6.3: Carbonate Components and Classification

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.

Figure \(\PageIndex{1}\): Illustration showing some fundamentals of carbonate rocks in thin section as they would appear in plane polarized light (Page Quinton via Wikimedia Commons; CC BY-SA 4.0).

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.

 

Figure \(\PageIndex{2}\): Schematic thin section (PPL) diagram showing the important components of carbonate rocks in thin section (Page Quinton via Wikimedia Commons; CC BY-SA 4.0).

 

Figure \(\PageIndex{3}\): The four main types of grains in carbonates (all images from Michael C. Rygel via Wikimedia Commons; CC BY-SA 4.0). A) Photomicrograph (PPL) of bryozoan fragments in the Mississippian Lodgepole Formation, SW Montana. Skeletal fragments can come in an impressive variety of shapes and sizes ... this diversity of form is even more pronounced by the 2D slice of the thin section. B) Photomicrograph (PPL) of ooids in the the Cambrian Mifflintown Formation, PA. C) Photomicrograph (PPL) of peloids in the bryozoan fragments in the Ordovician Benner Formation, PA. D) Photomicrograph (PPL) of lithoclasts in Cambrian carbonate from upstate NY.

 

Figure \(\PageIndex{3}\): Hand sample and thin section photographs showing coated grains.  A and B are ooids (field of view in A is ~20 cm), C and D are pisoids (field of view in C is ~12 cm), and E and F are oncoids (field of view in E is ~10 cm).  A and E are from James St. John via Wikimedia Commons and Flickr; CC BY-SA 4.0.  All other images are from Michael C. Rygel via Wikimedia Commons; CC BY-SA 4.0.

 

Figure \(\PageIndex{4}\): Examples of micrite (lime mud); all images from Michael C. Rygel via Wikimedia Commons; CC BY-SA 4.0.  A) Photomicrograph (PPL) of micrite in a lime mudstone in the Mississippian Lodgepole Fm., SW Montana. B) Photomicrograph (PPL) showing a more magnified view of the central part of the image in Fig. 2A. Note that even at this scale, the micrite appears homogeneous. C) Photomicrograph (PPL) of micrite with coarse spar infilling gastropods and ostracods. Specimen is from the Gastropod Limestone Member of the Cretaceous Kootenai Fm., SW Montana. D) Photomicrograph (PPL) of micrite with scattered ostracods and calcispheres. Specimen is from the Mississippian Lodgepole Fm., SW Montana.

 

Figure \(\PageIndex{5}\): Photomicrograph (PPL) showing an oolitic grainstone with interstitial areas filled with spar (white) and/or blue epoxy (which infilled formerly open spore spaces during preparation of the slide; Michael C. Rygel via Wikimedia Commons; CC BY-SA 4.0).

 

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 \(\PageIndex{6}\): The Folk classification scheme assigns names to limestones based on the most abundant grain(s) and the nature of the interstitial material (Page Quinton via Wikimedia Commons; CC BY-SA 4.0).

 

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 \(\PageIndex{9}\): The Dunham classification scheme assigns names to limestones based on the abundance of mud and the support mechanism of the grains (Page Quinton via Wikimedia Commons; CC BY-SA 4.0)

 

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.