Dismicrite

Dismicrite is a type of micritic limestone that contains irregular patches or pockets of sparry calcite (coarse crystalline calcite) within a fine-grained micritic matrix. The term is derived from :

• Dis : meaning disrupted or disturbed
• Micrite : Microcrystalline limestone, composed of carbonate mud

Features or Properties of Dismicrite

Texture : Fine grained (micritic) with sparry calcite patches.

Composition : Mainly micrite (carbonate mud, CaCO₃) with coarser sparry calcite zones.

Appearance : Spotty or mottled due to contrast between micritic and spar.

Formation : Forms due to early diagenetic processes, gas escape structures, or biological activity.

Porosity : Can have vugs (irregular cavities) due to early dissolution and calcite recrystallizaiton.

Fig. “microscopic image of Dismicrite” (all images from Michael C. Rygel via Wikimedia Commons; CC BY-SA 4.0)

Formation processes

Micrite can be formed by a number of processes –

• Gas escape & soft sediment Disturbance
  • Microbial activity in carbonate mud produces gas.
  • Gas disrupts the micrite creating pockets where coarser calcite precipitates.
• Partial Recrystallization
  • Some micrite undergoes diagenetic transformation into sparry calcite.
• Burrowing Organisms
  • Bioturbation by marine organisms (burrowing invertebrates) can create disrupted textures in micritic sediments.

“Dismicrite can be found in Shallow marine carbonate platforms, Lagoonal and tidal flat environments, reef-associated micritic limestones”

Importance of Dismicrite

1. Indicator of early diagenesis in carbonate rocks.
2. Caninfluence reservoir quality in petroleum geology (affects porosity & permeability).
3. Helps interpret paleoenvironmental conditions (microbial activity, water chemistry).