Fact-checked by Grok 2 weeks ago

Ooid

An ooid is a small, spherical to ellipsoidal sedimentary grain, typically ranging from 0.25 to 2 mm in diameter, composed of a central nucleus—such as a detrital grain, skeletal fragment, or pellet—surrounded by one or more concentric layers of precipitated calcium carbonate, primarily in the form of aragonite or calcite.^[1]^[2] These grains exhibit a laminated internal structure, with cortical layers that may include organic material, and are distinguished from larger coated grains like pisoids by their size and typically more uniform sphericity.^[1]^[3] Ooids form through a process of incremental precipitation in high-energy, shallow marine environments, where constant agitation from waves, tides, or currents causes the nucleus to roll repeatedly, allowing successive thin layers of carbonate to accrete on its surface.^[3]^[2] This occurs predominantly in warm, tropical waters with supersaturated calcium carbonate conditions, such as those on platforms like the Bahamas Banks or the Cameroon shelf, though ooids can also develop in lacustrine, terrestrial, or deeper shelf settings during periods of low sea level.^[2] Modern ooids are mostly aragonitic, while ancient examples often preserve as calcite or magnesian calcite due to diagenetic alteration; their growth is episodic, driven by short bursts of rapid precipitation rather than continuous abrasion.^[2]^[4] Geologically, ooids are a key component of oolitic limestones and grainstones, serving as important non-skeletal grains that record past seawater chemistry, alkalinity, and hydrodynamic conditions.^[4]^[1] They have been preserved in the rock record since the Neoarchean, approximately 2.5 billion years ago, with abundance peaking in the early Paleozoic and varying through the Phanerozoic in response to global climatic and sea-level changes.^[5] In reservoir geology, ooid grainstones exhibit high porosity (up to 50%), making them significant hydrocarbon reservoirs, such as in Upper Jurassic formations.^[2] Ooid size and distribution further proxy ancient environmental stresses, like elevated alkalinity during glacial periods or the Early Triassic.^[4]

Definition and Characteristics

Definition

An ooid is a small, typically ≤2 mm in diameter, spheroidal to subspherical coated sedimentary grain composed primarily of concentric layers of calcium carbonate, either aragonite or calcite, surrounding a central nucleus.^[6]^[7] These grains form through accretionary processes in marine or lacustrine environments, resulting in a layered structure that distinguishes them from other allochems.^[1] Ooids are differentiated from similar coated grains such as pisoids, which exceed 2 mm in diameter and often exhibit more irregular concentric layering, and oncoids, which are subspherical to irregular in shape with asymmetric, microbially influenced laminations.^[8]^[9] When ooids are cemented together by calcium carbonate, they produce oolitic limestone, a textured rock characterized by the visible spherical grains embedded in a finer matrix.^[10] The term "ooid" derives from the Greek word for egg-shaped, reflecting their characteristic form, and these grains were first systematically described in the 19th century, with early detailed accounts provided by Charles Lyell in 1855 on the structure of oolitic limestones.^[11] Ooids play a key role in sedimentary geology as indicators of shallow, agitated carbonate depositional environments, contributing to the formation of significant limestone deposits throughout geological history.^[4]

Physical and Chemical Properties

Ooids typically range in size from 0.25 to 2 mm in diameter, though rare giant ooids can reach up to 35 mm, as documented in middle Cambrian deposits on the North China Platform.^[12] Their shape is generally smooth and rounded or subspherical, resulting from repeated abrasion and accretion that promotes a uniform exterior.^[2] Internally, ooids exhibit a concentric structure composed of 5 to 20 laminae surrounding a central nucleus, with each lamina typically 5 to 50 μm thick; these layers alternate in texture, often showing micritic and sparitic bands that trap minor detrital particles.^[13]^[14] Chemically, modern ooids are primarily composed of aragonite (CaCO₃), while ancient ooids are predominantly calcite due to diagenetic alteration, with both polymorphs incorporating minor elements such as strontium (Sr) and magnesium (Mg) through substitution in the crystal lattice—aragonite ooids often show elevated Sr/Ca ratios up to 1–2 mmol/mol.^[15]^[16] The density of ooids is approximately 2.7–2.8 g/cm³, intermediate between pure calcite (2.71 g/cm³) and aragonite (2.93 g/cm³), reflecting their mineralogy and minor inclusions.^[15] Porosity within individual ooids is generally low (less than 5%), but it is influenced by the cortical fabric, with tangential arrangements yielding denser packing and radial fabrics allowing slightly higher intercrystalline voids.^[2] Under optical microscopy in thin sections, ooid cortices display distinct fabrics: tangential fabrics consist of equant, randomly oriented crystals that appear mosaic-like under crossed polars, while radial fabrics feature elongated fibers or needles radiating from layer boundaries, producing characteristic extinction crosses when sectioned through the center.^[17]^[18] These properties aid in identifying original mineralogy and growth conditions, with radial fabrics more common in low-energy settings.^[19]

