Exercises

Exercises in carbonate petrology

cementation, diagenesis, neomorphism & porosity

This group of exercises links to the thin section catalog and tracks how carbonates undergo cementation, diagenesis, neomorphism and develop porosity. If a better understanding is required than is provided in this set of exercises you are referred to the more extensive descriptions provided on this site under the heading of carbonate Petrology.

cementation at deposition

These exercises track how carbonates begin their cementation at deposition and continue being cemented through their burial history. Evidence of cementation in the depositional setting includes the generation of oolites and grapestones. The mineral matter of the cement may originate through dissolution of some of the sediment itself, or may be allocthonous.

1. Cementation in the subtidal as in sample (Int/1/Hol/Bah-27716), in the beach as in samples (Pel/25/Hol/Bah-27686) from Staniard Creek on Andros Island, and from the supratidal as in sample (Pel/26/Hol/Bah-27692) from Staniard Creek, (this rock was cemented in the field yet thin section shows little evidence of this) in sample (Mi/2/Hol/Bah-11944) from Williams Island to the West of Andros and in sample (Int/3/Hol/Abud-25612) from Abu Dhabi.

2. Further evidence of cementation in beach rock can be seen in samples (B/2/Hol/Eni-26246), from Eniwetok in the Marshall Islands of the Pacific. What is the cement mineralogy?

3. Evidence of submarine cementation can be seen in reef rock as in a vermicullar gastropod reef from the North of Andros Reef tract sample (B/5/Hol/Bah-27677) and sample (B/6/Hol/Bah) from Joulters Cays Reefs. Sketch some of the cements. B/6 contains a large coral. Name the cement types (mineralogy and morphology).

4. Further evidence of submarine cementation can be seen in the reef rock of  (B/33/Plei/BAH- 27680) which is from Andros Island in the Bahamas. Sketch this fabric. Notice isopachus or equal thickness rim cement & fill of corals which is more irregular - than in the voids between the grains. This is characteristic of the phreatic zone or water table. What is the cementation history of B/33?

7. Sometimes Mg calcite cements only form on calcite fossils and aragonite cements form on aragonite fossils. What is happening in (B/15/Plie/Eni- 26153). Compose and complete a table: Fossil; mineralogy; cement mineralogy.

8. Note how coarse aragonite cement can be in (B/18/Plei/Eni-26166) in the gastropods. List the mineralogy of the grain types and compare with mineralogy of the cements. (B58/Pm/Tx-15) - Fibrous cement, isopachous, little alteration. (B57/Pm/N.Mx.-BNK) - Dolomitized rim cement, pore-filling calcite spar. (INT 19/Miss/Col-572) - Syntaxial overgrowth cement.


diagenetic cements of the vadose zone

1. a. (0/33/Hol/Bah-27673) is from Beach rock at Joulters Cay. Draw some meniscus cements at grain contacts. Mineralogy?

1. b. Int/17/Hol/Bah-27689 is from capillary crust of Staniard Creek, Andros the Bahamas. Note how patchy it's development is.

diagenesis

l. (B/10/Plei/Eni-26163) is a sample from Eniwetok showing first phases of freshwater cementation in reef rock. This rock was a biomicrite. micrite altered to microspar, rim cements to a drusy rim.

2. a. (B/16/Plei/Eni-26165) is from 165.5 ft Eniwetok. Note dissolution of Halimeda flakes and fill by low magnesian calcite. What is happening in coral? Note abundaformnt low magnesian calcite cement.

2.b. (B/17/Plei/Eni-26158) is from 160 ft at Eniwetok. Freshwater cement is filling voids but note how despite dissolution, grains which were aragonite still retain their fabric.

2. c. (B/19/Plei/Eni-26171) shows how aragonite fossils dissolve before aragonite cements. How Halimeda flakes may have voids filled by low magnesium calcite and the aragonite skeleton of the flake is dissolved. Also, note dissolution of aragonite cement. In (B/20/Plei/Eni-26173) the aragonite cement is being dissolved and taking a toothy appearance. What has happened to Halimeda, submarine cements, micrite in Halimeda tubes?

2 d. (B/21/Plie/Eni-26202) various gastropods first phases of dissolution,(B/22/Plei/Eni-26194) and (B/23/Plei/Eni-26223). Watch the changes in the halimeda, gastropods and marine cements.

