tongue layer. A layer of different rock types that overlaps one another is called an interfingering layer. When layers of different rock types overlap each other, they are called interfingering. A layer that is deposited on the side of a steep slope such as a cliff is called an abutment. In some cases, a submarine landslide fold formed shortly after deposition is sandwiched between unfolded strata, forming intralayer folds due to deposition. Not to mention strata with unconformities, except for cases where suspended muddy particles sink uniformly and gradually pile up, the rate of deposition is not uniform even in strata that appear to have been deposited continuously. In the case of alternating layers of mudstone and sandstone in turbidity current deposits, even thin layers of mudstone less than a few centimeters thick took hundreds of years to deposit, while layers of sandstone nearly a meter thick can be deposited in less than a day. Some depositional forms, such as graded bedding and cross-bedding, indicate the direction of the strata deposition, i.e. which layer was on top at the time of deposition. They are useful for analyzing the folded structure of strata that have been significantly deformed by crustal movements.

[Toshio Kimura and Akihiro Murata]

Sedimentary basin

In the past, as in the present, strata were deposited in thick layers in river basins, estuaries, lakes, lagoons, and oceans. A depression-like area where strata are deposited thicker than the surrounding areas is called a sedimentary basin. Sedimentary basins are usually formed within land areas or in inland seas surrounded by land by subsidence. They are often accompanied by uplift of the surrounding areas and become a source of clastic material. In the ocean, the strata are not particularly thicker than the surrounding areas, but there are huge sedimentary basins in the center of the ocean. Even in areas adjacent to land, if an uplift zone forms a little away from the land, a sedimentary basin can form, such as an island arc foreland basin. In the wide seas surrounding land, a large amount of clastic material is supplied from the land, so a sedimentary area with a wedge-shaped sedimentary body that is thick on the land side and thin on the sea side is formed, regardless of whether there is a sedimentary basin or not. The delta deposits at the river mouth are a small example of this. In these wide seas adjacent to land, when there is subsidence on the sea side and uplift on the land side, the sedimentary body can sometimes become huge, such as the sedimentary layer during mountain building.

Sedimentation in a sedimentary basin does not necessarily occur by gradually filling in the area from the center. The way sedimentation occurs varies from place to place even within a single sedimentary basin, depending on the supply and direction of debris, the type and direction of transport, and the type of crustal movement occurring in the surrounding area.

[Toshio Kimura and Akihiro Murata]

The sedimentation cycle

When new strata begin to deposit in a sedimentary basin, often a layer rich in conglomerate or sandstone is deposited, and gradually a layer mainly made of mudstone is deposited on top of that. Sometimes a layer made of coarse-grained rock is placed on top of that. It is often thought that this is due to the initially shallow sea gradually becoming deeper and then shallower, i.e., the occurrence of marine transgression and regression. This series of depositional phenomena from marine transgression to marine regression is called a depositional cycle. The layers during the regression are often removed by erosion, and the starting layer of the next depositional cycle is often deposited directly on the transgressive layer in an unconformable manner, so the depositional cycle is often called the period up to the transgressive layer. The depositional cycle as a whole is usually more than 200 to 300 meters thick, and is often mainly made of marine strata. In contrast, a phenomenon is known in coalfields where unit cycle layers several meters to tens of meters thick are repeatedly formed, with freshwater strata with coal beds in the lower half and shallow sea layers in the upper half. This is called a cyclothem (a small cycle of sedimentation).

Sedimentary cycles are also caused by global sea level rise, but thick strata of cyclic layers are caused by relative sea level changes associated with large-scale land-building or local crustal movements. Cyclotems are also thought to be caused by instability in the earth's crust. In Japan, they are sometimes attributed to periodic volcanic activity.