Formation and Growth

Nucleus and Initial Formation

The nucleus of an ooid acts as the foundational core for subsequent concentric layering, typically comprising either biogenic or lithic materials. Biogenic nuclei include shell fragments, foraminifera tests, mollusc fragments, and fecal pellets, while lithic nuclei consist of quartz grains or peloids.^[20] These nuclei generally measure 0.1–0.5 mm in diameter, with modern examples predominantly ranging from 0.25 to 0.5 mm.^[20] Initial coating occurs through the precipitation of the first calcium carbonate lamina onto the nucleus in supersaturated seawater, most commonly within shallow marine environments. This onset of formation relies on nucleation sites along the irregular surfaces of the nucleus, where elevated carbonate ion concentrations promote mineral deposition.^[20] Agitation in these settings aids the initial rolling of the nucleus, exposing fresh surfaces for coating.^[21] In modern Bahamian ooids from Joulters Cays, nuclei are chiefly peloids and fecal pellets derived from local carbonate sediments.^[21] Conversely, Jurassic oolites, such as those in the Middle Jurassic Great Oolite (Bathonian), frequently exhibit algal nuclei, including fragments of red, green, and blue-green algae that provided suitable cores for early lamina development.^[22]

Growth Mechanisms

Ooids accrete concentric layers through repeated cycles of chemical precipitation, mechanical rolling, and minor abrasion within agitated shallow-water environments. This process involves the nucleus or prior layers serving as substrates for new mineral deposition, followed by gentle tumbling that redistributes growth sites and removes irregularities, resulting in the formation of each cortical lamina over short timescales of days to weeks.^[13]^[23] The underlying chemical mechanism driving this accretion is the supersaturation of seawater with respect to calcium carbonate, primarily from elevated concentrations of Ca²⁺ and HCO₃⁻ ions, which promotes the precipitation of CaCO₃ onto the ooid surface. This reaction proceeds as follows:

\text{Ca}^{2+} + 2\text{HCO}_3^- \rightarrow \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O}

Precipitation occurs episodically during periods of transport, with each layer reflecting net growth after dissolution or minor removal.^[24] Mechanical rolling in wave- or current-agitated waters ensures uniform layer addition by preventing preferential overgrowth on any single side, while abrasion contributes to rounding and sphericity by smoothing protrusions. However, abrasion plays only a minimal role overall, particularly in the development of giant ooids where increased mass would theoretically amplify wear but does not significantly alter morphology.^[23]

Growth Modes

Ooid cortices exhibit distinct internal fabric patterns that reflect the crystallographic orientation of mineral crystals during incremental growth around the nucleus. These fabrics, observed through thin-section petrography or scanning electron microscopy, provide insights into the depositional dynamics and mineralogical composition influencing ooid development. The primary growth modes—radial, tangential, and micritic—dominate the literature on ooid microstructures, with variations arising from precipitation kinetics and environmental perturbations.^[25] In the radial mode, crystals precipitate perpendicular to the nucleus or preceding laminae surfaces, resulting in elongated, fibrous calcite structures that extend outward like lattice extensions from a central point. This fabric is characterized by concentric banding where each layer maintains radial symmetry, often with thicknesses varying due to periodic growth interruptions. Radial fabrics are prevalent in ancient calcitic ooids and modern examples from low-energy lacustrine settings, such as those in the Great Salt Lake, where minimal agitation allows unimpeded normal growth without significant abrasion.^[25]^[26]^[19] The tangential mode features needle-like or elongated aragonite crystals oriented parallel to the lamellar surfaces, forming through the accretion of crystal clusters or sprays that align tangentially during deposition. This results in a mosaic of prismatic grains with little radial continuity, often producing a more porous cortex compared to radial structures. Tangential fabrics dominate in modern marine aragonitic ooids from high-energy environments, such as Bahamian platforms, where wave agitation promotes repeated rolling and selective preservation of surface-parallel precipitates.^[27]^[19]^[26] Micritic mode involves the formation of fine-grained, cryptocrystalline envelopes composed of submicron carbonate particles, arising from rapid precipitation of amorphous precursors or post-depositional micritization via microbial degradation of organic matrices. These envelopes appear as dense, homogeneous layers in cross-sections, sometimes incorporating borings or fossil inclusions that enhance preservation, as documented in Jurassic ooids where ooimmuration encapsulates microfossils within micritic cortices. Micritic fabrics are common in both ancient and modern ooids influenced by organic mediation, contrasting with the coarser crystalline modes by their isotropic texture and higher susceptibility to diagenetic alteration.^[28]^[29]^[30] Transitions between growth modes are evident in many ooid cross-sections, where abrupt shifts from radial to tangential or micritic fabrics record changes in local conditions, such as fluctuations in water energy or chemistry during cortex accretion. For instance, initial radial growth may give way to tangential fabrics under increasing agitation, highlighting the sensitivity of fabric development to hydrodynamic regimes.^[28]^[19]

