Carbon Rock b Klt

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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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