Allochems

Allochems are transported carbonate grains in limestones — particles that are not simple chemical precipitates in place but that have undergone some degree of mechanical transport before deposition. Folk (1959) introduced the term to emphasize that many carbonate rocks are not passive crystalline precipitates but instead contain grains that behaved like siliciclastic particles during their transport and depositional history. ^[1] Carbonate grains typically range in size from coarse silt (0.02 mm) to sand (up to 2 mm), though larger particles such as fossil shells also occur, and they can be divided into five basic types, each distinguished by shape, internal structure, and mode of origin: carbonate clasts, skeletal particles, ooids, peloids, and aggregate grains. ^[1] The nature and relative abundance of these grain types in a limestone is the foundation of every major limestone classification system because each type carries distinct information about the depositional environment.

Carbonate Clasts (Lithoclasts)

Carbonate clasts are rock fragments derived either by erosion of ancient limestones exposed on land or by erosion of partially or completely lithified carbonate sediments within a depositional basin. ^[1] Clasts from outside the basin are called extraclasts; those eroded from sediments within the same basin are called intraclasts. ^[1] The distinction carries interpretive weight because extraclasts imply derivation from an older, distant source whereas intraclasts are produced penecontemporaneously within the basin — a different transport and diagenetic history. When the distinction cannot be made, the neutral term lithoclast (or limeclast) is used. ^[1] Extraclasts may have iron-stained rims from weathering or recrystallized veins inherited from the parent rock. ^[1] Lithoclasts range from very fine sand to gravel size, though sand-size fragments are most common, and they generally show some rounding from transport. ^[1]

Skeletal Particles

Skeletal fragments are by far the most common type of allochem, occurring as whole microfossils, whole larger fossils, or broken fragments, and they are so abundant in some limestones that they make up most of the rock. ^[1] The specific kinds of skeletal particles present depend on both the age of the rock and paleoenvironmental conditions: trilobite remains characterize early Paleozoic rocks while Cenozoic rocks commonly contain abundant foraminifers, and colonial corals indicate shallow, high-energy, well-oxygenated water while branching bryozoa indicate quiet water. ^[1] The combination of evolutionary constraints and environmental preferences makes skeletal assemblages powerful dual tools — for both biostratigraphy and paleoecology.

Ooids

Ooids are coated carbonate grains consisting of a nucleus (a shell fragment, pellet, or quartz grain) surrounded by one or more thin concentric layers (the cortex) of fine calcite or aragonite crystals. ^[1] They form under strong bottom currents and agitated-water conditions where calcium bicarbonate saturation is high, with precipitation building up the coating as grains are intermittently suspended by currents. ^[1] See the dedicated Ooids page for full detail on radial vs concentric structure, pisoids, oncoids, and sea-level mineralogical controls.

Peloids

Peloids are structureless grains of microcrystalline or cryptocrystalline calcite or aragonite, typically silt to fine-sand size (0.03–0.1 mm). ^[1] The term is deliberately non-genetic — peloids can form as fecal pellets, by endolithic algal micritization of ooids or skeletal fragments, by bacterial precipitation, or by reworking of mud aggregates. ^[1] Fecal pellets — the most common variety — are small, oval to rounded, and uniform in size, and are distinguished from ooids by their lack of internal concentric or radial structure and from intraclasts by their uniform shape, good sorting, and small size. ^[1]

Aggregate Grains

Aggregate grains are irregularly shaped carbonate particles consisting of two or more carbonate fragments — pellets, ooids, or fossil fragments — joined together by a dark, organic-matter-rich carbonate-mud matrix. ^[1] Those resembling a bunch of grapes are called grapestones, while those with a smoother appearance are called lumps — with lumps evolving from grapestones by continued cementation and micritization. ^[1] Aggregate grains in modern environments are composed mainly of aragonite but in ancient limestones are dominantly calcite. ^[1] They are only rarely reported in ancient limestones, possibly because diagenetic compaction distorts their shapes beyond recognition. ^[1]

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