Influencing Factors

Environmental Conditions

Ooid formation requires waters highly supersaturated with respect to carbonate minerals, primarily driven by evaporation in warm, shallow tropical seas that concentrate ions and promote precipitation of aragonite or high-Mg calcite cortices around nuclei.^[31] Optimal conditions include temperatures of 20–30°C, which enhance kinetic rates of mineral precipitation, and salinities ranging from 35 to 45 ppt, reflecting normal marine to slightly hypersaline settings where evaporation exceeds freshwater input.^[32] Additionally, a pH of 8–9 favors aragonite precipitation by increasing the carbonate saturation state (Ω_aragonite > 4), often achieved through local degassing of CO₂ or reduced acidity in these environments.^[33] Hydrodynamic conditions are crucial, with moderate agitation from waves or tides providing the energy needed to continuously roll ooids, allowing concentric layering without excessive abrasion or burial. This agitation maintains ooids in suspension above the seafloor, promoting uniform growth, but must be balanced to avoid fragmentation; excessive energy can limit size by increasing abrasion rates.^[34] Classic examples include the ooid shoals of the Trucial Coast in the Persian Gulf, where tidal currents and waves in water depths of 1–5 m drive formation in a semi-enclosed, evaporative basin, and the Great Salt Lake in Utah, where wind-generated waves agitate shallow (0.5–2 m) margins to produce intraclast-nucleated ooids.^[35]^[36] Ooids typically develop in restricted platforms or lagoons at depths less than 10 m, where wave base intersects the seafloor and limits dilution by open-ocean waters, concentrating supersaturated fluids.^[37] These low-energy to moderate-energy subenvironments, often in tropical latitudes (20–30°N/S), facilitate the necessary geochemical gradients while protecting ooids from rapid transport into deeper waters.^[4] Recent research highlights how variations in seawater chemistry serve as proxies for ooid characteristics, with a 2022 study demonstrating that ooid size can be used to reconstruct Phanerozoic seawater carbonate mineral saturation state (Ω), enabling insights into ancient ocean conditions from ooid deposits.^[15]

Biological Influences

Biological influences play a significant role in ooid development, particularly through microbial communities that mediate mineral precipitation and structural modification. Bacteria and algae within ooid cortices promote carbonate precipitation by producing extracellular polymeric substances (EPS), which serve as organic scaffolds for the nucleation of amorphous calcium carbonate (ACC) that subsequently transforms into crystalline aragonite. In Bahamian ooids, scanning electron microscopy reveals biofilms composed of bacteria, cyanobacteria, diatoms, and fungi, with EPS exudates facilitating the deposition of nanograins (20-150 nm) around microbial cells, as evidenced by nuclear magnetic resonance spectroscopy detecting ACC signatures.^[38] Recent analyses of ooids from the Great Salt Lake and Triassic Germanic Basin further confirm the presence of organic matter, including microbial-derived EPS, embedded in cortices, underscoring the biotic enhancement of precipitation processes. Bioerosion by endolithic organisms further shapes ooid fabric, creating micritic envelopes that influence cortex integrity and growth patterns. In Bahamian ooids from the Great Bahama Bank, boring algae, cyanobacteria, and fungi produce microborings up to 50 μm deep, such as ichnotaxa like Fascichnus and Saccomorpha, which micritize the cortex during resting or burial stages when light penetration is limited.^[39] Classic studies on Holocene ooids demonstrate that these euendolithic microbes form micrite envelopes by infilling borings with fine carbonate particles, altering the originally aragonitic laminae and promoting tangential crystal fabrics, as observed via scanning electron microscopy. Skeletal debris from marine organisms commonly serves as the initial nucleus for ooid formation, providing a biogenic substrate that initiates concentric layering. Bioclasts, such as fragments of foraminifera, mollusks, or other calcareous skeletons, act as nucleation sites in shallow marine environments, where they are coated by precipitated carbonate layers.^[40] This biotic contribution to ooid initiation is widespread, with reviews noting that skeletal material commonly comprises ooid nuclei, facilitating the overall organomineralization process.^[40] Modern examples illustrate these biological influences, such as in Shark Bay, Western Australia, where cyanobacterial mats in hypersaline settings accelerate ooid growth through organic-rich cortices. Ooids from Carbla Beach exhibit bimineralic (aragonite-magnesite) laminae with embedded microbial filaments and EPS, promoting rapid precipitation in mat-covered subtidal sands, as revealed by synchrotron X-ray fluorescence mapping of organic biomarkers. These mats enhance accretion rates by stabilizing grains and inducing localized supersaturation, contrasting with purely abiotic environments.