[Toshio Kimura and Akihiro Murata]

Sedimentary environment of the strata

Just as there is a distinction between coastal and offshore, depositional environments are not uniform even within a single sedimentary basin. Therefore, even in strata of the same age and period, sedimentary layers and biological groups differ depending on the location. This is called simultaneous heterofacies. The petrological and biological characteristics that indicate differences in depositional environments are called lithofacies and biofacies, respectively. They are also called coastal facies and offshore facies. When characterized by an abundance of sandstone or mudstone, they are called sandstone facies and mudstone facies. Fossils that indicate the depositional environment are index fossils. These are heterofacies that occurred within a short period of time, but heterofacies that were created due to differences in tectonic environments over a long period of time over a continental area are called tectonic facies. For example, a group of strata that were deposited over a long period of time in a stable shelf environment is called a stable shelf facies.

[Toshio Kimura and Akihiro Murata]

Key Layer

A geological layer that has particularly prominent characteristics and is thought to have been deposited over a wide area at almost the same time is called a key layer. Quartz sandstone layers deposited near the coast of a stable shelf have such characteristics, and are often used as key layers in geological surveys of stable shelf facies areas. Their prominent topography makes them even more useful as key layers. In contrast, in unstable areas, coastal facies are often not strictly contemporaneous layers. For example, basal conglomerates often change their depositional site over time as the marine transgression progresses. Although the distinctive rock quality is helpful in geological surveys, it cannot be used as a key layer indicating contemporaneity. In Japan, which was an unstable area, volcanic ash layers that were deposited simultaneously across different environments often serve as good key layers.

[Toshio Kimura and Akihiro Murata]

Classification and comparison of strata

Strata always show simultaneous heterogeneity over a wide area. In other words, strata of different rock types are deposited in different places. Therefore, strata are divided into rock layers and periods according to their petrological properties. When describing strata and marking them on a geological map, the basic unit of rock layers is a formation, which consists of one or more types of sedimentary rock, has characteristics that make it easy to distinguish it from other layers, and is large enough to be marked on a geological map. There are also sublayers, which are smaller units than formations. Formations come together to form larger units called groups. The boundaries of groups are often determined by the presence of unconformities. Several groups are also called groups of formations. These units are called type localities, and the names of the places where they typically develop are added to them, such as the Kiyosumi Formation (the character "formation" is often omitted) and the Kazusa Group. In Japan, the same rock layer units can often be applied to a wide area of Mesozoic and Paleozoic strata, where large sedimentary basins were widespread. The opposite is true in the Neogene strata of the Cenozoic era, where the sedimentary basin was narrow.

The units of time division are fossil zones, periods, worlds, eras, and epochs. The determination of whether or not strata in different locations are from the same era is called correlation. Comparison is performed using index fossils (standard stones). Therefore, fossil zones are the basic unit of strata correlation, and are called by the species or genus name of the index fossil that indicates it. Periods, worlds, eras, and epochs consist of a single, a few, or a large number of fossil zones. They may be named after the place name of the type region, or they may be named after the research history there. Period and world boundaries are determined where there is a slightly larger change in the fossil zones of the type region due to biological evolution or environmental change, and era boundaries are determined where there is an even larger change. At the boundaries of eras, major changes in animal fossils occurred worldwide. In type regions, the boundaries of eras are often determined according to changes in the biota caused by environmental changes in the area, rather than by biological evolution that proceeded in the same environment. For this reason, there is often debate in the academic community about the location of the era boundaries. Different environments each have index fossils for different groups of organisms. When different strata intersect, their index fossils can be used to compare them. Even in regions that do not have the same index fossils as the international type regions, comparisons with the international period divisions are made based on such research. Strata that have been compared and dated are called stages, series, systems, or kingdoms. For example, the Miocene, Jurassic, and Palaeozoic strata are called the Miocene, Jurassic, and Palaeozoic, respectively.

