Diagenetic concretions are masses of minerals formed by diagenesis (the process of sediment turning into sedimentary rock), and can be formed in situ or weathered out. They vary in morphology: they may have visible sedimentary laminations (fine discrete layers), and they can be spherical, discoidal, or elongate in shape. They do not displace sedimentary layers. Their interiors may be solid, rinded, or layered.
Spherical concretions can form in self-organized (non-random) or random spacing within the three dimensions of the host rock, and their spatial distributions are linked to how the spheroids formed.
Clusters of concretions (elongate along sedimentary laminae) can be biotic when nucleated on organics concentrated on bedding planes. Also, clusters of concretions are observed on organic nuclei such as preserved bones. Irregular shapes can also preserve biotic nuclei, such as entombing shells or other fossils.
PRO evidence supports hypotheses that the spheroids described could be formed by biotic processes vs. CON evidence that argues against them being made by biological processes.
Biological Signal Strength - Background
PRO evidence supports hypotheses that the spheroids described could be formed by abiotic processes vs. CON evidence that argues against them being made by abiotic processes.
Abiotic Prevalence - Background
PRO evidence supports hypotheses that spheroids could be generated abiotically in a given environment. CON evidence argues against the generation of abiologically formed spheroids.
Abiotic Signal Strength - Background
PRO evidence supports hypotheses that the abiotic spheroid spatial distributions described here could be preserved and detectable in a given environment vs. CON evidence that argues against the preservation and detectability of abiotic spheroid spatial distributions.
In mudrocks, where fluid flow (and thus reactant transfer) is restricted, spheroidal concretions can have diverse mineralogies between concretions within an outcrop and may be more likely to involve biomediated reactions. [Contributor: Sally Potter-McIntyre]
[Congruence, General]
Spheroidal concretions often result in biologically-involved precipitation [Sally Potter-McIntyre]
Spheroidal concretions, particularly in mudrocks, where fluid flow (and thus reactant transfer) is restricted, is much more likely to result in biologically-involved precipitation.
Evidence is Sourced from:
Title
The internal structure of carbonate concretions in mudrocks: a criticalevaluation of the conventional concentric model of concretion growth
Authors
Mozley, Peter S
Abstract
Carbonate concretions have traditionally been viewed as forming concentrically, through the progressive addition of carbonate to the outer edge of the growing concretion. This conventional model of concretion growth is based mainly upon center-to-edge textural and geochemical trends that are consistent with concentric growth. However, a number of recent studies of concretions in mudrocks have documented complex zonation that is not compatible with the conventional concentric model. In some cases complexly zoned concretions contain late-stage cements that precipitated throughout the concretion body. None of the center-to-edge relationships used to support the conventional concentric model exclude more complex modes of origin. As concretions that fit the conventional model may be the exception rather than the rule, high-resolution petrographic analysis of concretions should always be used to characterize internal structure and mode of growth prior to interpretation of geochemical and textural data.
Evidence for mudrock‐hosted carbonate concretions being biologically mediated [Contributor: Sally Potter-McIntyre]
Where fluid flow (and thus reactant transfer) is restricted, mudrock‐hosted carbonate concretions are much more likely to be biologically mediated (e.g., involving sulfate reduction).
Evidence is Sourced from:
Title
Mudrock-hosted carbonate concretions: a review of growth mechanisms andtheir influence on chemical and isotopic composition
Authors
Raiswell, R and Fisher, QJ
Abstract
Existing interpretations of cement textures and isotopic compositions may significantly under‐estimate the depth and duration of concretionary growth. Minus‐cement porosities can commonly under‐estimate depths of concretionary growth for some, or all, of the following reasons; (i) cements might not passively replace host sediment porosity, (ii) non‐cement carbonate phases (such as replaced bioclastic carbonate) can be significant, (iii) sediment compaction models over‐estimate rates of porosity loss at shallow (<500 m) depths and (iv) cementation can create a framework that prevents compaction and preserves porosity. Cement textures can be used to distinguish two modes of growth; concentric growth, where successive layers of cement are added to the outer surface (radius increases with time), and pervasive growth, where cement crystals grow simultaneously throughout the concretion volume (little or no radius increase with time). Cement textures of siderite concretions are mostly consistent with pervasive growth, but many calcite microsparite concretions show no diagnostic textural features and could grow either concentrically or pervasively. Concretionary cementation, whether concentric or pervasive, occurred such that there was accessible porosity which could be filled by later cements. Pervasive growth in particular is associated with the retention of substantial amounts of porosity which may be filled by chemically and isotopically distinct phases. The resulting chemical gradients across concretions may then reflect variations in the relative proportions of early and later cements more than variations in porewater composition.
Carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from −30‰ to +15‰, and thus overlaps 13C‐depleted values normally considered characteristic of sulphate reduction. Many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis, indicating a deeper and more prolonged cementation history. The necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters (the equilibrium model), or be generated in situ by organic matter decay (the local‐equilibrium model), or created where external fluids are introduced (the fluid‐mixing model).
Bioturbation influenced the morphology of concretions in the Morrison Formation. [Contributor: Sally Potter-McIntyre]
Bioturbation, among other diagenetic factors recognized, influenced concretion formation in Brushy Basin Member of the Jurassic Morrison Formation in the Four Corners regions of the U.S.
Evidence is Sourced from:
Title
Concretion formation in volcaniclastic host rocks: evaluating the roleof organics, mineralogy, and geochemistry on early diagenesis
Authors
Potter-McIntyre, SL and Chan, MA and McPherson, BJ
Abstract
The upper part of the Brushy Basin Member of the Jurassic Morrison Formation in the Four Corners regions of the U.S. was deposited in an ephemeral alkaline saline lake system with copious input of volcanic ash that resulted in dynamic fluid–rock interaction during early diagenesis. Three broad diagenetic facies—defined by color and associated bioturbation features—are interpreted: red, green, and intermediate. Diagenetic facies reflect meter-scale paleotopography: red facies represent shallow water to subaerial, oxidizing conditions; green facies reflect saturated conditions and reducing pore-water chemistry shortly after deposition, and intermediate facies represent a combination of—or rapid transition between—the previous two conditions. Three categories of concretions are characterized based on mineralogy: carbonate, iron (oxyhydr)oxide, and phosphate concretions. Variation in concretion mineralogy and morphology in the Brushy Basin Member suggests that alkaline saline fluid chemistries created diagenetic microenvironments within a larger lake system to influence concretion precipitation. However, porosity and permeability are more important factors affecting concretion size, morphology, and mineralogy than host-rock composition. Diagenetic fluid–rock interactions in volcaniclastic sediments can be spatially variable even on tens- of-centimeters scale.
Clusters of concretions can form on organic precursors such as bones, shells, or small masses of organic material. [Contributor: Sally Potter-McIntyre]
Reviewer Andy Czaja: " This argument seems to state that the features have a biological origin because they form on/around biological precursors. But the abiotic signal strength argument pointed out concretions that had self-organized spatial arrangement were abiological, even though they nucleated on organic material in mudrocks. This seems contradictory, unless I am missing something."
eConcentric growth occurs when a organic matter nucleates the precipitation reaction.
Concentric growth occurs when a organic matter nucleates the precipitation reaction.
Concentric growth occurs when a small piece of organic matter biogenically nucleates the precipitation reaction. The concretions grow radially with an increasing radius. Pervasive growth occurs as the crystals nucleate and grow concurrently throughout the body of the concretion. Pervasive growth is particularly common in siderite (iron carbonate) concretions. Pervasive growth concretions initially develop as plastic, porous, weakly cemented masses that become more rigid with further cementation.
Evidence is Sourced from:
Title
Characterization of Navajo Sandstone concretions: Mars comparison and criteria for distinguishing diagenetic origins
Authors
Potter, Sally L and Chan, Marjorie A and Petersen, Erich U and Dyar, M Darby and Sklute, Elizabeth
Abstract
The eolian Jurassic Navajo Sandstone spheroidal hydrous ferric oxide (HFO) concretions are divided into two size classes: macro-concretions of N 5 mm diameter and micro-concretions of b 5 mm diameter. Three internal structural end-members of macro-concretions are described as rind, layered, and solid. Two end-members of micro-concretions are rind and solid.
Chemical and mineralogical gradients (μm- to mm-scale) are identified with QEMSCAN (Quantitative Elemental Mineralogy using a SCANning electron microscope) and visible to near infrared (VNIR) reflectance spectroscopy. Three HFO phases are identified using VNIR reflectance spectroscopy.
i. An amorphous HFO phase is typically located in the rinds.
ii. Goethite is present along interior edges of rinds and throughout the interiors of layered and solid
concretions.
iii. Hematite is present in the centers of rind concretions.
