Fragments of partially lithified carbonate sediment eroded from within the depositional basin and redeposited. Classification Schemes Two primary systems dominate carbonate petrography:
Marine organisms actively build skeletons and shells using calcium carbonate. Key contributors include corals, mollusks, foraminifera, and green algae.
A trigonal polymorph containing less than 4 mol% MgCO3MgCO sub 3 . LMC is the most thermodynamically stable form of CaCO3CaCO sub 3 at earth-surface temperatures and pressures. Dolomite ( CaMg(CO3)2CaMg(CO sub 3 close paren sub 2 origin of carbonate sedimentary rocks pdf new
. This causes widespread dissolution, creating secondary porosity, vugs, and karst networks. As the fluid becomes saturated deep underground, it precipitates stable low-magnesium calcite, cementing the remaining rock. The Deep Burial Zone
The transformation of soft carbonate sediment into indurated sedimentary rock involves a complex progression of chemical, physical, and mineralogical changes known as diagenesis. Because carbonate minerals are highly unstable when exposed to fluids outside their initial depositional environment, they undergo rapid alterations across three primary realms. A trigonal polymorph containing less than 4 mol%
High rates of seafloor spreading lead to extensive hydrothermal venting. This process strips magnesium from seawater, resulting in a low marine molar ratio (
The replacement of calcite by dolomite, often enhancing porosity and creating valuable reservoir rocks for oil and gas. interconnected intercrystalline secondary porosity
Because the crystal lattice of dolomite is significantly more compact than that of calcite, stoichiometric replacement results in a theoretical 13% reduction in solid mineral volume. This volume reduction generates extensive, interconnected intercrystalline secondary porosity, turning tight limestones into high-quality reservoir units.
[ Dissolved Inorganic Carbon (DIC) Pool ] │ │ Biotic │ │ Abiotic / Microbial Pathways ▼ ▼ Pathways ┌───────────────────┐ ┌─────────────────────────┐ │ Enzymatic Control │ │ Alkalinity Pumps │ │ (Shells, Skeletons)│ │ (Photosynthesis, SLR) │ └───────────────────┘ └─────────────────────────┘ │ │ └───► ▼ ◄──┘ [ CaCO3 Crystal Nucleation ] │ │ High Mg/Ca ▼ ▼ Low Mg/Ca (Aragonite) (Calcite) Biomineralisation (Biotic Production)