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Carbonate Rock Classification and Interpretationof Depositional Environments
Aims
To introduce you to the means of describing carbonate
rocks and then to classify them using Dunhams and
Folks classifications
To introduce you to the use of this classification in
making simple palaeoenvironmental interpretations of
carbonate rocks
Task 1 Sample C or D orientation exercise
Identify the principal grain types (allochems) in sample
(CS) C or D
Types of carbonate grains are listed below
Identify the nature of the intergranularmaterial
Is it crystalline cement, lime mud (micrite), porespace or a mixture of types?
Classify the rock according to Dunhams classification
(Figure 1) and then state an equivalent classification in
Folks scheme
Describe sample fully on the sheet provided
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Task 2 Carbonate Rocks and Depositional Environment
You have been provided with eight carbonate rocks ([CS] A H,
note that sample E has three different lithologies). These are
taken from a quarry in which a succession of Jurassiccarbonate rocks is exposed. The log of this succession (a log
shows changes in grain size and sedimentary structures with
height above the base of the stratigraphic section) is shown in
Figure 5. The height at which each sample was collected is
indicated. The aim of this exercise is to determine the
environment of deposition of the succession using the rock
textures and compositions, as well as sedimentary structures
(shown on the log).
For each sample, try to:
Identify the principle grain types (allochems) in each
sample (see Table 1 and Figures 3 & 4)
If grains are present, identify the type ofintergranularmaterial (cement, matrix or pore space)
Classify these rocks according to Dunhams
classification using the flow chartand Figure 1, and state
an equivalent in Folks scheme
Once you have done this try to indicate the most likely
environmental deposition of each rock on the diagram
indicating carbonate depositional environments on an idealised
carbonate ramp (Figure 2).
The rocks were taken from a quarry section where they were
found one above another as shown in the log of the quarry
(Figure 5). Because sedimentary rocks are normally formed bysuccessive layers of sediment being added on top of previously
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deposited material, this tells us that sample A is the oldest and
sample H is the youngest (deposited last).
We can use this to infer how the environment changed in the
area of the quarry at the time when the rocks were deposited.
Now attempt to answer the following questions
and provide reasoning and evidence for your answers!
1. What happened to the water depth with time? Explain
your reasoning.
2. What happened to the energy conditions with time and
why?
3. Several of the samples were deposited in peritidal
(around the tides) environments from subtidal lagoon
to supratidal flats. These deposits commonly form
shallowing-up cycles which show repeated successions
of rock types and sedimentary structures. Thesefeatures are related to the length of time the sample was
exposed (never in the subtidal lagoon, but during low
tides in intertidal flats, etc.). Analogy to cores from
modern day tidal flats has allowed geologists to make
precise palaeoenvironmental interpretations of such
successions. A typical shallowing-up succession is
shown in Figure 6.
Which samples do you think represent deposition in
which peritidal environments?
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Carbonate Rocks in Thin Section
The following thin sections correspond to the hand specimens
you began to study last week
Thin Section Hand Specimen
PSI (CS) B
MBS8 (CS) C
MBS20 (CS) D
PORT I (CS) G
Objectives
(1) Complete full descriptions of each thin section
concentrating on
Identifying the principle grain types, their state of
preservation, etc.
Identification of the intergranular material
(2) Refine your palaeoenvironmental interpretations foreach sample
Make a list of criteria that you have used to determine
the environment of deposition in each case
(3) Identify the nature of any diagenetic products present in
the limestones.
These can be classified under the following headings
a) Cements
b) Microbial micritization
c) Dissolution
d) Neomorphism (recrystallisation without significant
change in mineralogy)
e) Compaction (including fracturing)
For each thin section, try to identify which of these processes
has affected the rock during diagenesis.
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Table 1 Carbonate Grain Types (allochems)Bioclasts
Grains formed by the utilisation of calcium carbonate byanimals/plants in the construction of parts or all of their
skeletons. May be whole or fragmented.
Examples of common bioclasts, and their appearance, isshown in Figure 3
Coated Grains
Grains formed by precipitation of calcium carbonate mud onto anucleus grain (e.g. a shell fragment, quartz grain, etc.).Precipitation may be a largely inorganic process (ooids) orthrough trapping of carbonate mud by blue green bacteria
(oncoids).
Ooids are generally sand sized grains (i.e. 2mm) andhave crinkly, uneven laminae around the nucleus (Figure4)
PeloidsPeloid (Figure 4) is a descriptive term used for sand sizedgrains which are internally composed of lime mud (micrite).They may have three origins
1. Micritization (replacement of the grain structure withmicrocrystalline calcite) of other grains
2. Faecal pellets3. Cements often associated with microbial activity
Lithoclasts and intraclasts
Grains which are reworked fragments of other rock types:Lithoclasts: rock fragments unlike the rock which they arecontained withinIntraclasts: rock fragments which are similar to that which theyare contained withinIntraclasts are fairly common in carbonate sediments because
they can cement very rapidly on the sea floor. The newlyformed limestones can be ripped up during storms to generatethe intraclasts
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Table 2 Intergranular Components of Carbonate Rocks
Three types of material commonly occur between grains in
carbonate rocks:1) Cements
Crystalline precipitates around grains usually carbonatecements or chert. Often looks sugary in hand specimen
2) Matrix
Finer grained sedimentary material between grains especiallylime mud. Often looks dull in hand specimen
3) Pore Space
Gaps between/within grains at depth these spaces are usuallyfilled with fluids (water, oil, gas)
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Figure 1. Dunhams Classification (with additional terms for coarser grainedcarbonates)
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Figure 2. Palaeoenvironmental model of a carbonate ramp. Typical rock
types are indicated. A copy of this figure is available in the virtual box.
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Figure 3. Some common Bioclasts in Carbonate Rocks (From M. Tucker,
1991, Sedimentary Petrology)
Figure 4. Types of Ooids, Oncoids, Peloids and Aggregate Grains (From M.Tucker, 1991, Sedimentary Petrology)
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Figure 5. Log of the Jurassic carbonate rock succession. The stratigraphic
location of each sample is indicated.
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Which samples (A-H) do you think were deposited in a tidal flat
setting like this?
Where on the Jurassic tidal flat might they have formed?
Figure 6. Cross section from a modern semi-arid carbonate tidal flat
showing the relative positions of different structures from the subtidal lagoon(0% exposure) to supratidal soil zone (100% exposure). The small log (1-
3mm thick) shows the likely sedimentary succession which can be expected
to result from the progradation of such a tidal flat. Note how the deposits
show successively shallow structures. From Wright V.P. & Burchette, T., in
Sedimentary Environments Processes, Facies & Stratigraphy, ed. H. G.
Reading, Blackwell, page 350.A copy of this figure is available in the
virtual box.
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