Lec 9 - carbonate diagenesis

what are the 6 major processes for carbonate diagenesis

Cementation, microbial micritization, neomorphism, disolution, compaction, dolomitization

What is cementation

Major diagenetic process that depends on the water chemistry. PCO₂ and Mg/Ca ratio and carbonate supply rate

microbial micritization

formation of micrite envelopes around bioclasts and completely micritized grains

What is neomorphism

replacement and recrystallization processes where there may have been a change in mineralogy

what are the two types of neomorphism

aggrading neomorphism and calcitization

what is aggrading neomorphism

coarsening of crystal sizes in a lime mud (micrite)

what is calcitization

replacement of aragonite shells and cements by calcite

what is dissolution

the replacement of aragonite shells and cements by calcite (produces secondary porosity)

Compaction

close packing as a result of burial

What is Dolomitization

CaMg(CO₃)₂ precipitated in near surface and burial environments

3 major diagenetic environments

• near-surface meteoric

• marine

• burial

near-surface meteoric

Diagenesis caused by rain or ground water. Occur when shoreline progrades, sea level fall, or limestone is uplifted

Marine diagenesis

diagenesis takes place on seafloor in both shallow and deep water in the intertidal-supratidal zone

Burial

tens to hundreds of meters down to several km where metamorphic dehydration reactions and recrystallization takes over.

what is intertidal-supratidal diagenesis

cementation in intertidal produces beach rock, most common in tropics and subtropics

where does beach rock formation take place

a few tens of cm below the surface of the beach

what type of cements form in intertidal-supratidal diagenesis

aragonite and/or high Mg calcite.

• Aragonite occurs as fringes from 10-200 microns thick of acicular crystals oriented normal to the grain.

where does Shallow subtidal cementation occur

occurs in low latitude shallow marine areas

what processes are involved in shallow subtidal cementation

• cementation, in high energy areas

• microbial micritization, more common in quieter water

what happens in stagnant environments during shallow subtidal cementation

cementation is restricted to intraskeletal cavities such as within gastropods.

How does Aragonite manifest in marine cements

mostly as acicular fringes and needle meshwork, but botryoid is also common

how does High Mg calcite occur

occurs as bladed cements, 20-100 microns long and <10 microns wide. Forms isopachous fringes

where does significant marine carbonate dissolution occur?

shallow seafloor in higher latitudes where seawater is undersaturated with CaCO3

describe the relationship between saturation and temperature for CaCO3

saturation state of CaCO3 decreases with decreasing temperature

Properties of marine cement in limestones

• first-generation cement

• may form isopachous fringes around grains or cavity walls

• fibrous

• non-ferroan and non-luminescent

• succeeded by clear calcite spar

How commonly are original aragonite cements preserved in ancient limestones?

Very rarely preserved - most are altered to calcite during diagenesis.

Describe the shape of calcite crystals replacing aragonite cements.

Irregular to equant often cutting across the original acicular aragonite pattern

What occurs during meteoric diagenesis

• carbonate dissolution

• cementation

• formation of soils

what is calcite spar

cement that occupies majority of original pore space in limestone. Clear equant calcite

features of Sparite

• located between grains

• clear with few inclusions

• drusy fabric

• preferred growth direction

• presence of planar intercrystalline boundaryes

what cement is typical for near-surface meteoric diagenesis

drusy sparite

For drusy sparite to form what are the 3 possible sources of CaCO3

• pore water

• pressure dissolution

• dissolution of CaCO3 from skeletal aragonite

Types of aggrading neomorphism

• microspar: pseudospar formation from micrite

• calcitization: of originally aragonitic skeletons, ooids, and cement

Chemical compaction

result of increased solubility at grain contacts and along sediment interfaces under an applied stress

what are 3 common textures that result from chemical compaction

fitted fabrics, stylolites, and pressure-dissolution seams

mechanism for dolomitization - direct precipitation

seawater saturated

mechanism for dolomitization - replacement

one-to-one exchange

Dolomitizing fluids

fluids provide the neede input without needing to remove anything