In geology, stratum is called each of the layers into which sediments, sedimentary rocks, pyroclastic rocks and metamorphic rocks are divided when these layers are due to sedimentation processes.[1] The branch of geology that studies strata is called stratigraphy.[2].
It must be taken into account that other different geological phenomena can give rise to layers, which will then not be called strata. This is the case, for example, of slabs that form during metamorphism when great pressures affect the rocks, causing cuts perpendicular to the compression force. Finally, igneous intrusions "Intrusion (geology)") can form dykes "Dyke (geology)") or interstratified layers that appear as if it were just another stratum, although it must be taken into account that the dykes can have a lenticular shape when they form a mantle or sill "Sheet (geology)") which, when they become quite domed, are usually called laccoliths.
Characteristics
Typically, a stratum is generally one of a series of parallel layers that lie on top of each other to form enormous thicknesses of strata. The bedding surfaces (stratification planes) that separate the strata represent episodic breaks in deposition associated with either periodic erosion, the cessation of deposition, or some combination of both. Stacked together with other strata, individual strata can form stratigraphic units. compounds that can extend over hundreds of thousands of square kilometers of the Earth's surface. Individual strata may cover equally large areas. Strata are usually seen as bands of material of different color or structure exposed in cliffs, road cuts, quarries, and river banks. Individual bands can vary in thickness from a few millimeters to several meters or more. A band may represent a specific mode of deposition "Deposition (sediment)"): river silt, beach sand, coal swamp, sand dune, lava bed, etc.
Stratification
Strata are typically formed as layers of relatively uniform power (thickness), limited by stratification surfaces, which are more or less clear interfaces with respect to the most recent stratum (the one located above or superjacent) and the oldest one (the one located below or infrajacent). In the description of the strata, the term "base" or "wall" is used to refer to the oldest part (geometrically the lower part, if there are no tectonic alterations in the position) and "roof" for the most recent. Strata are the basic study units of stratigraphy.
Stratum
Introduction
In geology, stratum is called each of the layers into which sediments, sedimentary rocks, pyroclastic rocks and metamorphic rocks are divided when these layers are due to sedimentation processes.[1] The branch of geology that studies strata is called stratigraphy.[2].
It must be taken into account that other different geological phenomena can give rise to layers, which will then not be called strata. This is the case, for example, of slabs that form during metamorphism when great pressures affect the rocks, causing cuts perpendicular to the compression force. Finally, igneous intrusions "Intrusion (geology)") can form dykes "Dyke (geology)") or interstratified layers that appear as if it were just another stratum, although it must be taken into account that the dykes can have a lenticular shape when they form a mantle or sill "Sheet (geology)") which, when they become quite domed, are usually called laccoliths.
Characteristics
Typically, a stratum is generally one of a series of parallel layers that lie on top of each other to form enormous thicknesses of strata. The bedding surfaces (stratification planes) that separate the strata represent episodic breaks in deposition associated with either periodic erosion, the cessation of deposition, or some combination of both. Stacked together with other strata, individual strata can form stratigraphic units. compounds that can extend over hundreds of thousands of square kilometers of the Earth's surface. Individual strata may cover equally large areas. Strata are usually seen as bands of material of different color or structure exposed in cliffs, road cuts, quarries, and river banks. Individual bands can vary in thickness from a few millimeters to several meters or more. A band may represent a specific mode of deposition "Deposition (sediment)"): river silt, beach sand, coal swamp, sand dune, lava bed, etc.
Stratification
Strata are typically formed as layers of relatively uniform power (thickness), limited by , which are more or less clear interfaces with respect to the most recent stratum (the one located above or superjacent) and the oldest one (the one located below or infrajacent). In the description of the strata, the term "base" or "wall" is used to refer to the oldest part (geometrically the lower part, if there are no tectonic alterations in the position) and "roof" for the most recent. Strata are the basic study units of stratigraphy.
Horizontal and uniform strata can move and deform in the presence of tectonic forces, undergoing changes in position (tilting, which can even lead to inversion) and shape (folding). Furthermore, some strata appear from their very origin inclined towards each other and of unequal thickness, as occurs in cross stratification. The genesis of cross stratification is quite simple in the processes of filling by sediments carried by rivers (sands, clays) when they reach the foothills of a mountain range and form waste cones, terraces and other minor relief forms. These sediments are arranged according to gravity and the fact that they cross each other is due to a change in the course of the river that is diverted with each major flood, which is often sporadic as is typical in arid climates, for the simple fact that the previous flood produced an accumulation that the waters of the river have to surround due to its greater height, now being deposited with a different dip (that is, an inclination), direction and thickness. Over time, these sands or clays can consolidate and form sedimentary rocks (sandstone, for example, as is the case in the image of Antelope Canyon) but they have preserved the original arrangement of the sands that formed said rocks. A stratigraphic discontinuity can also be seen in the center of the image.
In short, these stratification surfaces reflect heterogeneities of the sedimentation process, with abrupt changes in the nature of the sediment or more or less prolonged interruptions of the deposition process.
Joints and failures
The strata can be more or less plastic or rigid and this has obvious consequences on their resistance to deformation. When they are very rigid (sandstone or limestone rocks, for example), the internal movements of the lithosphere can break or split these strata and joints are formed, which are always perpendicular to the surface of the strata, and faults, when there is a noticeable displacement in a vertical or horizontal sense of all the strata along a fault line of much larger dimensions than in the case of joints or along a fault mirror when it is a normal fault, that is, when a part of the strata is elevated with respect to the same strata that remain relatively depressed on the other side of said fault mirror.