A synthesis of petrographic, mineralogical and chemical analyses suggests that concretions grow pervasively (as opposed to radially expanding). Our model proposes that concretions precipitate initially as an amorphous HFO that sets the radius and retains some original porosity. Subsequent precipitation fills remaining pore space with younger mineral phases. Inward digitate cement crystal growth corroborates concretion growth from a set radius toward the centers. Internal structure is modified during late stage precipitation that diffuses reactants through semi-permeable rinds and overprints the interiors with younger cements. Physical characterization of textures and minerals provides diagnostic criteria for understanding how similar concretions (“blueberries”) form in Meridiani Planum, Mars. The analogous Navajo Sandstone concretions show similar characteristics of in situ self-organized spacing, spheroidal geometries, internal structures, conjoined forms, and precursor HFO phases that dehydrate to goethite or hematite. These characteristics indicate a common origin via groundwater diagenesis.
Carbonate concretions typically form around some nucleus (a shell fragment, organic matter or a grain).
Concretion size and spacing reflect host- rock characteristics as well as fluid chemistry, oxidation potential, pH, flow paths, and timing. The presence of bacteria can enhance the rate of precipitation. In some geological settings, carbonate concretions typically form around some nucleus (a shell fragment, organic matter or a grain). The lack of obvious macro nuclei in the Navajo concretions suggest a self- organized distribution for the spherical concretions, at some optimal nearest-neighbour spacing in zones along reaction fronts. Individual concretion populations are fairly consistent with similar size and spacing in a given portion of a reaction front.
Evidence is Sourced from:
Title
A possible terrestrial analogue for haematite concretions on Mars
Authors
Chan, Marjorie A and Beitler, Brenda and Parry, WT and Ormo, Jens and Komatsu, Goro
Abstract
Recent exploration has revealed extensive geological evidence for a water-rich past in the shallow subsurface of Mars. Images of in situ and loose accumulations of abundant, haematite-rich spherical balls from the Mars Exploration Rover ‘Opportunity’ landing site at Meridiani Planum1–3 bear a striking resemblance to diagenetic (post-depositional), haematite-cemented concre- tions found in the Jurassic Navajo Sandstone of southern Utah4,5. Here we compare the spherical concretions imaged on Mars to these terrestrial concretions, and investigate the implications for analogous groundwater-related formation mechanisms. The morphology, character and distribution of Navajo haematite concretions allow us to infer host-rock properties and fluid processes necessary for similar features to develop on Mars. We conclude that the formation of such spherical haematite concretions requires the presence of a permeable host rock, ground- water flow and a chemical reaction front.
Mudrock‐hosted carbonate concretions may form in the presence of organic material like soft body remains [Sally Potter-McIntyre]
Clusters of concretions can form on preserved soft body material or skeletal remains.
Evidence is Sourced from:
Title
Mudrock‐hosted carbonate concretions: a review of growth mechanisms and their influence on chemical and isotopic composition
Authors
Raiswell, R Fisher, QJ
Abstract
Existing interpretations of cement textures and isotopic compositions may significantly under‐estimate the depth and duration of concretionary growth. Minus‐cement porosities can commonly underestimate depths of concretionary growth for some, or all, of the following reasons; (i) cements might not passively replace host sediment porosity, (ii) non‐cement carbonate phases (such as replaced bioclastic carbonate) can be significant, (iii) sediment compaction models over‐estimate rates of porosity loss at shallow (<500 m) depths and (iv) cementation can create a framework that prevents compaction and preserves porosity. Cement textures can be used to distinguish two modes of growth; concentric growth, where successive layers of cement are added to the outer surface (radius increases with time), and pervasive growth, where cement crystals grow simultaneously throughout the concretion volume (little or no radius increase with time). Cement textures of siderite concretions are mostly consistent with pervasive growth, but many calcite microsparite concretions show no diagnostic textural features and could grow either concentrically or pervasively. Concretionary cementation, whether concentric or pervasive, occurred such that there was accessible porosity which could be filled by later cements. Pervasive growth in particular is associated with the retention of substantial amounts of porosity which may be filled by chemically and isotopically distinct phases. The resulting chemical gradients across concretions may then reflect variations in the relative proportions of early and later cements more than variations in porewater composition.
Carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from −30‰ to +15‰, and thus overlaps 13C‐depleted values normally considered characteristic of sulphate reduction. Many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis, indicating a deeper and more prolonged cementation history. The necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters (the equilibrium model), or be generated in situ by organic matter decay (the local‐equilibrium model), or created where external fluids are introduced (the fluid‐mixing model).
