Petrology 186-212, Francis 2014
Petrology Lab 7: Carbonates and other Bio-precipitates
The chart below is a synthesis of Durham (1962) and Folks (1962) classifications of
Carbonate Rocks:

Allochthonous carbonates composed of transported carbonate clasts and mud

Autochthonous carbonates composed of organically-bound carbonate in ~ growth
position.

Crystalline carbonates whose primary textures have been destroyed by
recrystallization.
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Petrology 186-212, Francis 2014
Fossils and other allochems
Unlike the siliciclastic rocks, the clastic grains of carbonates range from fossil fragments, to
coated grains (ooides and pisolites), fecal pellets, and intraclasts (grains of pre-existng
carbonate rocks). As an introduction to the variety of fossil fragments, during coffee break,
try to identify as many fossils as possible in the table in the lounge, using the fossil guide
handed out in the lab.
Interclasts:
Broken fragments of pre-existing carbonate rocks.
Grapestone:
Cemented aggregates of coated grains. They develop in transgressive
environments where the water depth becomes deeper than that in which
the individual ooids will grow.
Coated grains:
Round grains that are made up of concentric layers of aragonite needles.
They form as a sand-sized sediment in shallow, agitated water (0 - 2 m) by the direct
precipitation of aragonite needles onto organic material coating the grain. They are
typically concentrically layered like a hailstone, sometimes with a fragment as a
nucleus, with each layer composed of tangential aragonite needles. Ooids in older
rocks may have a radial arrangement of crystals within individual layers. This may
reflect direct precipitation from seawater (high-Mg calcite) or recrystallization during
diagenesis (calcite). Dating studies indicate that individual ooids may take up to
several 1000 years to grow. The growth of any given layer is thought to be arrested
because of poisoning by Mg(OH)2, following which the organic coating returns,
which in turn traps more aragonite to form a new layer.
pisolites ( > 2 mm)
ooids ( < 2 mm)
Peloids:
Fecal pellets and other pellets of micrite.
Fossils:
Broken fragments of fossil parts.
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Petrology 186-212, Francis 2014
1. Allochthonous (allochemical) Carbonates(clastic)
Allochthonous carbonate rocks are made of allochems (clastic carbonate grains) and are
subdivided in much the same way as the siliciclastic rocks; according to grainsize into:
mudstones
grainstones
rudites
The major differences between allochthonous limestones and siliciclastic rocks are in the
nature of the clastic fragments (allochems), the degree of sorting, and the matrix or cement
that holds these rocks together. Typically the degree of sorting is much poorer than in
siliciclastic rocks, and there is a significant percentage of either carbonate mud (micrite)
and/or sparry calcite cement.
The allochemical carbonates are formally named according to their predominant
allochem type, the relative abundance of either micrite or sparry calcite cement,
and grain size (eq. biomicritic packstone or oosparite grainstone, intrasparitic
rudite).
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Petrology 186-212, Francis 2014
2. Autothonous (orthochemical) Carbonates (Boundstones)

Reef complexes are essentially living buckets, in which the walls of the bucket are
composed of boundstone consisting of corals and other anchored species such as
bryozoans. The shallow inner lagoon is filled with fine grainstones and micritic
mudstone, while the steep outer flanks are composed of coarser grainstones and
rudestone, formed from talus off the reef front.

Finely laminated boundstones are formed by mats of blue green algae that grow in
low lying intertidal to supratidal zones. These organic mats trap carbonate mud when
flooded, eventually forming a finely laminated micrite, in which the original organic
seams have been replaced by porous septa or sparite cement. One of the few clastic
sedimentary rocks in which the size of porosity is larger than the grains. Algae mats
and stomatolites were the dominant carbonates in the Precambrian, before the
development of reefs. Stromatolites represent localized buildups or cabbage-shaped
heads formed by a similar mechanism to that responsible for the algal mats.
This station contains a number of different boundstones. Try to determine the dominant
fossil type(s) in the reef samples, and look for the special cases of algal mats or
stromatolites
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Petrology 186-212, Francis 2014
3. Crystalline Carbonates and Dolomites
Because of the solubility of CaCO3, limestone is easily recrystallized under diagenetic and
low-grade metamorphic conditions. Extensive recrystallization destroys the original clastic
texture of these rocks, and where extensive, such rocks are simply referred to as crystalline
limestone or dolomite. During the recrystallization process, aragonite is typically converted
to calcite, and in some cases dolomite.
Although dolomite is a common rock in the stratigraphic record, particularly in the Paleozoic
where it is the dominate carbonate rock, modern day dolomitic sediments are relatively rare
and largely restricted to sabkha environments (salt-encrusted supra-tidal flats) where they
appear to form by the alteration of initial aragonite precipitates. Most dolomites are thought
to have formed either early by the penecontemporaneous alteration of unconsolidated
calcareous sediments or somewhat later during the diagenesis of carbonate rocks.
This station contains a variety of crystalline limestones and dolomites. In some cases, the
original clastic or fossil textures are preserved, but commonly dolomites are extensively
recrystallized and the original textures are partially or totally obliterated. Such rocks are best
termed crystalline dolomite. The presence of a light brown weathering surface often signals
the presence of dolomite rather than calcite or aragonite. The acid test is diagnostic, as is
density.
4. Evaporites, cherts, & iron formation:
This station contains a variety of chemical and biochemical precipitates and evaporites.
Examine each specimen, determine its mineralogy, and classify with the help of the class
notes on biochemical precipitates.

Chert:
Bedded cherts are characterized by layers of chert several centimeters in thickness
separated by thin seams of siliceous shale. They form in areas where there is little other
siliciclastic or carbonate sedimentation, such as deep water marine environments, below
the carbonate compensation level. Commonly associated with pillowed volcanic rocks,
turbidite sandstones, and other deep water sediments. Chert also forms as nodules in
shelf-type carbonate rocks during diagenesis due to the remobilization of silica from the
skeletons of diatoms and/or radiolarian, as well as sponge spicules.

Iron Formation
Iron formations are Fe - rich deposits characterized by the cyclic alternation of Fe-rich
and silica-rich bands on the scale of centimeters to meters. Although some are Paleozoic,
the majority are Precambrian in age. This age distribution might reflect a lower oxygen
fugacity for the Precambrian atmosphere that kept Fe in its more soluble Fe2+ state.
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