Carbonate Diagenesis

Carbonate sediments are far more chemically reactive after deposition than siliciclastic sediments, and the minerals they contain — aragonite, high-magnesian calcite, and calcite — are vulnerable to dissolution, recrystallization, and replacement in ways that silicate grains are largely not. ^[1] This reactivity means the mineralogy of a carbonate rock may be completely transformed from its original depositional state: an aragonitic mud may convert entirely to calcite during early diagenesis or burial, and that calcite may itself be replaced by dolomite at a later stage. ^[1] Original depositional textures such as carbonate grains and micrite may be destroyed in the process, while porosity may be reduced by compaction and cementation or enhanced by dissolution — sometimes both in the same rock at different stages of its burial history.

The Three Diagenetic Realms

Carbonate sediments move through the same broad diagenetic stages as siliciclastic sediments — eogenesis (shallow burial), mesogenesis (deep burial), and telogenesis (uplift and unroofing) — but their diagenesis is conveniently described in terms of three distinct realms that differ in fluid chemistry, temperature, and the processes that dominate. ^[1]

The marine realm encompasses the seafloor and the very shallow marine subsurface, characterised by seawater temperatures and normal-salinity marine waters. ^[1] The principal diagenetic processes here are bioturbation of sediments, boring of carbonate shells and other grains by organisms, and cementation in warm-water settings — particularly in reefs, platform-margin sand shoals, and carbonate beach sands (beachrock). ^[1]

Marine carbonate sediments are transferred into the meteoric realm either by sea-level fall, by progressive infilling of a shallow carbonate basin, or by late-stage uplift and unroofing of deeply buried carbonate rock. ^[1] The meteoric realm is defined by the presence of freshwater and is divided into two zones: the unsaturated vadose zone above the water table, where pore spaces are not completely filled with water, and the water-saturated phreatic zone below it. ^[1] Meteoric waters are typically highly charged with CO₂, making them chemically aggressive (acidic); aragonite and high-magnesian calcite dissolve readily in these corrosive waters because they are more soluble than calcite. ^[1] The dissolution of aragonite and high-magnesian calcite may simultaneously saturate the water with respect to calcite, causing calcite to precipitate — a process called calcitization — in which less stable minerals are progressively replaced by more stable calcite, and open pore spaces may fill with calcite cement as well. ^[1]

After passing through shallow diagenesis and possibly the meteoric realm, carbonate sediments enter the subsurface realm, where increased pressure, higher temperature, and compositionally different pore fluids drive further physical and chemical change: physical compaction, chemical compaction (pressure solution at grain boundaries), additional dissolution, cementation, aragonite-to-calcite transformation, and replacement of calcite by dolomite or other minerals. ^[1]

Biogenic Alteration

Organisms alter carbonate sediments in all the ways they alter siliciclastic sediments — boring, burrowing, and ingesting — destroying primary sedimentary structures and leaving behind mottled bedding and organic traces. ^[1] In addition, small organisms such as fungi, bacteria, and algae create microborings in skeletal fragments and other carbonate grains; fine-grained (micritic) aragonite or high-magnesian calcite may then precipitate into these holes. ^[1] When boring and micrite precipitation are intense enough in warm-water environments, carbonate grains can be reduced almost completely to micrite, a process called micritization; when boring is less intensive, only a thin micrite rim — called a micrite envelope — forms around the grain. ^[1] Larger organisms such as sponges and mollusks create macroborings in skeletal grains and carbonate substrate, while organisms such as fish, sea cucumbers, and gastropods break down carbonate grains to smaller pieces in yet another way. ^[1]

Cementation

Cementation operates in all three diagenetic realms but in different styles in each, producing a distinctive suite of cement textures that can be used to infer the diagenetic environment.

In the marine realm, cementation occurs mainly in warm-water, grain-rich settings: reefs, platform-margin sand shoals, and carbonate beach sands. ^[1] Where seafloor sediments become well cemented they are called hardgrounds; cemented carbonate beach sand is called beachrock. ^[1] Seafloor cement is commonly aragonite (less commonly high-magnesian calcite) and takes several textural forms. ^[1] Beachrock contains meniscus cements — bridges of cement between grains held by capillary water during low tide when the beach drains — and pendant cements that form along grain bottoms where drops of water hang. ^[1] Isopachous rinds, which completely envelop grains in a uniform layer, form in subaqueous conditions where grains are constantly bathed in water, and aragonite cement may also occur as a mesh of needles or as fibrous radial crystals with a botryoidal form. ^[1]

In the meteoric realm, cementation plays a secondary role to dissolution, and the cement is almost exclusively calcite derived from the dissolution of less stable aragonite and high-magnesian calcite. ^[1] In the vadose zone, meniscus and pendant cements dominate; in the phreatic zone, the characteristic cements are isopachous, blocky, or syntaxial rim cements, where optically continuous calcite precipitates around single-crystal echinoderm fragments in the same way that overgrowths form around quartz grains in sandstones. ^[1]