Variations and Types

Size and Shape Variations

Ooids exhibit a wide range of sizes, with some as small as 0.1 mm in diameter to standard forms between 0.25 mm and 2 mm, with rare giant ooids exceeding 10 mm. At the upper extreme, giant ooids can exceed 35 mm in diameter, as documented in a 2025 study of exceptional middle Cambrian examples from the North China Platform, where they formed in a storm-influenced coastal environment facilitated by microbial mats and organomineralization under high temperatures and carbonate saturation.^[12] Deviations from the ideal spherical morphology occur in various forms, including ellipsoidal and irregular shapes, which arise primarily from insufficient agitation during growth, leading to uneven cortical layering.^[4] Compound ooids, resembling pisoid-like clusters, develop when multiple nuclei aggregate or when secondary coatings form around existing grains, resulting in clustered or botryoidal structures that deviate from simple concentric forms.^[41] Geometric models of ooid growth describe the evolution of these shapes through iterative processes of precipitation and erosion, predicting equilibrium morphologies based on balanced accretion and abrasion rates.^[23] A key metric in these models is the sphericity index, defined as S = 4\pi A / P^2, where A is the projected area and P is the perimeter of the ooid in two dimensions; values approaching 1 indicate near-perfect sphericity, while lower values reflect elongation or irregularity.^[23] This index quantifies how growth dynamics lead to time-invariant shapes under constant environmental forcing.^[42] Larger ooid sizes, including giant variants, are associated with high-supersaturation conditions that accelerate precipitation rates relative to abrasion, allowing for extended growth before equilibrium is reached.^[43]

Compositional Variations

Ooids display notable compositional variations in mineralogy and chemistry, driven by depositional environments, seawater chemistry, and diagenetic processes. These differences distinguish carbonate-dominated types from rarer non-carbonate variants and highlight post-formational alterations. Among carbonate ooids, mineralogy shifts between aragonite and calcite, often tied to the strontium (Sr) content. Modern marine ooids primarily precipitate as aragonite, incorporating elevated Sr levels (typically 1000–5000 ppm) due to its orthorhombic crystal structure that favors Sr substitution.^[44] In ancient settings, such as Neoproterozoic ooids from the Libby Formation, primary aragonite precursors exhibit Sr concentrations of 2400–2900 ppm, whereas preserved calcite forms show lower Sr, reflecting either direct precipitation in calcite seas or diagenetic stabilization.^[44]^[45] Dolomitic ooids, uncommon in marine contexts, form in evaporative lacustrine environments through microbial mediation. In the Pliocene Shizigou Formation of China's Qaidam Basin, these ooids consist of micritic dolomite cortices (~50.5 mol% Mg) around nuclei of dolomite, clay, or quartz, developed in brackish lakes with high evaporation rates exceeding precipitation by over 20 times annually.^[46] Non-carbonate ooids occur in specialized settings and include phosphatic, iron oxide, and siliceous types. Phosphatic ooids, composed of apatite-rich grains like pellets and ooids, dominate in phosphorite deposits, such as Late Cretaceous reworked phosphorites where they form medium- to coarse-grained phosphate components up to 35 wt% P₂O₅.^[47] Iron oxide ooids, primarily goethite (FeO(OH)) with ~80.5 wt% FeO, precipitate around volcanic or biogenic nuclei in hydrothermal vents, as seen in modern Aeolian Arc examples; these serve as analogues for ancient oolitic ironstones in Precambrian banded iron formations.^[48] Siliceous ooids are rare, typically associated with iron oxides in exhalative fluid systems, containing up to 11.5 wt% SiO₂ in their cortices.^[48] Stable isotopic compositions, particularly δ¹³C and δ¹⁸O, in ooid cortices record seawater chemistry variations. Modern aragonitic ooids from marine settings equilibrate with surface seawater, yielding δ¹³C values of ~3-5‰ and δ¹⁸O of ~ -1 to +1‰, while values in hypersaline lakes show evaporative enrichment in δ¹⁸O; Proterozoic ooids show depleted δ¹⁸O (down to -9.75‰) indicative of lower seawater δ¹⁸O.^[49] These signatures persist in low-abrasion growth regimes, as demonstrated in 2024 analyses of Bahamian ooids preserving primary isotopic equilibrium with δ¹³C of 4.7-5.2‰ and δ¹⁸O of -0.5 to 0.6‰.^[50] Diagenetic neomorphism commonly alters ooid composition, converting aragonite to calcite via intrafabric dissolution and reprecipitation along thin solution films. In fossil ooids, this process reduces Sr content and forms blocky calcite fabrics, often over millennia.^[51]^[52] For example, Great Salt Lake ooids undergo neomorphic replacement of primary aragonite (>90%) with coarser radial calcite or Mg-phases, introducing minor dolomite enclaves (<5 μm) without fully erasing microbial influences.^[53]

