Carbonate Rock Diagenesis

Introduction

Information and diagrams redrafted after Tucker and Wright (1990); Moore (2001); James and Choquette (1990).

Diagenesis definition

All those processes taking place after initial deposition, leading to the stabilisation and lithification of the sediment.

Carbonate sediment is in geochemical equilibrium with the environment in which it forms (e.g., seawater). When it later comes into contact with other fluids – shallow marine burial fluids, meteoric, deep burial fluids, it is subjected to mineralogical and fabric transformation, stabilisation and lithification.

Controls on carbonate diagenesis

  1. Original mineralogical composition (aragonite and HMC unstable, LMC more stable)
  2. Diagenetic fluids (marine, meteoric, burial)
  3. Burial history (burial brines, T, P)

Diagenetic environments

Marine

Diagenetic processes
Diagenetic products

Micritisation

Micrite envelopes, micritized grains (peloids)

Cementation
  • Aragonite and HMC with acicular, botryoidal, fibrous and bladed cements;
  • isopachous rims in marine phreatic setting;
  • syntaxial cement.
  • Micrite peloidal cement

Cementation in high-energy intertidal settings

Beachrock

Low sedimentation rates, current/waves, cementation and boring

Hardgrounds

Dolomitization (intertidal, hypersaline lagoon)

Dolomite crusts, penecontemporaneous replacive mimetic dolomite

Meteoric

Diagenetic processes
Diagenetic products

Dissolution

Karst, caves, vug porosity

Cementation (vadose)

Meniscus, pendant cement, LMC

Cementation (phreatic)

Isopachous rims to mosaics of granular, blocky, equant LMC; syntaxial cement.

Stabilisation
  • Aragonite dissolution-replacement by LMC;
  • biomouldic and oomouldic porosity;
  • moulds infilled by equant LMC cement

Pedogenic processes

Caliche, rhizocretion

Dolomitization (mixing and phreatic zone)
  • Dolocrete (groundwater calcrete);
  • mixing zone dolomite

Dedolomitization

Transformation into calcite of previously formed dolomite.

Burial

Diagenetic processes
Diagenetic products

Dissolution

Vug porosity

Cementation
  • Isopachous rims and mosaics of equant granular and blocky calcite spar (LMC); drusy, poikilotopic and syntaxial fabric.
  • Fe-calcite (blue when stained with K ferricyanide) in reducing conditions of burial.

Neomorphism/Calcitisation

Replacement by LMC

Silicification

Replacement of carbonate minerals by SiO2 as amorphous silica or chalcedony.

Dolomitization
  • Fe-dolomite (mauve to green when stained with K ferricyanide) in reducing conditions of burial.
  • Baroque (saddle) dolomite due to hydrothermal fluids, T = 80-120°C.