Concretions are easily preserved because of the preferential cementation that renders them harder than the surrounding host rock. If the host rock is sufficiently denuded, the concretions are typically still preserved and collected in topographic low spots. [Contributor: Sally Potter-McIntyre]
eIn lab studies of iron oxide concretion formation, the precipitation of hydrated iron sulfate, driven by the increasing pH, caused the greenish rind to become red and harder. [Sally Potter-McIntyre]
In lab studies of iron oxide concretion formation, the precipitation of hydrated iron sulfate, driven by the increasing pH, caused the greenish rind to become red and harder. [Sally Potter-McIntyre]
Laboratory tests with reactions of FeSO4 or Fe(NO3)3 with KOH were performed to show how the precipitation of hydrated iron sulfates or iron-hydroxides may form rinds around an initial, spherical source of iron (i.e. nucleation center). The precipitation of hydrated iron sulfate, driven by the increasing pH, caused the greenish rind to become red and harder. Fe ions from the iron source of the interior of the ‘concretion’ were thought to diffuse toward the rind, causing inward growth. As precipitation proceeded, the interior became depleted in green iron precipitate and the sand again became more visible. The experiment showed that variations in the amount of reactant, and the ability to seal off the fluids may be controlling factors in how concretions grow, including effects on their cementation and hardness.
Evidence is Sourced from:
Title
Models of iron oxide concretion formation: field, numerical, and laboratory comparisons
Authors
Chan, MA Ormö, Jens Park, AJ Stich, Michael SOUZA‐EGIPSY, Virginia Komatsu, G
Abstract
Well-exposed Jurassic Navajo Sandstone iron oxide concretions preserve important diagenetic records of groundwater flow and water–rock interactions. Field relationships, precipitation patterns, and geometries of the Navajo concretions provide the basis for input parameters in numerical computer simulations and laboratory chemical bench tests. Although field geometries are very difficult to replicate, numerical simulations and laboratory experiments examine end results such as nucleation and growth of iron oxide concretions, produced from known input
parameters. Three numerical simulations show the development of periodic self-organized nucleation centers through Liesegang-type double-diffusion of iron and oxygen. This numerical model simulates a scenario where oxygen is provided by shallow fresh water and iron is sourced from deeper reduced formation water. Concretions form in the region where the two waters interact with each other. Model sensitivities show that advection of water is an important mechanism for supplying the iron, and that acidic conditions in the iron-charged water can
cause iron to stay in solution longer to produce nucleation centers that are farther from the input source. Laboratory bench tests with reactions of FeSO4 or Fe(NO3)3 with KOH show how the precipitation of hydrated iron sulfates or iron-hydroxides may form rinds around an initial, spherical source of iron (i.e. nucleation center). These rinds may show inward growth depending on the concentration of the iron source in relation to the surrounding fluid. A number of complex factors such as concentration and flux, time, and multiple events can create banded patterns during rind growth. Comparisons of the terrestrial examples with numerical and laboratory models have strong implications for understanding similar hematite concretions on Mars.
eCopper Harbor Formation structures are examined and interpreted as abiotic concretions because biogenic origin hypothesis is rejected. Conglomerates examined accumulated in a relative topographic low area.
Copper Harbor Formation structures are examined and interpreted as abiotic concretions because biogenic origin hypothesis is rejected. Conglomerates examined accumulated in a relative topographic low area.
Given the morphological, mineralogical, distributional, and sedimentological evidence reported herein, the Copper Harbor structures are most parsimoniously interpreted as authigenic calcite concretions. A biogenic hypothesis for their origin is rejected. However, a concretionary origin for these structures is not wholly inconsistent with a biological precursor. The morphology of the structures is unusual for concretions. The valley in which the conglomerates examined accumulated formed as a relative topographic low within a generally elevated region.
Evidence is Sourced from:
Title
Macroscopic structures in the 1.1 Ga continental Copper HarborFormation: concretions or fossils?
Authors
Anderson, Ross P and Tarhan, Lidya G and Cummings, Katherine E andPlanavsky, Noah J and Bjørnerud, Marcia
Abstract
Continental siltstones of the Mesoproterozoic Copper Harbor Formation, Michigan contain
macroscopic structures of a size and morphological complexity commonly associated with fossils of eukaryotic macroorganisms. A biogenic origin for these structures would significantly augment the Proterozoic continental fossil record, which is currently poor, and also add to a growing body of sedimentological and geochemical data that, albeit indirectly, indicates the presence of life in continental settings early in Earth’s history. These three dimensional structures occur abundantly within a single cm-scale siltstone bed. Along this bedding plane, these structures are generally circular-to-ovoid, range up to several centimeters in diameter, and most specimens possess a transecting lenticular element. Structures exhibit sharp, well-rounded external margins and, in contrast to the surrounding aluminosilicate-rich matrix, are calcitic in composition. Surrounding sedimentary laminae are deflected by and cross cut the structures. A fossiliferous origin is considered but rejected and an authigenic concretionary origin is favored based on these characters. However, a concretionary origin does not exclude the possibility of a biogenic precursor that served as a locus for early diagenetic calcite precipitation. This study
highlights the need for careful analysis of morphological, mineralogical, distributional, and sedimentological characteristics when considering the origin of enigmatic structures; morphological complexity alone is an insufficient criterion for assignment of biogenicity. The unusual morphology of these concretions augments known concretion morphologies generally comparable to unusual fossil forms, and draws into question the biogenicity of similarly cryptic Proterozoic structures including, in particular, those of the 2.0 Ga Francevillian Formation of Gabon.