Geological Significance

Occurrence in the Rock Record

Ooids have been documented in the geological record for approximately 2.9 billion years, with the oldest known occurrences in the Neoarchean (~2.9 Ga) Pongola Supergroup of South Africa.^[54]^[55] Throughout the Phanerozoic Eon, ooids exhibit a dominance in marine sedimentary sequences, with abundance peaks during intervals of "calcite seas"—periods characterized by seawater chemistry favoring low-magnesium calcite precipitation—particularly in the Ordovician, Jurassic, and Cretaceous.^[54]^[56] These peaks reflect elevated carbonate supersaturation and extensive shallow-marine carbonate platform development during times of high global sea level and tropical shelf expansion.^[54] Contemporary analogs for ooid formation occur in shallow, agitated subtropical to tropical marine environments, providing insights into ancient processes. In the Bahamas, the Joulters Cays on the Great Bahama Bank represent a prolific site of ooid sand accumulation across expansive shoals covering hundreds of square kilometers.^[57] Similar formations are observed along the Trucial Coast (modern-day United Arab Emirates) in the Persian Gulf, where ooid sands accumulate in high-energy tidal zones.^[58] In Shark Bay, Western Australia, organic-rich bimineralic ooids form in hypersaline, microbially influenced subtidal settings.^[59] Stratigraphically, ooids are prominent in oolitic limestones of various formations, such as the Eocene Green River Formation in the Uinta Basin of Utah and Colorado, where they occur in lacustrine nearshore deposits of the Douglas Creek Member.^[60] In the Middle Jurassic Carmel Formation of southwestern Utah, ooids characterize high-energy shoals and associated hardgrounds in a restricted carbonate lagoon.^[61] Overall, ooids are primarily distributed in tropical shallow-water carbonate systems, with rare occurrences in deep-sea settings due to the requirement for agitation and supersaturation; non-marine examples, such as those in Great Salt Lake, Utah, are exceptional and tied to hypersaline lake margins.^[62]^[53]

Paleoenvironmental and Economic Importance

Ooids serve as valuable proxies for reconstructing ancient seawater chemistry, particularly through the geochemical signatures preserved in their cortices. The concentric laminae of ooid cortices record variations in seawater chemistry, such as alkalinity and saturation states.^[63] Additionally, the minor role of abrasion in ooid growth, as demonstrated by three-dimensional modeling of ooid shapes, enhances the reliability of these cortical records for inferring seawater alkalinity and saturation states, with abrasion contributing less than 1% to shape evolution even in larger specimens.^[63] The abundance of ooids in sedimentary records provides insights into paleoclimate conditions, often peaking during warm, arid periods that promote high carbonate supersaturation and evaporation in shallow, agitated waters. For instance, enhanced ooid formation during such climates is linked to increased seawater alkalinity, facilitating widespread precipitation on tropical to subtropical platforms.^[15] This association underscores ooids' utility in identifying episodes of greenhouse climates, where their proliferation signals elevated temperatures and reduced freshwater input.^[64] Economically, oolitic grainstones form significant hydrocarbon reservoirs due to their high primary porosity and permeability. In the Upper Jurassic Arab Formation of the Arabian Peninsula, oolitic grainstones exhibit porosities typically ranging from 20% to 30%, enabling substantial oil accumulation in fields like Ghawar.^[65] Similarly, in the Permian Basin of the United States, oolitic limestones within the San Andres Formation contribute to reservoir quality, with preserved intergranular porosity supporting hydrocarbon storage and flow.^[66] Recent research has expanded ooids' paleoenvironmental applications, particularly through the analysis of giant ooids as indicators of ocean evolution. A 2025 study on middle Cambrian giant ooids (exceeding 35 mm in diameter) from the North China Platform demonstrates their formation under high-temperature, high-carbonate saturation conditions influenced by phosphorus influx and storm activity, serving as proxies for shifts in global ocean chemistry during the early Paleozoic.^[67]

Ooid Immuration

Fossil Encasement Processes

Ooimmuration refers to the taphonomic process by which small fossils become encased within ooids, a phenomenon distinct from standard ooid nucleation where inorganic or detrital particles initiate concentric layering.^[30] This term was coined to highlight the unique preservation of organic remains through lithification, potentially involving biological mediation.^[30] The mechanism begins with small fossils, such as ostracods, calcareous algae, or gastropods, acting as nuclei in supersaturated carbonate waters typical of shallow, agitated marine settings like ooid shoals.^[30] During ooid growth, these biogenic particles may also become entrained and coated mid-formation as layers of aragonite or calcite precipitate rapidly around them, effectively sealing the fossils within the cortex.^[30] Microbial films adhering to the fossil surfaces can accelerate this precipitation by promoting nucleation sites, enhancing the encasement efficiency in dynamic environments.^[30] A prominent example of ooimmuration is documented in the Middle Jurassic (Bajocian) Carmel Formation of southwestern Utah, where ooids contain encrusted gastropods, ostracods, and algal fragments preserved through this coating process.^[30] In these deposits, the rapid cortical accretion in high-energy conditions shielded the fossils from physical abrasion and biological degradation during early diagenesis.^[30] Additional examples have been reported in Neoproterozoic deposits, such as the Kunihar Formation in the Lesser Himalaya, India.^[68]

Preservation and Taphonomic Implications

Ooimmuration enhances the taphonomy of encased fossils by providing a protective carbonate coating that shields delicate structures from dissolution, bioerosion, and mechanical abrasion in high-energy depositional settings. The concentric layers of the ooid act as a barrier, preventing chemical breakdown of aragonitic or otherwise vulnerable skeletal material during early diagenesis.^[30] Microendolithic borings within the ooid cortex increase permeability, facilitating the diffusion of carbonate ions that promote internal mineralization and stabilize the fossil nucleus against further degradation.^[30] This process introduces taphonomic biases in microfossil assemblages, preferentially preserving small, mobile, or soft-bodied taxa that serve as suitable nuclei for ooid accretion while underrepresenting larger or non-nucleating organisms in oolitic deposits. Geologically, ooimmuration in ooid shoals serves as an indicator of rapid burial events, often driven by storm-generated tempestites that entomb communities before extensive decay or transport. Such preservation has been noted in Paleozoic lagerstätten associated with ancient carbonate platforms.