In recent years, as the ages of each era have become clearer, comparisons are also made by measuring radiometric dates on volcanic ash layers sandwiched between strata. Late Cenozoic strata are also compared by magnetic stratigraphy. Similar rocks are produced in similar geological environments, not just sedimentary environments. Even nearby locations from the same era often have different geological environments. For example, the rock types of neighboring volcanoes can be different. Therefore, rock bodies in separate areas cannot be compared based solely on the similarity of rock type. Proper comparisons can tell us the distribution of geological environments at a certain point in time within a wide area, and comparisons with other points in time can tell us the changes in the geological environment. This is the basis for understanding geological phenomena.

[Toshio Kimura and Akihiro Murata]

Relationship between the two strata
©Shogakukan ">

Relationship between the two strata

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

層状をなした岩体を地層という。地層の多くは堆積(たいせき)岩からなる。しかし火山活動と密接な関係をもつ溶岩（火山岩）や火山砕屑(さいせつ)岩も地層とよばれる。また、サンゴ礁石灰岩など堆積岩でありながら小塊状岩体をつくるものも、より広い範囲内で層状部をつくって分布するので、地層とよばれる。

［木村敏雄・村田明広］

層理の形成過程

地層は海底、陸上を問わず地表に物質が堆積するか、または地表を流れてできる。火山砕屑岩を含めて、堆積岩をつくる砕屑物や化学物質は、そのときどきに存在する堆積表面、すなわち地表面に平行に堆積して層理（成層）をつくる。地表面は微小なものを含めて、多少とも凹凸をもつ。凹凸の斜面が急で大きいところには堆積はおこらない。斜面に堆積するか否かは堆積物がもつ安息角、すなわち、すべりをおこさずに堆積可能な最大の傾斜角による。安息角は粗粒堆積物ほど大きく、水を多く含むほど小さくなる傾向がある。したがって海底では泥層の安息角は小さく、数度以内の傾斜の斜面でも安定を失い地すべりや崩壊をおこす。

　一方、陸上での粗粒角礫(そりゅうかくれき)岩は、かなりの傾斜角をもつ安定斜面をつくることができる。また、陸上で水の影響を受けないところでは、火山灰はしばしば地表の起伏なりに成層する。すなわち、堆積の当初から地層はかなりの傾斜をもつことがある。しかし、堆積岩の主要な堆積の場である水底では、凹部に堆積がおこりやすく、そこはしだいに埋め立てられて水平部が大きく広がろうとする傾向をもつ。このようにして大きな広がりをもった水底では水平層が堆積する。これは「初源堆積水平の法則」とよばれる。このように水平層が次々に上に重なって堆積すると、上に重なる地層が下の地層より若いことになる。これは「地層累重の法則(ちそうるいじゅうのほうそく)」（または累重の法則）とよばれ、相接する二つの地層の新旧を判別するときの基準となる。

［木村敏雄・村田明広］

単層と葉層

明瞭(めいりょう)な層理面に挟まれ、かつほぼ類似の岩質からなる層を単層という。単層の中にみられる、より細かい層は葉層とよばれる。これは普通1センチメートルよりも薄いものについていう。葉層はそれを包み込む単層に平行であるが、斜交して斜交葉理をつくることがある。地層はまた、単層をつくらずレンズ状となることがある。レンズ状の地層はその周辺でしだいに薄くなり、ついに消滅する。同様の消滅を一方向にのみおこした単層は、舌状層とよばれる。異なる岩質の単層が交互に重なり合った地層は互層である。また、異なる岩質の地層が互いに側方に移り変わる関係にあるとき「指交interfingering」という。崖(がけ)のような急な斜面の側方に堆積した地層はアバットabutという。堆積後まもなく生じた海底地すべり褶曲(しゅうきょく)層が、褶曲しない地層の間に挟まれて、堆積による層内褶曲を形成していることもある。不整合を挟む地層はいうまでもないが、浮遊した泥質粒子が一様に沈下してしだいに積み重なったような場合を除くと、連続して堆積したとみえる地層でも堆積の速さは一様でない。混濁流（乱泥流）堆積物中の泥岩層と砂岩層との互層についていうと、泥岩は数センチメートル未満の薄層でも数百年の堆積期を要しているのに、砂岩は1メートル近くの厚層でも1日未満で堆積したものがある。級化層理、斜交層理など堆積形態のなかには、堆積時にどちらが上位であったかの地層堆積の向きを示すものがある。地殻変動によって著しく変形した地層の褶曲構造を解析するのに役だつ。

