GRAINS: Non-skeletal Grains: Ooids, Pisoids, and Other Coated Grains
2003. "GRAINS: Non-skeletal Grains: Ooids, Pisoids, and Other Coated Grains", A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis, Peter A. Scholle, Dana S. Ulmer-Scholle
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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 (cortical layers) with radial and/or concentric orientation of constituent crystals. Nuclei typically consist of detrital terrigenous grains, skeletal fragments, or pellets and peloids, and coatings can have a variety of compositions. A rock composed dominantly of ooids is termed an “oolite”. That term is commonly misused, however, to describe the constituent ooid grains.
Spastolith or deformed ooid - An ooid or other coated grain that has been deformed, generally by shearing the concentric laminations away from each other or from the nucleus. In rarer cases, the deformation is tectonic.
Superficial ooid - An ooid with an incomplete or very thin cortical coating; specifically one in which the thickness of the accretionary coating is less, commonly far less, than the radius of the nucleus.
Pisoid - A small spheroidal particle with concentrically laminated internal structure, larger than 2 mm and (in some usages) less than 10 mm in diameter. A pisolite is a rock containing abundant pisoids.
Oncoid - In North American usage, an oncoid is a coated grain of algal (but not red algal) or microbial origin that is coarser than 2 mm in diameter; a spheroidal form of microbial stromatolite showing a series of concentric (often irregular or scalloped) laminations. These unattached stromatolites are produced by mechanical turning or rolling, exposing new surfaces to microbial/algal growth. Common European usage is less genetic, and in that usage a microbial/algal origin is not a prerequisite. An oncolite is a rock composed of oncoids; the term, however, is often used improperly as a synonym for “oncoid”.
Rhodoid (rhodolith) - An irregularly laminated calcareous nodule composed of encrusting coralline algae arranged in more or less concentric layers about a core; spheroidal but knobby surfaced, and up to several centimeters in diameter; form in warm to cool, clear, shallow sea water down to depths of 150-200 m.
Calcareous ooids and pisoids are known from the Late Archean to Recent; specific coated grains, such as Girvanella oncoids or red algal nodules (rhodoids) are restricted by the age ranges of the constituent organisms (listed in chapters on organic grains).
Modern calcareous ooids are known with aragonite or Mg-calcite compositions (or combinations of both), and there is evidence that these same compositions existed throughout Phanerozoic time, perhaps with specific temporal preferences (e.g., Sandberg, 1983; Wilkinson and Given, 1986). Laminae of organic material are found interlayered in most ooid cortices and help preserve structure during diagenesis.
Calcareous cave and soil pisoids typically have low-Mg calcite compositions. Other ooids/pisoids (covered in later chapters) can have ferruginous (especially hematite or chamosite), siliceous, bauxitic, phosphatic, evaporitic (gypsum, halite) or other coatings.
Ooids and other coated grains require conditions suitable for inorganic or microbial precipitation and for biological encrustation of grains. They also require repeated rotation of grains to allow the formation of concentric coatings. Thus, the best environments for ooid formation are tidal deltas and bars, or beaches (marine or lacustrine) where surficial grains are kept in daily motion. Because reefs or bioherms “compete” with ooids in high-energy settings, biologically stressed areas (with abnormal salinities or temperatures) can favor ooid formation by inhibiting organism growth and enhancing rates of carbonate precipitation. Because of their rounded shape ooids are easily reworked into adjacent environments (especially eolianites).
Other coated grains (superficial ooids, pisoids and oncoids) can be formed in soils and caves (vadoids; cave pearls), in relatively deep-water, current-scoured platform areas (rhodoids), in shelf areas prone to periodic storm action, in partially protected lagoons, and in a wide variety of other settings.
Figures & Tables
A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis
This volume expands and improves the AAPG 1978 classic, A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities(AAPG Memoir 27). Carbonate petrography can be quite complicated. Changing assemblages of organisms through time, coupled with the randomness of thin-section cuts through complex shell forms, add to the difficulty of identifying skeletal grains. Furthermore, because many primary carbonate grains are composed of unstable minerals (especially aragonite and high-Mg calcite), diagenetic alteration commonly is quite extensive in carbonate rocks. The variability of inorganic and biogenic carbonate mineralogy through time, however, complicates prediction of patterns of diagenetic alteration. This book is designed to help deal with such challenges. It includes a wide variety of examples of commonly encountered skeletal and nonskeletal grains, cements, fabrics, and porosity types. It includes extensive new tables of age distributions, mineralogy, morphologic characteristics, environmental implications and keys to grain identification. It also encompasses a number of noncarbonate grains, that occur as accessory minerals in carbonate rocks or that may provide important biostratigraphic or paleoenvironmental information in carbonate strata. With this guide, students and other workers with little formal petrographic training should be able to examine thin sections or acetate peels under the microscope and interpret the main rock constituents and their depositional and diagenetic history.
- carbonate rocks
- color imagery
- problematic fossils
- sedimentary rocks