References

[1]
Non-skeletal Grains: Ooids, Pisoids, and Other Coated Grains
Jan 1, 2003 · Ooid (oolith) - a spherical to ellipsoidal grain, 0.25 to 2.00 mm in diameter, with a nucleus covered by one or more precipitated concentric coatings.
[2]
Ooid - an overview | ScienceDirect Topics
Ooid is defined as a small, spherical carbonate grain that forms primarily in shallow tropical waters, influenced by local geochemical conditions and physical ...
[3]
Carbonates & Other Rocks
Apr 17, 2013 · Ooids. These are spherical sand sized particles that have a concentric or radial internal structure. · Peloids. These are spherical aggregates of ...
[4]
[PDF] The where, when, and how of ooid formation
May 7, 2024 · Ooids are a common constituent of carbonate rock throughout the last 3.2 billion years. The chemical and physical characteristics of ooids ...
[5]
Ooid Production and Transport on the Caicos Platform
Feb 29, 2008 · Ooids are spherical or ellipsoid concretions, usually less than 2 mm in diameter, of calcium carbonate and aragonite crystals arranged around a ...
[6]
Allochem - ALEX STREKEISEN
Ooliths or ooids are spherical or ellipsoidal grains, less than 2mm in diameter, having concentric laminae around a nucleus. Ooliths are usually composed of ...<|control11|><|separator|>
[7]
The mineralogy of carbonates; non-skeletal grains
Aug 30, 2019 · Ooids are spherical to subspherical grains, characterized by concentrically layered, micron-sized calcite or aragonite crystals.Missing: definition - - | Show results with:definition - -
[8]
Pisolites and Oncoids - SEPM Strata
Mar 6, 2013 · Pisolites, oncoids, and oncolites are enveloped by irregular layers. All these grains are frequently larger than ooids and commonly are over a centimeter in ...
[9]
Ooid Sand: Tiny Spherical Grains Formed in Shallow Water
Nov 16, 2011 · Ooids are small rounded accretionary mineralized bodies. They could be called sand grains, but they are no ordinary sand grains.
[10]
Calcareous Ooids: A Synopsis - SpringerLink
In 1855, Lyell gave an appropriate definition of “oolitic grains” (p. 12): “The variety of limestone called 'oolite' is composed of numerous small egg-like ...Missing: first | Show results with:first
[11]
The largest known ooids and their implications for sedimentology
Aug 29, 2025 · By most definitions, ooids range in size from 0.1 to 2 mm. Modern measurements from diverse locations—including the Bahamas (0.13 to 1 mm, ...
[12]
Ooid Cortical Stratigraphy Reveals Common Histories of Individual ...
Jul 11, 2020 · Ooids are sand grains that are composed of a series of concentric layers of calcium carbonate precipitated around a smaller particle. We ...
[13]
Giant ooids of microbial origin from the Zhangxia Formation ...
The thickness of the concentric laminae accounts for 2/3 to 3/4 of the diameter of the giant ooids.
[14]
Marine Ooid Sizes Record Phanerozoic Seawater Carbonate ...
Nov 15, 2022 · Calcite and aragonite mineral saturation states (ΩCa and ΩAr, respectively) link seawater carbonate chemistry with the thermodynamic driving ...
[15]
Mineralogy, O18/O16 Ratios, and Strontium and Magnesium ...
As is true for the concentrations, the uptake of Sr and Mg in the shells is shown to be sensitive also to changes in the concentrations of the two trace ...Missing: substitution | Show results with:substitution
[16]
Chapter III. Significance of primary radial fabrics associated with ...
Mar 4, 2022 · Spherulites and some ooids have radial fabrics that are directly related to the arrangement of calcite fibers or aragonite needles, the long ...<|control11|><|separator|>
[17]
[PDF] Carbonate Sediments and Rocks Under the Microscope
An ooid with a radial structure will therefore show a well-devel- oped extinction cross, at least when the plane of the section passes through or near to the ...<|control11|><|separator|>
[18]
Distribution and Controlling Growth Factors of Ooids in Qinghai Lake ...
Field and laboratory studies suggest that tangential ooids are inorganic precipitated during agitated conditions, whereas radial ooids are formed in quiet water ...
[19]
Microborings reveal alternating agitation, resting and sleeping ...
Oct 17, 2023 · Ooids have formed abundantly in shallow warm (sub-tropical) marine or lacustrine environments throughout much of Earth's history (Simone, 1980; ...
[20]
[PDF] Facies Anatomy and Diagenesis of a Bahamian Ooid Shoal
Ooid sands in the Joulters Cays area occur not only as high-energy bars and ... size, can become an ooid nucleus, but peloids are most common in Joulters.
[21]
[PDF] by GF ELLIOTT - The Palaeontological Association
A microflora of red, green, and blue-green algae is recorded from the Middle Jurassic Great Oolite ... algal, around a nucleus. Typical oncolites, while ...
[22]
