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GEOL 332 Lab 5
Sedimentary Rock Identification II
Name: _____________________________________________ Date: _______________
Sedimentary Rocks: Carbonates, et al.
Lab Equipment List: hand lens, ruler, pencil, and eraser.
Objectives
1) to become familiar with the properties important in recognizing and classifying sedimentary
rocks
2) to become familiar with the textures characteristic of sedimentary rocks;
Carbonate Sedimentary Rock Classification
In a simple model for the evolution of sedimentary rocks we find that if weathering, transportation, and
sorting go to completion all that remains are three end member rock compositions.

Siliciclastic Rocks: Quartz Arenite / Shale

Carbonate Rocks: Limestone / Dolomite
Limestones are not single composition rocks but a group of related rocks all composed of CaCO3 and
reacting with dilute HCl acid. Limestone [CaCO3] is also chemically related to Dolomite [CaMg (CO3)2].
Because all these rocks have CO3 in common they are called the Carbonates. The composition of most
Carbonates is derived from a combination of biological and chemical components.
Two Carbonate classification systems are used today, one by R.L. Folk and the second by R.J. Dunham.
The Dunham system is based on depositional texture (that is, the amount of matrix surrounding the
grains at the time of deposition). It uses such names as Mudstone, wackestone, packstone, grainstone,
and boundstone.
Carbonate rock names (Limestones and Dolomites) consist of a conjunction of two names, one
describing the ALLOCHEMS, the large pieces, the other describing the INTERSTITIAL MATERIAL.
Allochems are equivalent to gravel, sand, lithics or feldspars in the siliciclastics. Interstitial material is
equivalent to Clay or cements in clastics. There are four kinds of allochems:
1.
2.
3.
4.
Fossils - may be whole fossils, or broken and abraded fossils; all are called "bio" fragments
Oolites - small spheres
Pellets - fecal Pellets produced by invertebrate animals; look superficially like Oolites but are dull
Intraclasts - chunks of eroded Limestone deposited as a Conglomerate
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Interstitial Material
Micrite is "lime mud", the dense, dull-looking sediment made of Clay sized crystals of CaCO3. Micrite
forms from the breakdown of calcareous algae skeletons. It is not clear if all ancient Micrites formed in
the same way. Many Carbonates are composed of nearly 100% Micrite. Such rocks are simply called
Micrites.
With Carbonates containing allochems the question is whether Micrite is present or absent as an
interstitial material, and if present, by how much. If Micrite is present during deposition then it fills the
spaces between the allochems and the rock will be given a name which describes the allochems in a
Micrite matrix. For example, a rock with fossil fragments embedded in Micrite is called a "Biomicrite".
If the depositional environment has strong currents, only allochems may be deposited. If we could see
the sediment during deposition and all the allochems would be loose, like a pure sand or gravel. This is
analogous to a 100% siliciclastic sand on a beach with no silt or Clay. In this case, Micrite would be claysized and would be washed away. The rock formed is then composed only of allochems, held together
by clear to translucent Calcite crystals with rhombohedral cleavage (called SPAR or SPARITE) acting as a
cement. The spar is precipitated from fresh or marine water percolating through the sediment after
deposition, but before final cementation.
Classification of Carbonates
The classification of Carbonates using the allochem/interstitial material system (the Folk System) is very
systematic and straight forward. The allochem name is combined with the interstitial name (Micrite or
spar). The table below shows the major categories of Carbonate rocks based on their allochems and
interstitial material.
But what happens if there is more than one allochem in the rock, or there is a mixture of Micrite and
spar? You can easily build your own descriptive rock names. The name is built up by stringing together
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all the allochem names in order from least to most abundant, and then adding the interstitial material
name ("matrix" below for short).
For example, a rock like this:

Oolites + Fossils + Spar matrix = Oo bio sparite
o
The name is written as one word, Oobiosparite.
Another example (again allochems from least to most abundant):

Pellets + Oolites + Fossils + Micrite matrix = pel oo bio Micrite
o
The name is written as one word, Peloobiomicrite.
But what if there is both Micrite and spar matrix? The system is the same; just list them from least to
most abundant.

