Non-Skeletal Grain types

Terminology

Non-skeletal Grains

Information contained in this table has been extracted from Tucker and Wright (1990) and Flügel (2004).

Peloids

Grain Type

Size range between Ø 0.1-1mm; common Ø 100-500 μm.

Sub-rounded, spherical, ellipsoidal or angular grains composed of crypto- or microcrystalline carbonate (clay and silt-grade carbonate mud) with no internal structures.


Origin

Peloids are polygenetic.

Faecal pellets are a category of peloids of faecal origin (common those from crustaceans, e.g, crabs, shrimps, or worms and gastropods etc.).

Peloids can originate from rounded carbonate mud clasts, fragments of calcareous algae, micritization of carbonate grains by endolithic micro-organisms (bahamite peloids).

A group of peloids (< 100 µm) derives from direct precipitation either abiotically (peloidal cement as cavity fill; MacIntyre, 1985) or biologically induced/influenced precipitation (clotted peloidal micrite/automicrite; Flügel 2004).


Depositional Environment

Peloids are most common in tropical, shallow water, subtidal to intertidal, of low to moderate energy environments such as lagoons and tidal flats.

Precipitated peloidal automicrites are common in reefs and mounds.

Peloids can occur resedimented in deeper water (slope and basinal settings).

Ooids

Grain Type

Size range between Ø 200 μm-2 mm; common 500 μm-1 mm.

Spherical-sub-spherical or ovoid grains consisting of one to several regular concentric laminations around a nucleus (peloids, bioclasts, quartz grains, intraclasts). Laminae may exhibit tangential or radial crystal microfabric.


Origin

Ooids generally form in agitated waters in carbonate sand bed forms (such as dunes and ripples) where they are frequently moved by waves, tidal and storm currents. Recent marine ooids can be:

  1. aragonitic with tangential orientation of acicular crystals or needles (2 µm in length);
  2. aragonitic or calcitic with a radial microfabric;
  3. micritic with the laminae obliterated by micritization processes. Saline lake ooids are aragonitic with a radial microfabric.

Origin of ooids is assumed to be mostly due to abiotic precipitation around a nucleus kept in motion by hydrodynamic energy but biogenic precipitation influenced by micro-organisms and microbial biofilms is not excluded.


Depositional Environment

Ooids form and accumulate in water depth < 5m (up to 10-15m) in marine shoals (close to the edge of the platform), beaches, and tidal bars but also in high-energy saline lake margins.

Ooids can also form in low energy marine settings (lagoons, intertidal-supratidal belt), where they generally exhibit a radial fabric.

Ancient ooids exhibit both a tangential structure (high energy, aragonitic) and a radial structure (low energy, or made of primarily high Mg calcite, or the radial fabric is a diagenetic feature derived by replacement of aragonite with low Mg calcite).

Oncoids

Grain Type

Size range between Ø <1mm to few dms; common > 2 mm.

Coated grains with a nucleus more or less distinguishable, often fossil fragments, thick, the coating consists of irregular laminae non concentric, and partially overlapping. Lamination derives from one or more encrusting biogenic components. Sphericity does not increase with growth.


Origin

Oncoids are biogenically-coated grains with an algal-microbial-cyanobacterial coating (laminae may exhibit and embedd biogenic structures) in same cases associated with encrusting foraminifera, bryozoans and polychaete worms.

Rhodoids are a particular type of oncoids formed by branching or crustose coralline red algae common during the Cenozoic.


Depositional Environment

Algal-microbial nodules, including those formed by cyanobacteria, are common in non-marine and shallow-marine limestones. Oncoids can be produced in fresh water (lake and rivers) where calcareous tufas are precipitated. In marine carbonate shelves and ramps, oncoids occur from the intertidal zone to the margin and resedimented in slope and basinal settings. When oncoidal laminae are regularly concentric they indicate overturning by current action. Static forms show a marked asymmetry.

Cortoids/ Micrite coated grains

Grain Type

Size range between Ø <1mm to several cm; common 500 μm-1 mm.

Coated grains (mostly bioclasts but also other non skeletal grains) with a structureless, micritic, thin (usually <100 µm) envelope. The micrite coating can be destructive of the grain margin, constructive or both.


