Coated Grains and Their Types

Coated grains are carbonate grains composed of a nucleus surrounded by layers called a cortex.
Types of coated grains include ooids, oncoids, cortoids, and pisoids.

Calcareous ooids are small, spherical or oval carbonate particles with concentric layers.
Nuclei can be skeletal fragments, peloids, smaller ooids, or siliciclastic grains like quartz.
Ooids are sand- to silt-size particles, ranging from 0.1 mm to over 2 mm, with 0.5 mm to 1 mm being most common.
Ooids can have a pearly luster when formed in agitated water and a dull luster in quiet waters.
Ooids are formed in both nonmarine and marine environments.
Ooids with numerous laminae are considered mature, while those with few laminae are called superficial ooids.

Cortoids are coated grains consisting of fossils, ooids, or peloids with a thin envelope of dark micrite, often consisting of crystals 2 to 5 microns in size.
These envelopes can form through various processes, including destructive micritization due to microboring organisms.

Carbonate lithoclasts are detrital fragments of carbonate rock, also called limeclasts.
They range in size from very fine sand to pebbles or boulders, and can be well rounded to angular.
Very small lithoclasts can be confused with large peloids
Lithoclasts are classified into two main categories: intraclasts and extraclasts.

Intraclasts are lithoclasts formed within a depositional basin by fragmentation of weakly cemented carbonate sediment.
They are eroded from the seafloor and redeposited near their original location.
They are typically formed by physical disruption from waves, currents, or storms, but can also result from organic activity, burrowing, or sediment sliding.

Extraclasts are lithoclasts formed by erosion of older, lithified carbonate rock exposed on land outside the depositional basin where the clasts accumulate.

A small amount of carbonate sediment is formed in the flowing waters of rivers, streams, and springs.
These waters are often undersaturated with calcium carbonate, but saturation can occur in waters draining areas with underlying carbonate rocks.
Carbonate precipitates from streams and cold-water springs are commonly low-Mg calcite, while hot springs can produce aragonite.
Carbonate sediments formed in these freshwater environments are known as travertine.
Travertine is dense, finely crystalline, and ranges in color from tan to white or cream.
Porous, spongy varieties, often forming as encrustations on plant remains, are called tufa.

Travertine forms mainly in springs, spring-fed lakes, and areas with waterfalls or cascades.
Precipitation requires supersaturated ground waters or streams with calcium carbonate and CO2.
Cold-water springs can experience precipitation due to CO2 loss caused by higher temperatures at the spring's mouth and exposure to the atmosphere.
Tufas are believed to form through calcite precipitation onto plants like mosses and algae.
Bacteria can also play a role in travertine precipitation.

Carbon dioxide-rich groundwaters moving through carbonate formations dissolve parts of the formations, creating solution pipes, sinks, and caves.
These features are known as karst features.
The formation of speleothems like stalactites and stalagmites occurs in caves and reflects specific stages of carbonate diagenesis.

The major diagenetic processes affecting carbonate sediments and rocks include: micritization, dissolution, cementation, compaction, neomorphism, dolomitization, and replacement by non-carbonate minerals.

This diagenetic regime includes the seafloor and near-surface environment.
It is characterized by marine waters of normal salinity, although hypersaline waters are present in evaporative environments.
Mixed marine-meteoric waters can also be present at the strandline and in the shallow subsurface where marine and meteoric realms mix.

This diagenetic regime is defined by the presence of freshwater, including the unsaturated vadose zone above the water table and the saturated phreatic zone below the water table.
A zone of mixed marine-meteoric water exists between the meteoric and marine realms.
Marine carbonates can be brought into this regime through falling sea levels, sediment filling of shallow carbonate basins, or late-stage uplift and unroofing of deep carbonate complexes.

Also referred to as the marine phreatic or deep phreatic zone, this regime has pore spaces filled with waters that were originally marine or meteoric.
The composition of these deep pore waters is often modified from their original marine or meteoric composition due to burial.

Organisms in carbonate depositional environments rework sediment through boring, burrowing, and sediment-ingesting activities.
These activities can destroy primary structures and leave behind mottled bedding and organic traces.
Microorganisms like fungi, bacteria and algae create microborings in skeletal fragments and carbonate grains.
Fine-grained aragonite or high-magnesian calcite precipitate into these holes, a process called micritization.
Less intensive boring can result in a thin micrite rim or envelope around grains.
Larger organisms like sponges, molluscs, fish, sea cucumbers, and gastropods can also break down carbonate grains.

Cementation plays a crucial role in all diagenetic realms.
On the ocean floor, cementation occurs in warm-water areas within pore spaces of grain-rich sediments or cavities.
Reefs, carbonate sand shoals, and carbonate beach sands are prime locations for early cementation.
Cemented carbonate beach sand is called beachrock.
Seafloor cement is commonly aragonite or high-magnesian calcite, and can take several textural forms.
Beachrock can contain meniscus cements formed by capillary forces as water drains from beaches during low tide.