Exercises in carbonate petrology


The term dolomite may be used as both a mineral name and a name for a carbonate rock containing more than 50% dolomite. Where good evidence exists that the rock was once limestone, the adjective dolomitized may be used. Although staining is the best method to distinguish dolomite from calcite, development of good rhombohedral structure is typical of dolomites and uncommon in calcites. This criterion should be employed with caution, however. Much mud sized material is, in fact, dolomite. For each slide, include the following information: a) rock name; b) % dolomite c) size range and average size of dolomite crystals d) type of dolomite (I,II,III) e) description of dolomite (e.g. zoning).

1.(Dol/2/?-6086) is a good example of dolomitized limestone. What was original rock type? Can you see ghosts? Compare to (Dol/3/Jur/Ala-26549).

2. (Dol/1/Ord/Tenn-CEG) is a good example of a coarse grained dolomite. Measure the grain size. Is there any evidence for the original texture of this rock?

3. (B/114/Dev/Oh-CEQ). Explain the distribution of dolomite rhombs in this slide. Name the principal fossils found in this rock. Compare to (B/115/Sil/Ohio-FW2) and (B/117/?-Isim5783). Is there an order to the dolomitization of fossils? What is it? Note zonation in dolomite rhombs.

4. (N/2-Pm/Eng.-CLL) is composed of calcite. Note the good rhombic development of the grains. This is an example of dedolomitization -- the process of dolomite reverting back to calcite. Would staining help in recognizing this phenomenon?

5. (Dol/4/Jur/Ala) Is a dolomitized oolite with some calcite left. Explain this preferential dolomitization. Compare to Dol/3/Jur/Ala-26549. (Dol/5/Jur/Ala- 26551) is completely dolomitized. Note porosity. Compare to Dol/3/Jur/Ala-26549.

6. (Dol/10/Jur/Saudi Arabia) A carbonate grainstone has undergone patchy dolomitization. Give the following 1) Rock name; 2) % dolomite and calcite;

7. (Dol/8/Mio/Iran) Selective dolomitization. Which grain types have been selectively dolomitized over others. Can you suggest reasons for this?

8. Dol/11/Mio/Iran Very cloudy, almost micritic, mass of dolomite rhombs. a) what other minerals are present? b) discuss petrogenesis of this rock (use evidence)

9. Dol/12/PM/Tx is a limestone being dolomitized. Give rock names for the precursor rock; b) this rock; c) the rock if dolomitization is completed. Note the selective dolomitization. State what it is.

Friday, February 20, 2015
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