Origin

Bioclasts and non skeletal grains with a non laminated micritic envelope are often due to micritization processes by endolithic micro-organisms. Micritization produce both destructive and constructive micrite rims around the grains.

It occurs in shallow water environments mostly by action of photosynthetic and non micro-organisms (cyanobacteria, endolithic and epilithic algae) when carbonate grains remain a temporary amount of time on the sea floor.


Depositional Environment

Shallow platform top settings from intertidal to subtidal lagoon to back-reef facies at the platform margin. From low to high-energy platform areas.

Increased microboring activity might be indicative of increased nutrient levels.

Pisoids

Grain Type

Size range between Ø <1mm to several cm; common > 2 mm.

Coated grains with sub-spherical or irregular shape; nucleus often non biogenic and coating with concentric, dark micritic or hyaline carbonate (aragonite, calcite) laminae exhibiting tangential or radial microfabric.


Origin

Bioclasts and non skeletal grains with a non laminated micritic envelope are often due to micritization processes by endolithic micro-organisms. Micritization produce both destructive and constructive micrite rims around the grains.

It occurs in shallow water environments mostly by action of photosynthetic and non micro-organisms (cyanobacteria, endolithic and epilithic algae) when carbonate grains remain a temporary amount of time on the sea floor.


Depositional Environment

Pisoids can be produced in marine shallow water carbonate platform-top from supratidal (associated with tepees) to shallow subtidal ponds. They occur in hypersaline to freshwater areas, in particular in the vadose zone. Pisoids form also in caves and calcrete palaeosols.

Aggregate grains

Grain Type

Size range between Ø <500 μm to several mm.

Grains formed by individual carbonate particles (bioclasts, ooids, peloids, intraclasts) bound together by micrite or sparite cement.

The outline is irregular globular or rounded and depends on the shape of the bound grains and the amount of coating aggregating/binding the grains.


Origin

Grains are welded together by organic matter or precipitated aragonite or Mg-calcite micrite or microsparite. Binding and cementation is likely due to activity of microbial organisms in areas where grains are temporarily not moved by current and wave action.

Grapestones are aggregate grains where the binding material forms a meniscus between the grains.

Lumps are aggregate grains with abundant, almost coating binding material and they can include partially or completely micritized grains.

In botryoidal lumps, the separate grains are bound and coated together.


Depositional Environment

Aggregate grains form in protected shallow subtidal to intertidal areas. Grapestones form in low-energy settings, commonly behind and protected by shoals, where sediment is moved relatively infrequently allowing microbial binding and cementation and where sand-sized grains may be transported from the shoals during high-energy storm events.

Intraclasts

Grain Type

Size range between Ø <200 μm to dm.

Fragments of lithified or partly lithified syn-sedimentary (penecontemporaneous) carbonate deposits reworked within the area of deposition.

They can have variable size and shapes, angular to sub-rounded.


Origin

A common type of intraclasts is a micritic flake or chip, derived from desiccation of tidal-flat muds or disruption by storms of partially lithified or cemented subtidal calci-mudstone. Rip-up clasts are common from supratidal to subtidal settings and are eroded by currents in shallow water. Channel formation in shallow or deep water produces intraclasts and channel lag. Hardgrounds are rapidly lithified marine cemented surfaces that can produce intraclasts. Reef detritus produces intraclasts in back-reef and fore-reef zones.


Depositional Environment

Shallow marine environments where the intraclasts are formed and redeposited or deep-water settings due to syn-sedimentary erosion and current scour of penecontemporaneous sea floor.

Extraclasts/Lithoclasts

Grain Type

Size range between Ø <200 μm to m.

Fragments of lithified carbonate and non carbonate rocks not represented in the immediate depositional area and deriving from the disintegration of rocks external to or older of the sedimentary basin where they are deposited.

The can have variable size and shapes, angular to rounded.


Origin

Extraclasts are carbonate or non carbonate clasts eroded, transported and redeposited in a depositional environment different from the source. They can be rounded, contain older fossils and diagenetic overprint.


Depositional Environment

Extraclasts/Lithoclasts in limestone derive from rivers, rock-fall at coast, deposition in basinal areas via turbidity currents and debris flows of material eroded from shallower and older lithified carbonates.