［木村敏雄・村田明広］

堆積盆

過去においても現在と同様に、地層は河川流域、河口、湖沼、潟、海などに厚く堆積した。地層が周辺よりも厚く堆積した凹地状区域を堆積盆という。陸域内ないし陸域に囲まれた内海では普通、沈降によって堆積盆ができる。周辺区域の隆起を伴い、そこが砕屑物供給地となることも多い。海では、周辺よりもとくに地層が厚いわけではないが、大洋の中心部に巨大な堆積盆がある。陸域に接した区域でも、陸からやや離れたところに隆起帯ができると、島弧前縁盆地のように堆積盆ができる。陸域周辺の広い海では、陸域から多量の砕屑物の供給があるため、堆積盆があろうとなかろうと、陸側に厚く海側に薄くなる、くさび状の堆積体をもつ堆積域ができる。河口の三角州堆積体はその小規模のものである。これら陸に接した広い海では、海側に沈降、陸側に上昇があるとき、造山運動期の堆積層のように、その堆積体はときに巨大になる。

　堆積盆の中での堆積は、かならずしも中心からしだいに埋め立てていくようにはおこらない。砕屑物の供給や運搬の仕方と方向、周辺を含めた区域での地殻変動のおこり方によって、一つの堆積盆内でも場所により堆積の仕方は異なる。

［木村敏雄・村田明広］

堆積のサイクル

堆積盆に新しく地層が堆積し始めるとき、しばしば礫岩または砂岩に富む地層が堆積し、その上にしだいに泥岩を主とする地層が堆積する。ときには、さらにその上に粗粒岩からなる地層がのる。初め浅かった海がしだいに深くなりまた浅くなった、すなわち海進と海退とがおこったためであると解されることが多い。このような海進から海退までの一連の堆積現象を、堆積のサイクル（輪廻(りんね)）とよぶ。海退時の地層はしばしば侵食によって除去され、海進層の上に直接不整合で次の堆積サイクルの開始層が堆積することが多いので、海進層までを堆積のサイクルとよぶことが多い。堆積のサイクルは全体として、普通200～300メートル以上の厚さで、海成層を主体とすることが多い。これに対して、下半部が炭層をもつ淡水成層と、上半部が浅海層からなる厚さ数メートルから数十メートルの単位サイクル層が繰り返して現れる事象が、炭田地区などで知られている。これについてはサイクロセムcyclothem（堆積小サイクル）の名がある。

　堆積のサイクルは汎(はん)世界的海面上昇によってもおこるが、厚い地層からなるサイクル層は、広域の造陸運動または局地的な地殻変動に伴う相対的な海面変動によっている。サイクロセムも地殻の不安定に起因するとされている。日本では周期的におこる火山活動に帰せられることがある。

［木村敏雄・村田明広］

地層の堆積環境

沿岸、沖合いの区別があるように、一つの堆積盆内でも堆積環境は一様でない。したがって、ある同じ年代・期間の地層でも場所によって堆積層や生物群が異なる。これを同時異相という。そして堆積環境の違いを示す岩石学的、生物学的特徴をそれぞれ岩相、生相という。そして沿岸相、沖合相などといわれる。砂岩または泥岩が多いことで特徴づけられる場合には砂岩相、泥岩相とよばれる。堆積環境を示す化石が示相化石である。これらは短い期間内の異相であるが、大陸的広域の長期にわたる造構環境の違いによって生じた異相は構造相という。たとえば、安定陸棚環境に長期に堆積した地層群は、安定陸棚相とよばれる。

