Carbonate Sedimentation Processes
The shallow marine “carbonate factory” differs from siliciclastic sedimentation
in several fundamental ways:
1. Clastics produced by bedrock weathering and transported to site of
deposition; carbonates formed in place by biological/chemical precipitation
What is the significance of grain size in siliciclastics and carbonates?
2. Grain size of clastics reflects water energy only; grain size of carbonates
reflects size of skeletons or precipitated grains (and water energy)
What drives vertical facies successions in siliciclastic and carbonate sections?
3. Clastic facies shifts mostly driven by base level change; carbonate facies
changes can arise from intrinsic properties of carbonate sedimentation
Shallowing-upward peritidal cycle
Many carbonate producers are photosynthetic (algae) or symbiotic (foraminifera)
Rate of carbonate sediment production is much greater in shallow water
How do substrate consistency and lithification differ?
4. Clastics remain unconsolidated until they are deeply buried; carbonates
often cemented on or just below seafloor during early burial
Benthic mixed layer
Beachrock
In contrast to clastic sedimentary rocks, carbonates are composed nearly
entirely of calcium carbonate (CaCO3), either calcite or aragonite
Calcite
Stable polymorph at surface P, T
Aragonite
Metastable at surface P, T
But is the favored precipitate in modern ocean
Atomic radius of Ca in calcite ≈ Mg radius
Allows limited solid solution of Mg in calcite crystal lattice
Calcite with >4 mole % (usually <20%) is called highmagnesium calcite (HMC)
Cation radius in aragonite is larger
Incorporates strontium (Sr) in place
of Ca, up to several thousand ppm
Despite little mineralogical variation, carbonate rocks are composed of a great
variety of grain types (allochems)
1. Skeletal fragments
Shells made by marine invertebrates, algae, and protists
E – echinoderm
T – trilobite
B – brachiopod
Skeletal mineralogy varies among taxonomic groups
Groups were dominant sediment constituents at different times
2. Coated grains – ooids and pisoids
Spherical grains with concentric
layers of calcite/aragonite
(cortex) around a central nucleus
Precipitate from supersaturated
water (with microbial influence?) in
tropical shallow-water environments
Ooid shoals, Bahamas
3. Peloids
Rounded sand-sized grain (0.1-0.5 mm) composed of structureless micrite
(microcrystalline carbonate or “carbonate mud”)
Extremely common sediment grain on
shallow platforms, particularly in lowenergy lagoon environments
Polygenetic and origin difficult to
determine – may be fecal pellets,
micritized shell fragment or ooid, etc.
Micritization: endolithic microbes (algae, fungi, bacteria) bore into allochems
and their tubes are later filled by micrite
Micritic envelope
4. Aggregates and 5. Clasts
Aggregates of grains (usually ooids)
cemented by microbial activity
Intraclasts (from area of deposition)
and extraclasts (from older rocks)
Mud/silt winnowed but sand grains stable
enough to allow microbial colonization
Often indicate storm reworking of
cemented strata
Grapestone
Clast
6. Matrix and 7. Cement
Pore space between allochems is either filled by a matrix of fine-grained micrite
(carbonate mud) or by cement of precipitated calcite/aragonite
Lower energy, quiet water
Higher energy, micrite is winnowed
Micrite partly forms by inorganic precipitation of aragonite needles (“whitings”)
Calcareous green algae also produce aragonite needles
Whitings, Persian Gulf
Penicillus
Micrite aragonite needles