Self-organized spacing, measured as the distribution on two-dimensional surfaces of neighboring spheroids, is a common feature of abiotic spheroidal concretions. This is because they form in near stagnant water tables where precipitation is triggered by supersaturation within a pore space, where spacing is controlled by chemical self-organization, abundance of reactants, rate of reactant transfer, and time. [Contributor: Sally Potter-McIntyre]
[Congruence, General]
Navajo sandstone concretions display self-organization, attributed to abiotic reactions
Field observations and the lack of obvious macro nuclei in the Navajo concretions suggest a self- organized distribution for the spherical concretions, at some optimal nearest-neighbour spacing in zones along reaction fronts.
Evidence is Sourced from:
Title
Geochemical Self-Organization: Oxford University Press
Authors
Ortoleva, PJ
Abstract
This monograph offers an interdisciplinary approach to the analysis of geological systems which become spatially organized through the mediation of chemical processes. The treatment is based on a mathematical approach. The intended readership includes researcher and advanced undergraduate and graduate students in all branches of geology as well as scientists and mathematicians concerned with nonlinear dynamics, numerical analysis, self-organization, nonlinear waves and dynamics, and phase transition phenomena. The work could also serve as a basis for a special topics course in mathematics, chemistry or physics.
Abiotic models of end-members of macro- and micro-concretions are described [Sally Potter-McIntyre]
Self-organized spacing is a common feature of spheroidal concretions because they form in near stagnant water tables where precipitation is triggered by supersaturation within a pore space and spacing is controlled by chemical self-organization, abundance of reactants, rate of reactant transfer, and time.
Evidence is Sourced from:
Title
Characterization of Navajo Sandstone concretions; Mars comparison and criteria for distinguishing diagenetic origins
Authors
Potter, Sally L. Chan, Marjorie A. Petersen, Erich U. Dyar, M. Darby Sklute, Elizabeth
Abstract
The eolian Jurassic Navajo Sandstone spheroidal hydrous ferric oxide (HFO) concretions are divided into two size classes: macro-concretions of > 5 mm diameter and micro-concretions of < 5 mm diameter. Three internal structural end-members of macro-concretions are described as rind, layered, and solid. Two end-members of micro-concretions are rind and solid.
Chemical and mineralogical gradients (μm- to mm-scale) are identified with QEMSCAN (Quantitative Elemental Mineralogy using a SCANning electron microscope) and visible to near infrared (VNIR) reflectance spectroscopy. Three HFO phases are identified using VNIR reflectance spectroscopy.
i.
An amorphous HFO phase is typically located in the rinds.
ii.
Goethite is present along interior edges of rinds and throughout the interiors of layered and solid concretions.
iii.
Hematite is present in the centers of rind concretions.
A synthesis of petrographic, mineralogical and chemical analyses suggests that concretions grow pervasively (as opposed to radially expanding). Our model proposes that concretions precipitate initially as an amorphous HFO that sets the radius and retains some original porosity. Subsequent precipitation fills remaining pore space with younger mineral phases. Inward digitate cement crystal growth corroborates concretion growth from a set radius toward the centers. Internal structure is modified during late stage precipitation that diffuses reactants through semi-permeable rinds and overprints the interiors with younger cements.
Physical characterization of textures and minerals provides diagnostic criteria for understanding how similar concretions (“blueberries”) form in Meridiani Planum, Mars. The analogous Navajo Sandstone concretions show similar characteristics of in situ self-organized spacing, spheroidal geometries, internal structures, conjoined forms, and precursor HFO phases that dehydrate to goethite or hematite. These characteristics indicate a common origin via groundwater diagenesis.