［木村敏雄・村田明広］

鍵層

とくに目だつ特徴をもち、かつ広範囲にほぼ同時に堆積したとみられる地層は、鍵層(かぎそう)とよばれる。安定陸棚の沿岸近くで堆積した石英砂岩層はそのような特徴をもち、安定陸棚相区域の地質調査の際に、鍵層としてしばしば利用されている。地形的に突出して目だつので、鍵層としての有用性を増している。これに対して不安定区域では、沿岸相も厳密に同時層でないことが多い。たとえば基底礫岩は、海進の進行に伴って、時間の変化とともに堆積地を変えることが多い。その特徴ある岩質が地質調査の手助けとなるが、同時を示す鍵層とはなりえない。不安定区域であった日本では、異なる環境にまたがって同時に堆積した火山灰層がよい鍵層となることが多い。

［木村敏雄・村田明広］

地層の区分と対比

地層は広い範囲ではかならず同時異相を示す。すなわち、場所によって異なる岩質の地層が堆積している。したがって、地層の区分には岩石学的性質による岩層区分と時代区分とがある。地層の記載、地質図への記入にあたって、岩層区分の基本単位層となる累層(るいそう)は、単一または複数種の堆積岩からなり、他の層から容易に区別できる特徴をもち、しかも地質図に記入可能な大きさをもつものである。累層より小さい単位として部層がある。累層が集まって、より大きい単位としての層群をつくる。不整合の存在をもって層群の境界とすることが多い。また、いくつかの層群をまとめて累層群とよぶ。これらの単位層は、典型的に発達する場所を模式地として、その地名を付して、たとえば清澄(きよすみ)層（累の字はしばしば省略される）、上総(かずさ)層群のようによばれる。日本では、大きい堆積盆が広く広がっていた中・古生代の地層について、広い範囲に同一岩層単位が適用できることが多い。堆積盆が狭かった新生代新第三紀の地層では逆である。

　時代区分の単位は、化石帯および期、世、紀、代である。異なる位置にある地層が同時代のものであるか否かの認定を対比という。対比は示準化石（標準化石）によって行われる。したがって、化石帯が地層対比の基本単位で、それを示す示準化石の種名または属名でよばれる。期、世、紀、代は、単一、少数または多数の化石帯からなる。模式地域の地名でよばれることもあるし、そこでの研究史を反映した呼び名のこともある。生物進化あるいは環境変化に起因して、模式地の化石帯群にやや大きい変化があったところに期、世の境界が、もっと大きい変化があったところに紀の境界が定められている。代の境界には、動物化石群の世界的な大変化がおこった。模式地では、同一環境下で進行した生物進化によってではなくて、その区域の環境変化による生物群の変化に従って紀の境界が定められていることが多い。そのため、紀の境界の位置についてしばしば学界の論争がある。異なる環境のそれぞれには、異なる生物グループの示準化石がある。異相の地層が指交しているとき、それぞれの示準化石を利用して互いの対比が可能となる。国際的な模式地域と同じ示準化石がない地域でも、このような研究を基にして国際時代区分との対比が行われている。対比が行われ時代が確定した地層は階、統、系、界とよばれる。たとえば、中新世、ジュラ紀、古生代の地層は、それぞれ中新統、ジュラ系、古生界とよばれる。

　近年、各時代の年代が明らかになってきたので、地層に挟まれた火山灰層などの放射年代測定値によっての対比も行われる。新生代後期層については、磁気層序学による対比も行われる。堆積環境に限らず、似た地質環境下では似た岩石を生じる。また、同じ時代の近接した場所でも、しばしば互いの地質環境が異なる。たとえば、隣接する火山の岩質が異なることがある。したがって、岩質の類似のみをもって、離れた地区の岩体を対比することはできない。正しい対比によって、広い区域の中でのある時点における地質環境の分布を知りうるし、他の時点との比較によって、地質環境の変遷を知りうる。これが地質現象を理解する基本となる。

［木村敏雄・村田明広］