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)

Shallowing-upward peritidal cycle 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

Many carbonate producers are photosynthetic (algae) or symbiotic (foraminifera) Rate of carbonate sediment production is much greater in shallow water

BeachrockBenthic mixed layer 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

In contrast to clastic sedimentary rocks, carbonates are composed nearly entirely of calcium carbonate (CaCO 3 ), 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

Cation radius in aragonite is larger Incorporates strontium (Sr) in place of Ca, up to several thousand ppm 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 high-magnesium calcite (HMC)

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 Ooid shoals, Bahamas Precipitate from supersaturated water (with microbial influence?) in tropical shallow-water environments Spherical grains with concentric layers of calcite/aragonite (cortex) around a central nucleus

Rounded sand-sized grain ( mm) composed of structureless micrite (microcrystalline carbonate or “carbonate mud”) Polygenetic and origin difficult to determine – may be fecal pellets, micritized shell fragment or ooid, etc. Extremely common sediment grain on shallow platforms, particularly in low- energy lagoon environments 3. Peloids

Micritization: endolithic microbes (algae, fungi, bacteria) bore into allochems and their tubes are later filled by micrite Micritic envelope

4. Aggregates and 5. Clasts GrapestoneClast Intraclasts (from area of deposition) and extraclasts (from older rocks) Aggregates of grains (usually ooids) cemented by microbial activity Often indicate storm reworking of cemented strata Mud/silt winnowed but sand grains stable enough to allow microbial colonization

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 waterHigher energy, micrite is winnowed

Micrite aragonite needles Penicillus Micrite partly forms by inorganic precipitation of aragonite needles (“whitings”) Calcareous green algae also produce aragonite needles Whitings, Persian Gulf