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EES 2021
LAB 3 – CARBONATE ROCKS
Name________________________________________________
Date_________________
Carbonate rocks are fundamentally different than siliciclastic rocks in terms of their depositional
environments and chemistry. To deal with these differences, geologists have developed classification
schemes and facies models specific to carbonate rocks. While the study of carbonate rock depositional
environments and subsequent diagenetic alteration are subjects for at least a full semester class, we will
attempt to cover some of the basics here.
Components of Carbonate Rocks
Carbonate rock grains, called allochems, can be divided into 4 broad categories: skeletal allochems,
nonskeletal allochems, matrix, and cement. We will deal with skeletal, nonskeletal allochems and matrix
compositions for this lab.
Non-skeletal allochems
Non-skeletal allochems are components which are not derived from the physical remains of a living
organism (shell, coral bits, etc.). These include intraclasts, oolites, and pellets. Intraclasts are lithified
(or at least partially lithified) aggregates of carbonate sediment. Rip-up clasts are an example of an
intraclast grain. Oolites are spherical, concentric grains of carbonate sediment which coat a central grain
which acts as a nucleus. Wave agitation on shallow shoals promotes the growth of oolites much like a
snowball being rolled around in snow. Pellets are non-spherical, oblong shaped grains, which are
usually the product of fecal material.
Skeletal allochems
Skeletal allochems are transported bits and pieces of marine organisms. This includes (but is not limited
to) brachiopod values, pelecypod (bivalve) valves, gastropods, echinoderms, corals, and bryozoans. The
degree of articulation (how “together” the organism is), abrasion, and diversity of skeletal allochems
must be considered when interpreting a depositional environment. For example, a rock containing many
broken brachiopod and coral fragments must have been deposited in a high-energy depositional
environment, such as a reef margin or breaker zone. Conversely, a rock containing exclusively wellpreserved delicate organisms (such as bryozoans) must have been deposited in a low energy depositional
environment, such as a lagoon. Sedimentary structures must also be incorporated into interpretations
regarding depositional environments.
Matrix
Carbonate matrix composition can be divided into 2 classes: micrite and sparite. Micrite is essentially
lime mud (analogous to the muddy matrix in a greywacke sandstone). Micrite appears “muddy” in hand
sample and is a dull brownish color in thin section. Sparite has a crystalline texture (interlocking grains)
and appears clear in thin section when viewed in plain polarized light.
Classification
Because the nature of the origin of carbonate rocks is almost unequivocally different than that of
siliciclastic rocks, carbonate rocks are classified based on different characteristics than siliciclastic
rocks.
Folk’s (1959, 1962) scheme focuses on the composition of carbonate rocks. In Folk’s classification
scheme, the type of allochem serves as a prefix to the cement type in the rock. For example, a rock
containing mostly ooids in a sparry matrix would be classified as an oosparite. Folk’s classification
further separates rocks which have formed in-situ. This is the biolithite category and refers to reef rocks
and stromatolites. Rocks lacking allochems are simply called micrites or sparites (depending on the
matrix). Rocks with cavities (usually micrites with sparite filled cavities) are referred to as dismicrites.
Depositional Setting: Input the number that corresponds with the number on the figure. There are extra numbers
than there are depositional environments, so be careful on which answer you select.
SAMPLE #
ROCK NAME
DEPOSITIONAL SETTING
1
2
3
4
5
6
7
8
9
1
10
2
52
6
1
7
3
8
4