Shape evolution of ooids: a geometric model | Scientific Reports
Jan 29, 2018 · Here we investigate a rare and beautiful geophysical problem that combines both of these processes; the shape evolution of carbonate particles ...Results · Model Development · Combined Ooid Shape...
[23]
[PDF] Report (pdf) - USGS Publications Warehouse
A well preserved assemblage of oolites in the carbonate- bearing portion of the Libby. Formation contains a variety of fabrics, part of which is interpreted as ...
[24]
Chapter 7 Growth of Oöids, Pisolites and Grapestone - ScienceDirect
Calcite ooids and pisolites have a fabric of radial-fibrous crystals with a concentric pattern marking time-dependent discontinuities of layer thickness, fabric ...
[25]
Ooid fabric and fracture in the Great Salt Lake and the geologic record
Mar 3, 2017 · The radial fabric renders Great Salt Lake ooids weaker than tangential Bahamian-type ooids of the same size and shape. This produces broken ...
[26]
11.3 Cortical Fabrics in Calcite and Aragonite Ooids
Modern ooid fabrics may be classified as radial, tangential or random, with random fabrics arising either when elongate crystals lack a preferred orientation, ...<|separator|>
[27]
Ooid microfabrics: the origin and distribution of high intra-ooid porosity
Three main types of ooid cortex fabric are recognised: pristine radial, micritised radial and completely micritised.Missing: micritic | Show results with:micritic
[28]
[PDF] Role of organic matter in formation of stromatolites and micritic ooids ...
Aug 1, 2010 · The micritic cortex is the only zone to possess an abundance of filamentous and/or fossilized microbial remnants (Fig. 12). M. N. Page 46. 32.
[29]
OOIMMURATION: ENHANCED FOSSIL PRESERVATION BY ...
Oct 27, 2021 · This bedform model for ooid formation may be the best explanation for the radial fabric and relatively large size of ancient calcitic ooids.
[30]
Decoding the mechanism of formation in marine ooids: A review
One common thread in modelling the formation of marine ooids is the notion that ooid growth requires alternating agitated and resting stages. ... Synthesis and ...
[31]
Active Ooid Growth Driven By Sediment Transport in a High-Energy ...
Sep 30, 2018 · A convergence of evidence shows that active ooid growth occurs along the transport path in a high-energy shoal environment characterized by ...
[32]
The Enigma of Neoproterozoic Giant Ooids—Fingerprints of Extreme ...
Feb 6, 2020 · Kinetic parameters for aragonite and calcite precipitation at 25 °C, aragonite ... ooid formation at pH ≤ 8.2 at elevated pCO2 (Figure 2). The ...
[33]
Experimental evidence that ooid size reflects a dynamic equilibrium ...
Larger ooids are generally formed under more agitated conditions or over a relatively longer time period [15,18,19, 47] . In this study, ooid sizes ranged from ...
[34]
A Reconnaissance Survey of the Environment of Recent Carbonate ...
The physiographic setting of an area of recent carbonate sedimentation along the Trucial Coast in the Persian Gulf is described in detail. Previous work on the ...
[35]
[PDF] Great Salt Lake - Utah State Parks
Conditions conducive to oolitic sand formation are: ample amounts of calcium, shallow warm water, and lots of wave action to agitate the water. Calcium ...
[36]
Aragonite depositional facies in a Late Ordovician calcite sea ...
May 9, 2020 · Marine ooids are confined to shallow, generally <10 m water depth in tropical, tidal environments (Simone, 1980). Also present are ...
[37]
https://onlinelibrary.wiley.com/doi/10.1111/sed.12753
[38]
https://doi.org/10.1130/G39159.1
[39]
https://doi.org/10.1111/sed.13149
[40]
Experimental evidence that ooid size reflects a dynamic equilibrium ...
Jun 15, 2017 · Ooid size reflects a dynamic equilibrium that can be used to infer depositional conditions. Abstract. Ooids are enigmatic concentrically coated ...
[41]
[PDF] Non-Skeletal Grains
Non-skeletal grains include peloids, coated grains (ooids, pisoids, oncoids, rhodoliths), grain aggregates, and mechanical clasts. Peloids are small, fine- ...
[42]
(PDF) Shape evolution of ooids: A geometric model - ResearchGate
Jan 29, 2018 · Ooids found in the same location tend to have roughly similar maximal size2,20 and similar shape1,2. 2. Ooid shapes range from spherical to ...
[43]
Petrology and diagenetic evolution of Neoproterozoic ooids (Libby ...
Textural evidence suggests that Mg calcite ooids may have formed along with the ooids having an original aragonite mineralogy. Strontium concentrations of ...
[44]
Calcium Isotopic Constraints on the Transition From Aragonite Seas ...
May 2, 2022 · These new δ 44 Ca and Sr data provide further evidence that there was a protracted transition from aragonite seas to calcite seas in the Cambrian.
[45]
Origin of the Dolomitic Ooids Formed in the Pliocene Shizigou ...