Concretions studied showed evidence of periodic self-organization through diffusion of Fe and O. [Sally Potter-McIntyre]
Studies of Jurassic Navajo Sandstone iron oxide concretions resulted in numerical simulations showing the development of periodic self-organized nucleation centers through Liesegang-type double-diffusion of iron and oxygen. This numerical model simulates a scenario where oxygen is provided by shallow fresh water and iron is sourced from deeper reduced formation water.
Evidence is Sourced from:
Title
Models of iron oxide concretion formation: field, numerical, and laboratory comparisons
Authors
Chan, MA Ormö, Jens Park, AJ Stich, Michael SOUZA‐EGIPSY, Virginia Komatsu, G
Abstract
Well-exposed Jurassic Navajo Sandstone iron oxide concretions preserve important diagenetic records of ground-water flow and water–rock interactions. Field relationships, precipitation patterns, and geometries of the Navajo concretions provide the basis for input parameters in numerical computer simulations and laboratory chemical bench tests. Although field geometries are very difficult to replicate, numerical simulations and laboratory experi- ments examine end results such as nucleation and growth of iron oxide concretions, produced from known input parameters. Three numerical simulations show the development of periodic self-organized nucleation centers through Liesegang-type double-diffusion of iron and oxygen. This numerical model simulates a scenario where oxygen is provided by shallow fresh water and iron is sourced from deeper reduced formation water. Concretions form in the region where the two waters interact with each other. Model sensitivities show that advection of water is an important mechanism for supplying the iron, and that acidic conditions in the iron-charged water can cause iron to stay in solution longer to produce nucleation centers that are farther from the input source. Labor- atory bench tests with reactions of FeSO₄ or Fe(NO₃)₃ with KOH show how the precipitation of hydrated iron sulfates or iron-hydroxides may form rinds around an initial, spherical source of iron (i.e. nucleation center). These rinds may show inward growth depending on the concentration of the iron source in relation to the surrounding fluid. A number of complex factors such as concentration and flux, time, and multiple events can create banded patterns during rind growth. Comparisons of the terrestrial examples with numerical and laboratory models have strong implications for understanding similar hematite concretions on Mars.
With certain, more soluble, minerals, concretions may be dissolved while still in the subsurface, for example, carbonate concretions in an acidic fluid, complicating detection. [Contributor: Sally Potter-McIntyre]
eWith certain, more soluble, minerals, concretions may be dissolved while still in the subsurface, for example, abiotic carbonate concretions in an acidic fluid. [Sally Potter-McIntyre]
With certain, more soluble, minerals, concretions may be dissolved while still in the subsurface, for example, abiotic carbonate concretions in an acidic fluid. [Sally Potter-McIntyre]
Description of disolution of cements in subsurface. With certain, more soluble, minerals, concretions may be dissolved while still in the subsurface. One example is in a partly carbonate-cemented aquifer, the cooled formation waters are capable of dissolving the carbonate minerals and creating porosities that are higher than would otherwise be expected on the basis of burial depth.
Evidence is Sourced from:
Title
Secondary porosity: creation of enhanced porosities in the subsurface from the dissolution of carbonate cements as a result of cooling formation waters
Authors
Giles, MR De Boer, RB
Abstract
Theoretical calculations indicate that the cooling of pore waters initially in equilibrium with a carbonate mineral can result in the water becoming undersaturated with respect to that mineral. If the formation water has an initial pH below 6.9 then cooling will result in a water with a considerable potential to leach calcite. The temperature of formation waters may be forced to decrease where faults act as major vertical migration pathways for waters escaping from a compacting basin. Where faults cease to act as permeable pathways, the escaping formation water may be forced through an aquifer. In a partly carbonate-cemented aquifer the cooled formation waters are capable of dissolving the carbonate minerals and creating porosities that are higher than would otherwise be expected on the basis of burial depth.
Paleogeomorphology of the Ordovician Ordos Basin is described and concretions are noted.
After leaching and denudation in the weathering crust period, widespread distribution of the weathering crust reservoirs developed. Due to later denudation, many dolomite outcrops were still exposed. The Lower Ordovician Majiagou Formation in is composed of six members, at least one of which contains anhydrite concretions. In the Ordovician-Carboniferous, the Ordos Basin was uplifted and denuded as a whole due to the Caledonian and Early Hercynian movements.