Fossils + Spar matrix + Micrite matrix = bio spar Micrite
This system goes through other levels of refinement, such as in the table below where the abundance of
allochems is indicated. Other modifiers may deal with different sizes of allochems.
A classification such as this one works well if you want to construct rock names from observations. The
system, however, does not lend itself well to constructing keys for classification. A key requires the
establishment of arbitrary categories of rocks, and a system like the one above deals with all the myriad
combinations that are possible.
Most limestones are classified by Folk allochemical rocks if they contain over l0% allochems (transported
carbonate grains). Based on the percentage of interstitial material, the rocks may be further subdivided
into two groups: sparry allochemical limestones (containing a sparry calcite cement of clear coarsely
crystalline mosaic calcite crystals) and microcrystalline allochemical limestone (containing
microcrystalline calcite mud, micrite, which is subtranslucent grayish or brownish particles less than
about 5 microns in size). Further subdivision is based on the allochem ratios of Folk (1962) are shown in
Scholle & Ulmer-Scholle (2003).
Thus Folk's classification is most suited for thin section study. Remember that he terms rocks with
appreciable matrix as micrites while matrix-free rocks that contain sparry calcite cement are termed
sparites. As you can see sparites and micrites are further subdivided by means of their most common
grains.
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In contrast, Dunham's classification (figures above) and its modification by Embry and Klovan (1971) and
James (1984) deals with depositional texture. For this reason, his scheme may be better suited for rock
descriptions that employ a hand lens or binocular microscope. For example, if the grains of a limestone
are touching one another and the sediment contains no mud, then the sediment is called a grainstone. If
the carbonate is grain supported but contains a small percentage of mud, then it is known as a
packstone. If the sediment is mud supported but contains more than 10 percent grains, then it is known
as a wackestone, and if it contains less than 10 percent grains and is mud supported, it is known as a
mudstone.
If one compares the two classifications, a rock rich in carbonate mud is termed a micrite by Folk and a
mudstone or wackestone by Dunham. Moreover, a rock containing little matrix is termed a sparite by
Folk and a grainstone or packstone by Dunham. The wide range of percentage of mud matrix that a
carbonate may have and still be termed a packstone by Dunham sometimes reduces the utility of this
classification. Embry has modified Dunham's classification and Klovan (1971) to include coarse grained
carbonates (above figure). In their revised scheme, a wackestone in which the grains are greater than
2mm in size is termed a floatstone and a coarse grainstone is called a rudstone.
Both terms are extremely useful in description of limestones. Embry and Klovan to more graphically
reflect the role that the organisms performed during deposition also modified the boundstone
classification of Dunham. Terms such as bafflestone, bindstone, and framestone are useful in concept
but are extremely difficult to apply to ancient limestones where diagenesis and sample size limit one’s
ability to assess an organism’s function.
The last two pages of this lab includes a key to identify rocks based on their allochems and interstitial
material. Just be aware that its main weakness is that there are always rocks that do not fit easily into its
simple categories. There is also a chemical and biochemical rock identification key.
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Folk Sedimentary Rock Classification:
Folk’s (1959, 1962) classification of limestones, which uses prefixes to indicate the framework grains
present (bio- for fossils, pel- for peloids, oo- for Ooids, and intra- for intraclasts) and stems to indicate
whether the interstitial calcite is micritic or sparry. If the rock is originally bound together (as in a reef
rock), it is a biolithite.
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Textural maturity classification of limestones proposed by Folk (1962). Textural maturity classes are
based on the percentage of allochems present, their degree of sorting, and the extent of rounding (a
function of abrasion history). (Folk, 1965)
The Dunham classification of carbonate sedimentary rocks (Dunham 1962) with modifications by Embry
& Klovan (1971). This scheme is the most commonly used for description of limestones in the field and
in hand specimen.
Rock Identification

Step 1, fill out the table for the known sedimentary rock samples. (15 pts)

Step 3, fill out the table for the unknown sedimentary rock samples. (15 pts)
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(Dunham, 1962)
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References:
Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture. In: Ham, W. E.
(ed.), Classification of carbonate rocks: American Association of Petroleum Geologists Memoir, p. 108121.
Embry, AF, and Klovan, JE, 1971, A Late Devonian reef tract on Northeastern Banks Island, NWT:
Canadian Petroleum Geology Bulletin, v. 19, p. 730-781.
Folk, R.L., 1959, Practical petrographic classification of limestones: American Association of Petroleum
Geologists Bulletin, v. 43, p. 1-38.
Folk, R.L., 1962, Spectral subdivision of limestone types, in Ham, W.E., ed., Classification of carbonate
Rocks-A Symposium: American Association of Petroleum Geologists Memoir 1, p. 62-84.
Folk, R.L.; 1965, Petrology of Sedimentary Rocks, Hemphill.
James, N.P., 1984, Shallowing-upward sequences in carbonates, in Walker, R.G., ed., Facies Models:
Geological Association of Canada, Geoscience Canada, Reprint Series 1, p. 213–228.
Scholle, P. A. and Ulmer-Scholle, D. S, 2003, A Color Guide to the Petrography of carbonate Rocks: AAPG
Memoir 77, 474 p
Some of the material in this lab comes from http://www.sepmstrata.org/page.aspx?&pageid=610
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Flowchart to help identify Sedimentary Rocks
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