Sedimentary Petrology - Diagenesis of Carbonate Sediments
|Oobiosparite grainstone, Jurassic, N Yorkshire, UK.
Image: Anton Kearsley.|
- This web page is adapted from an online lecture delivered as part of
a general first year university Petrology module. It serves as an introduction
to this topic.
- Links and references have not yet been fully updated.
Diagenesis refers to the physical and chemical changes which take
place after the deposition of a sediment; it may include:
- dissolution and alteration of existing grains
- precipitation of new materials in pore spaces
- unstable minerals may be destroyed
- new minerals may grow
- existing crystals may become enlarged
Carbonates are much more susceptible to diagenesis than terrigenous clastic
sediments, and original structures and textures are often completely destroyed.
Diagenesis of carbonates is of great economic importance. Many important
oilfields are produced from carbonate reservoirs - e.g. the giant onshore
oilfields of the Middle East. Diagenesis controls the porosity and permeability
of carbonate reservoirs.
Tucker, 1991, pp. 133-154
Note: all references to Boggs, 1995 and Tucker, 1991 in these pages refer
BOGGS, S. 1995. Principles of Sedimentology and
Stratigraphy (2nd edition). Prentice Hall.
TUCKER, M.E. 1991. Sedimentary Petrology (2nd edition).
Diagenetic processes in carbonates
Can be summarised as:
1. solution of more unstable minerals (especially aragonite), creating
2. pore-filling by cements
3. alteration of original minerals to new ones, especially replacement
by neomorphic calcite:
- hi-Mg calcite ---> low-Mg calcite
- aragonite ---> low-Mg calcite
- fine-grained ---> coarse-grained (aggrading neomorphism)
Calcium carbonate cement and its recognition
chemically precipitated material, whether a new mineral, or an addition to
an existing mineral, may form a cement, which binds the grains of the sediment
together to form a rock.
It is important to distinguish between sparite cement (pore
fill) and neomorphic spar, an in situ replacement
of calcium carbonate in the solid state.
Cement is deposited in pores: these may be primary - voids between
or within grains, or they may be secondary, formed during diagenesis by
solution, or other chemical or physical changes.
Some of the features which characterize cements include:
- generally clear and clean appearance, with well-defined, often (but not
always) straight crystal boundaries;
- spar between the grains which does not penetrate into or cut across grains;
- sharp contacts between spar and particles;
- micrite matrix is present but unaltered; aragonite fragments 'replaced' by
- the presence of two or more generations of spar;
- straight crystal edges and frequent triple junctions with 180° angles
- long axes of crystals normal to grain surface;
- increasing crystal size away from grain surface.
The composition and crystal habit of calcium carbonate cements are
highly dependent on variations in physical and chemical conditions.
Carbonate cement types
The type of cement which forms depends particularly on Mg/Ca ratio
and salinity of the solution.
- In solutions where Mg/Ca ratios are low, calcite can crystallise freely,
forming large, rather equant crystals.
- Where Mg/Ca ratios are high, calcite and aragonite crystals are prevented
from growing sideways, so that fibrous, acicular, micritic and pelletal forms
Carbonate cements can be divided into several important types
Early submarine and beach cementation
is common in modern warm water carbonates:
- fringing or drusy cements formed in submarine environment; often fibrous
- micrite cements, formed in submarine environment. Often high-Mg calcite,
Freshwater (Meteoric Water) Cements
- blocky or mosaic cements, formed in freshwater, phreatic environment: often
the main pore-filling cement;
- vadose freshwater cements, formed above the water table, where pore spaces
were not completely filled with water;
- syntaxial or rim cements, growing in optical continuity with large single
calcite crystals, especially echinoderm grains
- Carbonate grains are more susceptible to compaction than quartz grains.
- Grains may behave in either a brittle or a ductile fashion. Early
cementation, common in carbonates, protects grains from compaction.
- Carbonate rocks and particles are very susceptible to pressure
solution. During compaction, strain is localized at grain contacts.
Pressure increases solubility. If pore water is present, this results in
localized dissolution at the grain contact.
As the stress is usually vertical, due to overburden pressure, there is
preferential solution of the upper & lower surfaces of grains, across a solution
film. This results in irregular, sutured contacts between the
Grain-to-grain pressure solution must occur before the filling of all the
pore spaces by cement - a pore-filling cement protects the grains against
compaction and pressure solution.
in more cemented rocks, pressure solution takes place along much more
extensive surfaces - these are stylolites. Dissolution often
starts along a bedding surface, due to overburden pressure. A burial depth of at
least several tens of metres is required for stylolite formation.
- Stylolites cross the whole rock, cutting grains, matrix and cement.
- They often have a zigzag form, due to differences in solubility of the different
components, e.g. skeletal fragments may be less soluble than micrite matrix.
- The less soluble parts form 3-dimensional 'fingers', extending into the
more soluble parts.
- Amplitude of stylolites may be from <1mm to >1m. The amplitude shows
the minimum thickness of dissolved material.
- Insoluble material becomes concentrated along the stylolite surface - often
clay & hydrocarbons. Thus, the stylolite surface may be an important permeability
barrier, affecting the flow in aquifers and petroleum reservoirs.
- Major effects: stylolites commonly result in up to 35% reduction of thickness
of a sequence.
- In areas of tectonic compression, vertical stylolites can contribute to
considerable lateral shortening.
- Where does the dissolved calcium carbonate go? Ions may migrate along the
solution film, or into pore spaces. The supply of CaCO3 from pressure
solution along stylolites is thought to be a major source of late diagenetic
- Most dolomite is of replacement origin:
- It replaces existing calcium carbonate
- Dolomite often destroys and cuts across the original textures of the
- Replacement may be complete or partial: if partial, dolomite may form
isolated grains or patches in zones of greater permeability.
This page was written by Roger Suthren
Last Modified: 24 April, 2015 21:10