Red beds
Encyclopedia
Red beds are sedimentary rocks, which typically consist of sandstone
, siltstone
, and shale
that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain thin beds of conglomerate
, marl
, limestone
, or some combination of these sedimentary rocks. The ferric oxides, which are responsible for the red color of red beds, typically occur as as coating on the grains of sediments comprising red beds. Classic examples of red beds are the Permian
and Triassic
strata of the western United States and the Devonian
Old Red Sandstone
facies
of Europe.
. Van Houten (1961) developed the idea to include the in situ (early diagenetic
) reddening of the sediment by the dehydration of brown or drab colored ferric hydroxides. These ferric hydroxides commonly include goethite
(FeO-OH) and so called "amorphous ferric hydroxide" or limonite
. In fact, much of this material may be the mineral ferrihydrite
(Fe2O3 H2O).
This dehydration or "aging" process is now known to be intimately associated with pedogenesis
in alluvial floodplain
s and desert
environments. Berner (1969) showed that goethite
(ferric hydroxide) is normally unstable relative to hematite
and in the absence of water or at elevated temperature will readily dehydrate according to the reaction:
Gibbs Free Energy
(G) is defined as some reactions are spontaneous because they give off energy in the form of heat (H
< 0). Others are spontaneous because they lead to an increase in the disorder of the system (S
> 0). Calculations of H and S can be used to probe the driving force behind a particular reaction. The Gibbs free energy of a system at any moment in time is defined as the enthalpy
of the system minus the product of the temperature times the entropy
of the system.
The Gibbs Free Energy for the reaction goethite ---> hematite (at 250°C) is -2.76kJ/mol and Langmuir (1971) showed that G becomes increasingly negative with smaller particle size. Thus detrital ferric hydroxides including goethite and ferrihydrite will spontaneously transform into red colored hematite pigment with time. This process not only accounts for the progressive reddening of alluvium but also the fact older desert dune sands are more intensely reddened than their younger equivalents.
silicates by oxygenated groundwater
s during burial. Walker’s studies show that the hydrolysis
of hornblende
and other iron-bearing detritus follows Goldich’s stability series. This is controlled by the Gibbs Free Energy of the particular reaction. For example, the most easily altered material would be olivine
: e.g.
A key feature of this process, and exemplified by the reaction, is the production of a suite of by products which are precipitated as authigenic
phases. These include mixed layer clays (illite
– montmorillonite
), quartz
, potassium feldspar and carbonates as well as the pigmentary ferric oxides. Reddening progresses as the diagenetic alteration becomes more advanced and is thus a time dependent mechanism. The other implication is that reddening of this type is not specific to a particular depositional environment
. However, the favourable conditions for diagenetic red bed formation i.e. +Eh
and neutral-alkaline pH
are most commonly found in hot, semi-arid areas, and this is why red beds are traditionally associated with such climates.
weathering
profiles. The color boundaries may cross-cut lithological contacts and show more intense reddening adjacent to unconformities. Johnson et al. (1997) have also showed how secondary reddening phases might be superimposed on earlier formed primary red beds in the Carboniferous
of the southern North Sea
. The general conditions leading to post-diagenetic alteration have been described by Mücke (1994). Important reactions include pyrite
oxidation:
and siderite
oxidation:
Secondary red beds formed in this way are an excellent example of telodiagenesis. They are linked to the uplift
, erosion and surface weathering of previously deposited sediments and require conditions similar to primary and diagenetic red beds for their formation.
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
, siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
, and shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain thin beds of conglomerate
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
, marl
Marl
Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...
, limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
, or some combination of these sedimentary rocks. The ferric oxides, which are responsible for the red color of red beds, typically occur as as coating on the grains of sediments comprising red beds. Classic examples of red beds are the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
and Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
strata of the western United States and the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
Old Red Sandstone
Old Red Sandstone
The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...
facies
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
of Europe.
Primary red beds
Krynine (1950) suggested that the red beds were primarily formed by the erosion and redeposition of red soils or older red beds. A fundamental problem with this hypothesis is the relative scarcity of Permian red colored source sediments to the south of CheshireCheshire
Cheshire is a ceremonial county in North West England. Cheshire's county town is the city of Chester, although its largest town is Warrington. Other major towns include Widnes, Congleton, Crewe, Ellesmere Port, Runcorn, Macclesfield, Winsford, Northwich, and Wilmslow...
. Van Houten (1961) developed the idea to include the in situ (early diagenetic
Diagenesis
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
) reddening of the sediment by the dehydration of brown or drab colored ferric hydroxides. These ferric hydroxides commonly include goethite
Goethite
Goethite , named after the German polymath Johann Wolfgang von Goethe, is an iron bearing oxide mineral found in soil and other low-temperature environments. Goethite has been well known since prehistoric times for its use as a pigment. Evidence has been found of its use in paint pigment samples...
(FeO-OH) and so called "amorphous ferric hydroxide" or limonite
Limonite
Limonite is an ore consisting in a mixture of hydrated iron oxide-hydroxide of varying composition. The generic formula is frequently written as FeO·nH2O, although this is not entirely accurate as limonite often contains a varying amount of oxide compared to hydroxide.Together with hematite, it has...
. In fact, much of this material may be the mineral ferrihydrite
Ferrihydrite
Ferrihydrite is a widespread hydrous ferric oxyhydroxide mineral at the Earth's surface, and a likely constituent in extraterrestrial materials. It forms in several types of environments, from freshwater to marine systems, aquifers to hydrothermal hot springs and scales, soils, and areas affected...