Joints can also occur in igneous or metamorphic rocks, as is the case of the polygonal joints of basaltic colonnades and in metamorphic rocks such as quartzite that appears associated with the sandstone sedimentary cover in the case of the Venezuelan tepuis.
Fossils
Being formed by sedimentary rocks, the strata usually contain fossils, that is, organic remains of animals and plants that serve to date the approximate age of said strata. This is done through the so-called index fossils, so called because they only existed in a certain Era or geological period. Obviously, finding said fossil in a stratum would serve to infer that said stratum was formed during the time in which the animal or plant existed. Dating is a very important application of stratigraphy. In the case of the sedimentary cover of the Roraima Formation of Venezuelan Guayana, which is formed by sandstone strata, there are no fossils of any type, which serves to date it as very old precisely due to this fact. It is more than 1,500 million years old, that is, they are sedimentary rocks prior to the Paleozoic Era, which is the era in which the first multicellular living beings are thought to have appeared. In this case, dating rocks from such strata requires dating methods of another type.
References
[1] ↑ a b Salvador, A. ed., 1994. International stratigraphic guide: a guide to stratigraphic classification, terminology, and procedure. 2nd ed. Boulder, Colorado, The Geological Society of America, Inc., 215 pp. ISBN 978-0-8137-5216-7.
[2] ↑ Neuendorf, K.K.E., Mehl, Jr., J.P., and Jackson, J.A. , eds., 2005. Glossary of Geology 5th ed. Alexandria, Virginia, American Geological Institute. 779 pp. ISBN 0-922152-76-4.
[3] ↑ Davies, N.S., y Shillito, A.P. 2021, True substrates: the exceptional resolution and unexceptional preservation of deep time snapshots on bedding surfaces. Sedimentology. publicado en línea el 22 de mayo de 2021, doi: 10.1111/sed.12900.
[4] ↑ Davies, N.S., y Shillito, A.P. 2018, Incomplete but intricately detailed: the inevitable preservation of true substrates in a time-deficient stratigraphic record. Geology, 46, 679-682.
stratification surfaces
Horizontal and uniform strata can move and deform in the presence of tectonic forces, undergoing changes in position (tilting, which can even lead to inversion) and shape (folding). Furthermore, some strata appear from their very origin inclined towards each other and of unequal thickness, as occurs in cross stratification. The genesis of cross stratification is quite simple in the processes of filling by sediments carried by rivers (sands, clays) when they reach the foothills of a mountain range and form waste cones, terraces and other minor relief forms. These sediments are arranged according to gravity and the fact that they cross each other is due to a change in the course of the river that is diverted with each major flood, which is often sporadic as is typical in arid climates, for the simple fact that the previous flood produced an accumulation that the waters of the river have to surround due to its greater height, now being deposited with a different dip (that is, an inclination), direction and thickness. Over time, these sands or clays can consolidate and form sedimentary rocks (sandstone, for example, as is the case in the image of Antelope Canyon) but they have preserved the original arrangement of the sands that formed said rocks. A stratigraphic discontinuity can also be seen in the center of the image.
In short, these stratification surfaces reflect heterogeneities of the sedimentation process, with abrupt changes in the nature of the sediment or more or less prolonged interruptions of the deposition process.
Joints and failures
The strata can be more or less plastic or rigid and this has obvious consequences on their resistance to deformation. When they are very rigid (sandstone or limestone rocks, for example), the internal movements of the lithosphere can break or split these strata and joints are formed, which are always perpendicular to the surface of the strata, and faults, when there is a noticeable displacement in a vertical or horizontal sense of all the strata along a fault line of much larger dimensions than in the case of joints or along a fault mirror when it is a normal fault, that is, when a part of the strata is elevated with respect to the same strata that remain relatively depressed on the other side of said fault mirror.
Joints can also occur in igneous or metamorphic rocks, as is the case of the polygonal joints of basaltic colonnades and in metamorphic rocks such as quartzite that appears associated with the sandstone sedimentary cover in the case of the Venezuelan tepuis.
Fossils
Being formed by sedimentary rocks, the strata usually contain fossils, that is, organic remains of animals and plants that serve to date the approximate age of said strata. This is done through the so-called index fossils, so called because they only existed in a certain Era or geological period. Obviously, finding said fossil in a stratum would serve to infer that said stratum was formed during the time in which the animal or plant existed. Dating is a very important application of stratigraphy. In the case of the sedimentary cover of the Roraima Formation of Venezuelan Guayana, which is formed by sandstone strata, there are no fossils of any type, which serves to date it as very old precisely due to this fact. It is more than 1,500 million years old, that is, they are sedimentary rocks prior to the Paleozoic Era, which is the era in which the first multicellular living beings are thought to have appeared. In this case, dating rocks from such strata requires dating methods of another type.
References
[1] ↑ a b Salvador, A. ed., 1994. International stratigraphic guide: a guide to stratigraphic classification, terminology, and procedure. 2nd ed. Boulder, Colorado, The Geological Society of America, Inc., 215 pp. ISBN 978-0-8137-5216-7.
[2] ↑ Neuendorf, K.K.E., Mehl, Jr., J.P., and Jackson, J.A. , eds., 2005. Glossary of Geology 5th ed. Alexandria, Virginia, American Geological Institute. 779 pp. ISBN 0-922152-76-4.
[3] ↑ Davies, N.S., y Shillito, A.P. 2021, True substrates: the exceptional resolution and unexceptional preservation of deep time snapshots on bedding surfaces. Sedimentology. publicado en línea el 22 de mayo de 2021, doi: 10.1111/sed.12900.
[4] ↑ Davies, N.S., y Shillito, A.P. 2018, Incomplete but intricately detailed: the inevitable preservation of true substrates in a time-deficient stratigraphic record. Geology, 46, 679-682.