The basin's annual evaporation is over 20 times greater than its annual precipitation [21]. The mean annual temperature is about 0–5 °C [23]. The Qaidam Basin ...
[46]
Origin and Evolution of the Late Cretaceous Reworked Phosphorite ...
Phosphatic grains dominate the phosphate components, which include pellets, ooids, and bone fragments. The phosphatic grains are medium to coarse and can be ...
[47]
Modern Iron Ooids of Hydrothermal Origin as a Proxy for Ancient ...
May 8, 2019 · Panarea ooids are characterized by the superposition of orange to dark brown 10 µm thick laminae commonly developed around a core of volcanic ...
[48]
Dual clumped isotopes (Δ47 and Δ48) reveal non-equilibrium ...
Aug 15, 2024 · The Δ47 and Δ48 values of the separated aragonite are consistent with the modern ooid sediments and in approximate equilibrium with the surface ...
[49]
Did major changes in the stable-isotope composition of Proterozoic ...
Jun 2, 2017 · The isotopically most enriched ooids are the best preserved normal marine components and may suggest that the δ18O of seawater was about -9.75‰ ...Missing: chemistry | Show results with:chemistry
[50]
Aragonite neomorphism via intrafabric dissolution and calcite ...
Dec 20, 2024 · Aragonite neomorphism occurs through intrafabric dissolution and calcite precipitation, not thin films, where calcite grows over microporous ...
[51]
Skeletal aragonite neomorphism — quantitative modelling of a two ...
The neomorphism of skeletal aragonite occurs by the simultaneous dissolution of aragonite and precipitation of calcite along thin solution films, ...
[52]
Microbial Activity and Neomorphism Influence the Composition and ...
Nov 23, 2022 · The radial microfabrics have formed the basis of conceptual models applied to understand the formation of radial fabrics in ancient ooids, but ...
[53]
https://thesedimentaryrecord.scholasticahq.com/article/56183-microbial-activity-and-neomorphism-influence-the-composition-and-microfabric-of-ooids-from-great-salt-lake-ut
[54]
[PDF] Seawater Chemistry, Biomineralization and the Fossil Record of ...
Note the increase at times of calcite seas, particularly in the Ordovician, Jurassic and Cretaceous. Data taken from. Mark Wilson's bibliography of inorganic ...Missing: peaks | Show results with:peaks
[55]
The sediment budget of Great Bahama Bank—Earth's largest ...
Jun 23, 2025 · On average, production rates ranged from 98–826 g m−2 yr−1 (Table S8), comfortably encompassing Broecker and Takahashi's (1966) estimate of 300– ...Missing: per | Show results with:per
[56]
[PDF] radiocarbon activity measurements of oolitic sediments from the ...
ooids of carbonate oolitic rocks are formed by successive concentric ... described by Evans (1973) from the Trucial Coast. According to Picha and ...
[57]
Organic‐rich bimineralic ooids record biological processes in Shark ...
May 24, 2023 · The dark ooids of Shark Bay, Western Australia, contain two different calcium carbonate minerals. The cortices of these ooids contain ...
[58]
[PDF] Green River Formation Southeastern Uinta Basin Utah and Colorado
The Douglas Creek Member is composed chiefly of sandstone, siltstone, and ·algal and oolitic limestone deposited in a nearshore-lacustrine environment. It.
[59]
A carbonate hardground in the carmel formation (middle jurassic, sw ...
Dec 17, 2008 · The Carmel hard‐ground was formed across a carbonate lagoon from an oolitic shoal seaward to a subtidal shelly facies landward.
[60]
[PDF] Marine oolites as proxies for palaeoenvironmental reconstructions ...
May 12, 2023 · Already in the 19th century, they were rec- ognized and carefully described (Lyell 1855, Sorby 1879).
[61]
What ooids tell us about ancient seawater chemistry - ScienceDirect
Jul 1, 2024 · We find that ooid abrasion plays only a small role in the formation of ooids, even in giant ooids, where the mass should cause abrasion to be most intense.
[62]
Evolution of a Late Oxfordian: early Kimmeridgian carbonate ...
Apr 23, 2015 · During the Bimammatum and Planula zones, the climate became progressively warmer and more arid, allowing enhanced carbonate production. This ...
[63]
Controls on porosity preservation within a Jurassic oolitic reservoir ...
This paper focuses on porosity evolution in the Arab D (Upper) reservoir in an onshore gas field, UAE. The reservoir has undergone porosity inversion.
[64]
Oolite-Bar Progradation, San Andres Formation, Midland Basin ...
Sep 21, 2019 · Permian sandstones of the Permian basin are characteristically more radioactive than carbonate rocks or evaporites. Dolomite also is more ...
[65]
The largest known ooids and their implications for sedimentology
Aug 29, 2025 · Marine ooids exhibit dynamic size changes that can potentially serve as proxies for ocean chemistry and environmental evolution.