The most common carbonate description schemes are comparable to those used for sandstones in that coarse- and fine-grained components are present at the time of deposition and that they can later be cemented together when minerals precipitate in pore spaces. Although you can gain a significant amount of information from carbonates in hand sample, thin sections are very useful for more nuanced descriptions and interpretations. Brief descriptions of the various components are provided in the sections that follow.
Components
Carbonate Grains
Carbonate grains
are analogous to the framework grains in a sandstone and consist of the following types:
Skeletal Particles (Fossils)
Skeletal particles
are composed of calcareous shells or skeletal material. Identification keys for fossils in thin section are provided in
Chapter 9.6
Coated Grains
Coated grains
form as carbonate is progressively added to a grain.
Ooids
are the most common type, they form when wave activity suspends a particle and allows for concentric layers of inorganic carbonate precipitation around a nucleus.
Oncoids
are a less common type formed where algae causes calcite to precipitate, but storms periodically reorient the grain causing precipitation to occur on alternating sides of the grain.
Pisoids
are superficially similar to ooids, but they are larger (>2 mm diameter) and grow because of
in situ
accumulation of calcite and aragonite in the vadose zone.
Peloids and Pellets
Peloids
are silt-to sand-sized particles composed of fine-grained lime mud (micrite); most are invertebrate fecal pellets.
Intraclasts
Intraclasts
are fragments of lithified carbonate that were incorporated into younger sediment (effectively rip-up clasts).
Micrite
Micrite
is fine-grained lime mud made of clay-sized crystals of microcrystalline calcium carbonate. Its analogous to the matrix in a sandstone in that its present at the moment of deposition. Micrite is most commonly formed from particles derived from algae or microfossils. Its also possible that it can from direct precipitation of these crystals in whiting events.
Spar
Spar
is relatively coarse-grained calcite cement that grows in the pore spaces of carbonates. The large, clear crystals are relatively easy to distinguish from the other carbonate components. Spar in carbonates is analogous to cement in siliciclastic sedimentary rocks.
Classification
Folk classification scheme
The
Folk classification scheme
is most useful for petrographic work. Samples are given a two part name; the first part is the most abundant grain type (bio-, pel-, oo-, or litho-) and the second indicates the nature of the interstitial material (-sparite or -micrite). Thus, a pelsparite would be a limestone composed mostly of peloids with with coarse spar between the shells and a biosparite would be most shell fragments with sparry calcite cement.
Figure \(\PageIndex{7}\): Photomicrographs (PPL) of different types of limestones in the Folk classification - part 1
(all images from
Michael C. Rygel
via
Wikimedia Commons
;
CC BY-SA 4.0
). A) Lithoclasts and skeletal material in an intrasparite from upstate NY. B) Lithoclasts and skeletal material in an intramicrite from upstate NY. Note that some of the dark micrite has started to recrystallize in the left half of the image. C) Oosparite from the Mississippian Lodgepole Formation, SW Montana. D) Oomicrite from the Silurian Keel Formation, Oklahoma. E) Biosparite. F) Biomicrite from the Mississippian Lodgepole Formation, SW Montana.
Figure \(\PageIndex{8}\): Photomicrographs (PPL) of different types of limestones in the Folk classification - part 2
(all images from
Michael C. Rygel
via
Wikimedia Commons
;
CC BY-SA 4.0
). A) Pelsparite from
the Mississippian Lodgepole Formation, SW Montana. B) Possible pelmicrite. In a situation where you have muddy peloids surrounded by micrite, the odds of being able to distinguish individual peloids after diagenesis is slim. However, the presence of pelsparite zones makes the "blobby" areas of micrite likely candidates for pelmicrite. C) Stromatoporoid boundstone from the Devonian Jefferson Formation, SW Montana. D) Mudstone Mississippian Lodgepole Formaton, SW Montana. E) Dismicrite. F) Recrystallized limestone from the Cambrian Pilgrim Formation, SW Montana.
Dunham Classification System
The
Dunham classification scheme
is most useful for hand sample and outcrop work. It is a four-fold scheme based on the presence/absence and relative abundance of mud:
(Lime) mudstones
have <10% grains
Wackestones
have >10% grains and are supported by mud
Packstones
are grain supported with interstitial mud.
Grainstones
are grain supported with little or no interstitial mud.
It is worth noting that with carbonates, shells are often the largest grains and thus maximum grain size could ultimately be controlled by biology. Because of this, the amount of mud is the best proxy for the amount of energy in the system.
Figure \(\PageIndex{10}\): Paired polished slabs and photomicrographs showing the main types of limestones in the Dunham classification scheme - part 1
(all images from
Michael C. Rygel
via
Wikimedia Commons
;
CC BY-SA 4.0
). A & B) Polished slab and photomicrograph (PPL) of a lime mudstone from the Mississippian Lodgepole Formation. C & D) Polished slab and photomicrograph (PPL) of a skeletal wackestone from the Mississippian Lodgepole Formation. Note that the micrite in the thin section has been partially dolomitized and many of the light colored patches between shells are actually dolomite crystals formed within the micrite. E & F) Polished slab and photomicrograph (PPL) of a skeletal packstone. Although the shells in the slab are somewhat widely spaced, this sample is grain supported because the shells are curved three dimensional forms that are touching each other in the third dimension.
Figure \(\PageIndex{11}\): Paired polished slabs and photomicrographs showing the main types of limestones in the Dunham classification scheme - part 2
(all images from
Michael C. Rygel
via
Wikimedia Commons
;
CC BY-SA 4.0
). A & B) Polished slab and photomicrograph (PPL) of a foram-rich skeletal grainstone. As with the packstone, this specimen is clast supported and although some foraminifera appear isolated, they are actually touching each other in the third dimension. C & D) Polished slab and photomicrograph (PPL) of a stromatoporoid boundstone. E & F) Polished slab and photomicrograph (PPL) of a recrystallized and dolomitized fossiliferous limestone. Although some fossils can still be identified in the polished slab, primary textures have been almost completely destroyed and the rock has a sandy appearance on weathered surfaces from the dolomite rhombs.