(Fe2O3 H2O).
This dehydration or "aging" process is now known to be intimately associated with pedogenesis
Pedogenesis
Pedogenesis is the science and study of the processes that lead to the formation of soil ' and first explored by the Russian geologist Vasily Dokuchaev , the so called grandfather of soil science, who determined that soil formed over time as a consequence of...
in alluvial floodplain
Floodplain
A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge...
s and desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
environments. Berner (1969) showed that goethite
Goethite
Goethite , named after the German polymath Johann Wolfgang von Goethe, is an iron bearing oxide mineral found in soil and other low-temperature environments. Goethite has been well known since prehistoric times for its use as a pigment. Evidence has been found of its use in paint pigment samples...
(ferric hydroxide) is normally unstable relative to hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
and in the absence of water or at elevated temperature will readily dehydrate according to the reaction:
- FeOOH (goethite)→ Fe2O3 (hematite) +H2O
Gibbs Free Energy
Gibbs free energy
In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...
(G) is defined as some reactions are spontaneous because they give off energy in the form of heat (H
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
< 0). Others are spontaneous because they lead to an increase in the disorder of the system (S
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
> 0). Calculations of H and S can be used to probe the driving force behind a particular reaction. The Gibbs free energy of a system at any moment in time is defined as the enthalpy
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
of the system minus the product of the temperature times the entropy
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
of the system.
The Gibbs Free Energy for the reaction goethite ---> hematite (at 250°C) is -2.76kJ/mol and Langmuir (1971) showed that G becomes increasingly negative with smaller particle size. Thus detrital ferric hydroxides including goethite and ferrihydrite will spontaneously transform into red colored hematite pigment with time. This process not only accounts for the progressive reddening of alluvium but also the fact older desert dune sands are more intensely reddened than their younger equivalents.
Diagenetic red beds
The formation of red beds during burial diagenesis was clearly described by Walker (1967) and Walker et al. (1978). The key to this mechanism is the intrastratal alteration of ferromagnesianMafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
silicates by oxygenated groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
s during burial. Walker’s studies show that the hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
of hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
and other iron-bearing detritus follows Goldich’s stability series. This is controlled by the Gibbs Free Energy of the particular reaction. For example, the most easily altered material would be olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
: e.g.
- Fe2SiO4 (fayalite) + O2 → Fe2O3 (hematite) + SiO2 (quartz) with E = -27.53kJ/mol
A key feature of this process, and exemplified by the reaction, is the production of a suite of by products which are precipitated as authigenic
Authigenic
An authigenic mineral or sedimentary rock deposit is one that was generated where it is found or observed. Authigenic sedimentary minerals form during sedimentation by precipitation or recrystallization instead of being transported from elsewhere by water or wind. Authigenic sediments are the main...
phases. These include mixed layer clays (illite
Illite
Illite is a non-expanding, clay-sized, micaceous mineral. Illite is a phyllosilicate or layered alumino-silicate. Its structure is constituted by the repetition of tetrahedron – octahedron – tetrahedron layers. The interlayer space is mainly occupied by poorly hydrated potassium cations...
– montmorillonite
Montmorillonite
Montmorillonite is a very soft phyllosilicate group of minerals that typically form in microscopic crystals, forming a clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite family, is a 2:1 clay, meaning that it has 2 tetrahedral sheets sandwiching a central...
), quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, potassium feldspar and carbonates as well as the pigmentary ferric oxides. Reddening progresses as the diagenetic alteration becomes more advanced and is thus a time dependent mechanism. The other implication is that reddening of this type is not specific to a particular depositional environment
Sedimentary depositional environment
In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock...
. However, the favourable conditions for diagenetic red bed formation i.e. +Eh
Eh
Eh is a spoken interjection in English, Armenian, Japanese, French, Italian, Greek, Spanish, Portuguese and Catalan that is similar in meaning to "Excuse me," "Please repeat that" or "huh?" It is also commonly used as a method for inciting an answer, as in "It's nice here, eh?" It is occasionally...
and neutral-alkaline pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
are most commonly found in hot, semi-arid areas, and this is why red beds are traditionally associated with such climates.
Secondary red beds
Secondary red beds are characterized by irregular color zonation, often related to sub-unconformityUnconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
profiles. The color boundaries may cross-cut lithological contacts and show more intense reddening adjacent to unconformities. Johnson et al. (1997) have also showed how secondary reddening phases might be superimposed on earlier formed primary red beds in the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
of the southern North Sea
North Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
. The general conditions leading to post-diagenetic alteration have been described by Mücke (1994). Important reactions include pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
oxidation:
- 3O2 + 4FeS2→ Fe2O3 (hematite) + 8S E = -789 kJ/mol
and siderite
Siderite
Siderite is a mineral composed of iron carbonate FeCO3. It takes its name from the Greek word σίδηρος sideros, “iron”. It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus...
oxidation:
- O2 + 4FeCO3 → 2Fe2O3 (hematite) + 4CO2 E = –346 kJ/mol
Secondary red beds formed in this way are an excellent example of telodiagenesis. They are linked to the uplift
Tectonic uplift
Tectonic uplift is a geological process most often caused by plate tectonics which increases elevation. The opposite of uplift is subsidence, which results in a decrease in elevation. Uplift may be orogenic or isostatic.-Orogenic uplift:...
, erosion and surface weathering of previously deposited sediments and require conditions similar to primary and diagenetic red beds for their formation.