Evidence is Sourced from:
Title
Paleogeomorphy evolution of the Ordovician weathering crust and its implication for reservoir development, eastern Ordos Basin
Authors
Wei, Xinshan and Ren, Junfeng and Zhao, Junxing and Zhang, Daofeng and Luo, Shunshe and Wei, Liubin and Chen, Juanping
Abstract
Gas reservoir development of the Ordovician weathering crust in the Ordos Basin is closely controlled by the pre-Carboniferous paleogeomorphy. Previous studies show that the paleogeomorphy is high in the west and low in the east, and the karst highland, karst slope and karst basin are developed from west to east. With further exploration in recent years, many karst breccia that represent strong karstification, are found in the east area which previously is considered to be the Ordovician karst basin. Thus, it is necessary to revaluate controlling factors of karst paleogeomorphy development from a viewpoint of the dynamic paleogeomorphy evolution, to investigate the paleogeomorphy evolution of the Ordovician weathering crust in geological history and guide further research and prediction of development law of reservoir spaces. In order to reconstruct the paleogeomorphy of the weathering crust in the top of Ordovician in the east Ordos Basin, paleogeography, thickness of residual strata and paleokarst characteristics are well studied. The result shows that a wide range of paleokarst highland occurred in the central to east part of Ordos Basin in the early period, and the karst reservoir spaces were formed; but in the late period, the east part of the basin gradually evolved into the paleokarst basin, the current pre-Carboniferous paleogeomorphy was thus formed, and the dissolution reservoir spaces formed in the early period were mostly filled, accordingly the reservoirs were tight. However fracture networks formed by cave collapse connect intercrystalline pores of dolomite, therefore, the reservoirs can still be well preserved locally and are worthy targets for hydrocarbon exploration.
In self-organized spheroidal concretions, which are formed abiotically, spacing is random and no obvious biosignatures are preserved. [Contributor: Sally Potter-McIntyre]
From reviewer Hank Rainwater: "I assume that self-organization leads to random spacing, but does this imply that spacing other than random is controlled by some other process that is relevant to the argument?"
eLack of obvious self-organized spacing suggests a random nucleation on organics which provide kinetically favorable nucleation sites. Self-organized nucleation occurs via diffusion.
Lack of obvious self-organized spacing suggests a random nucleation on organics which provide kinetically favorable nucleation sites. Self-organized nucleation occurs via diffusion.
Field samples studied here indicate that the lack of obvious self-organized spacing suggests a random nucleation phenomenon possibly due to the presence of organic material providing kinetically favorable sites (which could be randomly spaced in a detrital setting). Self- organized spacing is typical of some pervasive growth concretions where a nucleation reaction sets off a precipitation pattern that is self-organizing and size and spacing is controlled by reactant supply (references within). This self-organized nucleation occurs via Liesegang-type diffusion (see references within). Additionally, the differences in mineralogy yet chemical similarity of concretions suggest that diagenetic microenvironments are being affected by different chemical fluids and therefore precipitate different concretion mineralogies. These microenvironments may or may not be influenced by organics. In contrast, self-organized, abiotic concretions (e.g., Navajo Sandstone) exhibit uniform mineralogies.
Evidence is Sourced from:
Title
Concretion formation in volcaniclastic host rocks: evaluating the roleof organics, mineralogy, and geochemistry on early diagenesis
Authors
Potter-McIntyre, SL and Chan, MA and McPherson, BJ
Abstract
The upper part of the Brushy Basin Member of the Jurassic Morrison Formation in the Four Corners regions of the U.S. was deposited in an ephemeral alkaline saline lake system with copious input of volcanic ash that resulted in dynamic fluid–rock interaction during early diagenesis. Three broad diagenetic facies—defined by color and associated bioturbation features—are interpreted: red, green, and intermediate. Diagenetic facies reflect meter-scale paleotopography: red facies represent shallow water to subaerial, oxidizing conditions; green facies reflect saturated conditions and reducing pore-water chemistry shortly after deposition, and intermediate facies represent a combination of—or rapid transition between—the previous two conditions. Three categories of concretions are characterized based on mineralogy: carbonate, iron (oxyhydr)oxide, and phosphate concretions. Variation in concretion mineralogy and morphology in the Brushy Basin Member suggests that alkaline saline fluid chemistries created diagenetic microenvironments within a larger lake system to influence concretion precipitation. However, porosity and permeability are more important factors affecting concretion size, morphology, and mineralogy than host-rock composition. Diagenetic fluid–rock interactions in volcaniclastic sediments can be spatially variable even on tens- of-centimeters scale.
Certain soluble, minerals, concretions may be dissolved while still in the subsurface. Carbonate concretions in an acidic fluid are exemplified here.
Description of disolution of cements in subsurface. With certain, more soluble, minerals, concretions may be dissolved while still in the subsurface. One example is in a partly carbonate-cemented aquifer, the cooled formation waters are capable of dissolving the carbonate minerals and creating porosities that are higher than would otherwise be expected on the basis of burial depth.