In the subsurface burial realm, cements are also mainly calcite; the common types are syntaxial rims, bladed prismatic calcite, and coarse mosaic calcite, the last two often occurring in combination as drusy cement. ^[1] These burial cements are commonly coarse-grained and clear or white, and are generally referred to as sparry calcite cement; calcium carbonate for them is sourced, at least in part, from pressure solution of adjacent carbonate sediment. ^[1]

Dissolution

Dissolution requires conditions essentially opposite to those that favour cementation: unstable mineralogy (aragonite or high-magnesian calcite), cool temperatures, and acidic pore waters undersaturated with respect to calcium carbonate. ^[1] Dissolution is relatively unimportant on the seafloor but is particularly intense in the meteoric realm, where CO₂-rich and organically acidic meteoric waters percolate through the vadose zone into the phreatic zone, extensively dissolving aragonite and high-magnesian calcite — and even calcite itself if pore waters are sufficiently aggressive. ^[1] Dissolution concentrates especially along the water table (the boundary between the vadose and phreatic zones), which is why caves in carbonate rocks so commonly occur at that level. ^[1]

Dissolution is less intense in the deep-burial subsurface than in the meteoric realm for two reasons: most aragonite and high-magnesian calcite will already have been calcitized in the meteoric realm, and increasing temperature at depth decreases the solubility of all carbonate minerals. ^[1] Dissolution can still occur at depth if enough CO₂ is introduced to pore waters by burial decay of organic matter (decarboxylation), or if mixing of subsurface waters produces fluids undersaturated with respect to calcite; and carbonate sediments returned to the meteoric zone after uplift may undergo renewed extensive dissolution by CO₂-charged meteoric waters. ^[1]

Neomorphism

Neomorphism is the term coined by Folk (1965) to cover the combined processes of inversion and recrystallization. ^[1] Inversion refers strictly to the change of one mineral to its polymorph in the solid (dry) state; when aragonite transforms to calcite in the presence of water, it actually occurs by dissolution of less stable aragonite and nearly simultaneous precipitation of more stable calcite — a process most geologists call calcitization — and during diagenesis most aragonite is eventually calcitized. ^[1] Recrystallization indicates a change in crystal size or shape with little or no change in chemical composition or mineralogy; calcitization and recrystallization commonly go hand in hand. ^[1]

Neomorphism operates in all three diagenetic realms but is most important in the meteoric and subsurface environments. ^[1] It affects both carbonate grains and micrite, typically increasing crystal size, and when pervasive can convert an entire rock to coarse-grained sparry calcite — meaning a fine-grained (micritic) limestone can be completely transformed into a coarse-grained sparry rock. ^[1] One of the most difficult problems in carbonate petrography is distinguishing neomorphic spar (large clear calcite crystals produced by recrystallization) from sparry calcite cement (precipitated in pore spaces), since both appear optically similar in thin section. ^[1]

Replacement

Replacement involves dissolution of one mineral and nearly simultaneous precipitation of another mineral of different composition in its place, and can occur in all diagenetic environments. ^[1] Beyond dolomitization of CaCO₃, a range of noncarbonate minerals may replace carbonate minerals during diagenesis: microcrystalline quartz (chert), pyrite (iron sulfide), hematite (iron oxide), apatite (calcium phosphate), and anhydrite (calcium sulfate). ^[1] Replacement by anhydrite is particularly common in carbonate-evaporite sequences at depth, while replacement by chert (microcrystalline quartz) is common in both the meteoric and deep-burial environments, with silica documented replacing Paleozoic carbonates at burial depths ranging from 30–1,000 m; replacement by silica may be highly selective, preferentially affecting fossils and other carbonate grains over micrite. ^[1]

Physical and Chemical Compaction

Newly deposited carbonate sediments have initial porosities ranging from 40–80 percent; as burial proceeds, overburden pressure causes grain reorientation and tighter packing, reducing porosity and thinning beds. ^[1] At burial depths approaching 305 m (1,000 ft), grains may also deform by brittle fracturing and breaking or by plastic/ductile squeezing; and even at depths as shallow as 100 m, compaction can reduce the depositional thickness of carbonate sediments by as much as one-half, with accompanying porosity losses of 50–60 percent of original pore volumes. ^[1]

Chemical compaction begins at burial depths of approximately 200–1,500 m, where pressure solution at grain-to-grain contacts produces interpenetrating or sutured contacts; on a larger scale, pressure solution generates distinctive seams called stylolites, which are especially common in carbonate rocks and are marked by concentrations of clay minerals and other fine non-carbonate minerals (the insoluble residue left as carbonate dissolves). ^[1] Stylolites range in scale from microstylolites between grains (amplitude less than 0.25 mm) to large stylolites with amplitudes exceeding 1 cm; pressure solution with accompanying stylolite formation causes significant loss of porosity — perhaps as much as 30 percent of original pore volume — and further thinning of beds. ^[1]

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