Classification of Carbonates

Geol 370: Sedimentology and Stratigraphy
Topic 7: Classification of Carbonates

1. Photo by W. W. Little
The primary function of carbonate rocks is, through weathering and
erosion, to provide the cement found within clastic rocks.
Cement

2. Photo by W. W. Little
Carbonates are rocks composed of at least 50% calcite and/or
dolomite and are produced primarily through biological (limestone)
and diagenetic (dolostone) processes.
Carbonates

3. As with clastics, there are many schemes for classifying carbonate
rocks. The two most commonly used are referred to as “Dunham” and
“Folk,” named for the individuals who devised them. Because of the
difficulty of distinguishing between matrix and cement in carbonate
rocks, most current classifications are based on thin-section analyses.
Classification of Carbonate Rocks

4. The Dunham classification is similar to that of clastic rocks, in that it
clearly shows the role of energy in sediment accumulation. The
main distinction between rock types is based upon the relative
abundance of allochems (framework grains) and matrix. Cement is
treated as open pore space.
Dunham Classification

5. Some restrict mudstone through grainstone to carbonate rocks that
contain “framework” grains of sand size and smaller. Wackestones with
particles greater than sand size are sometimes referred to as floatstones.
Packstones and grainstones with grains larger than sand size can be
called rudstones.
Variation on the Dunham
Classification

6. Mudstones are composed of at least 90% carbonate mud. In hand-
sample, it is often extremely difficult to distinguish between mud
and cloudy calcite spar cement.
Mudstone

7. Carbonate mud originates through four processes: algal disaggregation,
mechanical and biological erosion, and direct precipitation from sea
water.
Origin of Carbonate Mud

8. Calcareous Algae
Halimeda and Penicillus are the two most common forms of
calcareous algae.
Penicillus Halimeda Carbonate Mud
Slide modified from Gahn, 2006

9. Wackstones are mud-supported carbonate rocks in which allochems
make up more than 10% of the rock volume but do not come into
regular contact with one another. They can appear to “float” in the
matrix.
Wackstone
Fossiliferous wackstoneCrinoidal wackstone

10. Packstone
Packstones are closed-framework, grain-supported carbonate rocks.
Crinoidal packstone Bivalve/ehinoidal packstone

11. Grainstone
Grainstones are open framework, grain-supported carbonate rocks.
Gastropodal grainstoneOolitic grainstone

12. An allochem is a carbonate particle that has been transported a short
distance and deposited as sediment.
Common Allochem Types

13. Intraclasts are intraformational lithic fragments derived through the
erosion of previously deposited carbonate sediment.
Intraclasts

14. Ooids are spherical “coated” carbonate grains consisting of concentric
laminations and are typically less than 1 mm in diameter. Coated grains
larger than a few millimeters are pisoids.
Ooids

15. Boundstones are carbonate rocks in which components have been
bound together through organic processes.
Boundstone

16. Bioclasts are the skeletal remains of organisms that lived
contemporaneously with depositional process that formed the deposit
that contains them.
Bioclasts (fossils)

17. Pelloids are composed of micro- to cryptocrystalline carbonate mud,
typically in elongate spheroidal shapes. Pelloids of fecal origin are
referred to as pellets.
Pelloids

18. Embry and Klovan (1971) expanded Dunham’s classification to
emphasize the importance of grain size in packstones and
grainstones and types of binding organisms in boundstones.
Expanded Dunham Classification

19. Floatstone
Floatstones are packstones in which at least 10% of the grains are
gravel size.
Rudist
Floatstone

20. Rudstone
Rudstones are grainstones in which at least 10% of the grains are
gravel size.
Rudist
Rudstone

21. Bafflestones are boundstones in which the binding organism acted
as a baffle (flow barrier).
Bafflestone
Algal
bafflestone

22. Bindstones are boundstones formed by encrusting organisms.
Bindstone
Modern stromatolitesStromatolitic dolobindstone

23. Bindstones are boundstones formed by organisms that produce a
rigid framework.
Framestone
Modern reefCoralline framestone
Photo by F. J. Gahn

24. Crystalline Carbonates
Crystalline carbonate rocks are those that do not retain their depositional
texture due to diagenetic alteration. A crystalline texture is unusual in
limestones but common in dolostones.

25. The Folk classification is more complex than that of Dunham and
incorporates relative abundances of allochems, matrix, and cement.
Allochem type also plays part in this classification.
Folk Classification

26. Micrite is equivalent to mudstone in the Dunham classification. As
the relative abundance of allochems increase, the type and
abundance of the allochem is used to modify the term micrite.
Micrite
Packed biomicriteMicrite

27. Sparite
Pelsparite
Biopelsparite
Biosparite
Sparite is equivalent to open
space in the Dunham
classification. As the
relative abundance of
allochems increase, the type
and abundance of the
allochem is used to modify
the term sparite.

28. Orthochemical carbonate rocks are
divided into two categories:
microcrystalline (equivalent to
Dunham’s crystalline category) and
autochthonous reef rocks/biolithite
(equivalent to Dunham’s
boundstone).
Orthochemical Carbonate Rocks

29. Biolithites are formed by organisms that bind sediment as part of
their growth process.
Biolithite/Reefrock

30. Dunham vs. Folk

31. Pores are cavities in the rock and can be related to depositional,
diagenetic, and tectonic processes.
Porosity

32. Primary intergranular porosity consists of depositional open space
between allochems that tends to decrease with burial through
compaction and cementation.
Intergranular Porosity

33. Primary intragranular porosity consists of depositional open space
within allochems that tends to decrease with burial through
compaction and cementation.
Intragranular (sheltered) Porosity

34. Secondary intercrystalline porosity consists of open space between
authigenic.
Intercrystalline Porosity

35. Secondary moldic porosity consists of open space within allochems
produced through dissolution.
Moldic Porosity

36. Secondary dissolution porosity consists of open space within or
between allochems formed by the removal of primary and
secondary material.
Dissolution (vug) Porosity

37. Dolomites form mostly through diagenetic processes that replace
Ca with Mg.
Dolomite