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Title
Secondary porosity: creation of enhanced porosities in the subsurfacefrom the dissolution of carbonate cements as a result of coolingformation waters
Authors
Giles, MR and De Boer, RB
Abstract
Theoretical calculations indicate that the cooling of pore waters initially in equilibrium with a carbonate mineral can result in the water becoming undersaturated with respect to that mineral. If the formation water has an initial pH below 6.9 then cooling will result in a water with a considerable potential to leach calcite. The temperature of formation waters may be forced to decrease where faults act as major vertical migration pathways for waters escaping from a compacting basin. Where faults cease to act as permeable pathways, the escaping formation water may be forced through an aquifer. In a partly carbonate-cemented aquifer the cooled formation waters are capable of dissolving the carbonate minerals and creating porosities that are higher than would otherwise be expected on the basis of burial depth.
Spheroidal diagenetic concretions are preferentially cemented mineral masses formed in the subsurface in sedimentary rocks. They range in size from sub-mm to meters in diameter. Concretions can form in many different morphologies, mineralogies, and substrates (e.g., sandstone, limestones, shales); however, this entry focuses on spheroidal morphologies.
Spheroidal concretions form in near stagnant water tables from diffusive transfer of reactants. The spheroidal form indicates formation in a static pore fluid, which causes symmetric forms (Parnell et al 2016). Concretions can exhibit self-organized (Fig. 1; non-random) or random spacing in three dimensions within the host rock.
Their spatial distribution is related to how the spheroids were formed. For example, chemical self-organization can lead to a random distribution of spheroids, or specific precipitation or chemical conditions can lead to more organized spheroids. Meanwhile, biotically-associated concretions [read more]Spheroidal diagenetic concretions are preferentially cemented mineral masses formed in the subsurface in sedimentary rocks. They range in size from sub-mm to meters in diameter. Concretions can form in many different morphologies, mineralogies, and substrates (e.g., sandstone, limestones, shales); however, this entry focuses on spheroidal morphologies.
Spheroidal concretions form in near stagnant water tables from diffusive transfer of reactants. The spheroidal form indicates formation in a static pore fluid, which causes symmetric forms (Parnell et al 2016). Concretions can exhibit self-organized (Fig. 1; non-random) or random spacing in three dimensions within the host rock.
Their spatial distribution is related to how the spheroids were formed. For example, chemical self-organization can lead to a random distribution of spheroids, or specific precipitation or chemical conditions can lead to more organized spheroids. Meanwhile, biotically-associated concretions can form in conjunction with organic precursors (e.g. bones, shells), creating clusters of concretions. Organic matter may be preserved within the concretions (e.g., as the nucleation point), offering insight into potential biogenicity (Fig. 2). However, not all concretions have obvious nuclei. Where iron reduction and spheroid formation occurs around detrital organic matter debris, spheroidal forms may not be present (Parnell et al. 2016).
Multiple factors affect concretion preservation. Concretions can be altered both mineralogically and structurally by generations of fluids with varying chemistries throughout diagenesis. Precipitation events can act as reactants and cause diffusion through higher initial porosity rinds to structurally alter interiors.
The most common concretions are carbonate precipitates. Carbonate concretions commonly nucleate on biofilms or exopolymeric substances (EPS) produced by microbes (Potter-McIntyre et al 2014). Two models for carbonate concretion growth are concentric growth (when a small piece of organic matter biogenically nucleates the precipitation reaction) and pervasive growth (crystals nucleate and grow concurrently throughout the body of the concretion; Mozley et al., 1996; Potter et al., 2011). Pervasive growth is thought to be caused by localized microbial activity associated with concentrations of organics in mudrocks and sandstone (Raiswell and Fisher 2000; Mozley and Davis, 2005).
[Contributors: Sally Potter-McIntyre, Jordan McKaig, Svetlana Shkolyar]
Figure 1: Example of abiotic calcite concretions. Calcite concretions are from Valley of Fire State Park, Nevada, USA, and exhibit a non-random spacing that is induced by geochemical self-organization (likely abiotic). Note that the larger concretions are spaced farther apart than the smaller ones because spacing is controlled by the reactants within a sphere surrounding each concretion. Image copyright: S. Potter-McIntyre.
Figure 2: Images of spheroids thought to have formed as a result of microbial reduction of iron (i.e., associated with biological activity). This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction. No changes were made to the original image identified as Figure 1 in Parnell et al. 2016, https://doi.org/10.1007/s11084-015-9466-x.