2.e. (B/24/Plei/Eni-26244) and (B/25/Plei/Eni-26245) look like ancient limestones. Note low magnesian calcite cement.

2 f. (B/26/Plei/Fla-IXF) is from Juno Beach, Florida. Note the progress of the dissolution of the mollusk fragments with retention of original fabric.

3. (B/31/Plei/Fla-27712) show final shapes of freshwater cementation of originally aragonitic sediments. Looks like porous ancient limestone, while (B/32/Plei/Fla-26608) has some aragonite left. Note how unaltered the high mg calcite foraminifera are.

4. The following thin sections trace the dissolution of aragonite and replacement by calcite in Pleistocene oolites. Sketch each stage.

a. (O/15/Plei/Bah-27702)      b. (O/14/Plei/Bah-27704)

c. (O/16/Plei/Bah-27706)      d. (O/17/Plei/Fla-27707)

e. (O/18/Plei/Bah-27703)


cement overgrowths on grains

l) Crinoids develop single crystal overgrowths. (B/131/-2154) is an example and the overgrowths are probably the source of the coarse cement. (B/134/Pm/Tex-323) is an example of crinoids with fibrous cement envelopes, which is very unusual. For both, list cement types and cementation history.

2) (B/135/Plei/Eni-26176) is an example of fibrous aragonite in optical continuity, with the aragonite mollusks! Unusual.

3) (B/136/Dev/WCanada) is an example of brachiopod cement overgrowth which are in optical continuity with the crystal fabric of the tests.


radiaxial cements

1. This an example of a "stromotactis" cavity as per Robin Bathurst which is filled by radiaxial cement. This cavity-fill cement may be primary, as some (including the authors of this page) believe, or a recrystallization fabric.

neomorphism - recrystallization and/or inversion

To quote from Folk: Neomorphism is a broad "term of Ignorance" denoting merely the change from one aspect of calcium carbonate into another, by whatever mechanism. If it is possible to determine the specific mechanism, then that more exact term should be used rather than the broad "neomorphism". Chief processes are: (1) Inversion, whereby aragonite changes to calcite, the two minerals remaining in essential contact (in sediments nearly always with a thin water-film allowing ion-transport between host and replacer); (2) true "recrystallization" where calcite of a particular crystal size, shape, or orientation, may be either aggrading or degrading; and (3) strain-recrystallization wherein a strained calcite lattice transforms to a mosaic of new unstrained calcite crystals. For details see Folk '65 SEPM SpP#13. Unfortunately, the term "recrystallization" has been used in the past for almost any method of formation of sparry calcite in carbonates--even for simple cementation. Solution-cavity fill, where a large gap in time of in space separates the solution of one type of carbonate and precipitation of another type, is not considered neomorphism; it is no different than simple cementation.

Of the processes grouped under neomorphism, inversion of aragonite to calcite is probably the most important and easiest to identify. True calcite to calcite recrystallization is probably rare, although it is certainly important in some localities. Neomorphism of carbonate mud to microspar, etc., is very common-- neomorphism is the proper word here because one does not know if the original mud was aragonite (most likely) of or calcite.

Is a very slovenly and all-to-common tendency to ascribe everything one can't immediately understand either to "algae" or vaguely to "recrystallization". Please be able to show proof.

1. (B/28/Plei/Ber-27561) and (B/30/Plei/Ber-27576) from Bermuda are really exciting examples of carbonate undergoing inversion. Sketch pelecypod tests showing transition.

2. (Al/16/Hol/GSL Ut-EYB) is an algal head from the Great Salt Lake in Utah; compare to (B/116/PmNMx-BNZ)? (Tansill formation) which shows spectacular overgrowth which could be replacing adjacent texture. Do they? Sketch these. What fossil fragments have these overgrowths.

3. (N/3/K/Tx/B/BZP) (Edwards formation) Spectacular cave fill. Code? Note the clay-rich areas. What was original mineralogy? What happened during diagenesis?

4. (N/4/Pm/NMx-B2A) is from Tularosa near Almagordo, New Mexico. This is a cavity in an algal mound. What caused fabric? Use plane light. Explain diagenesis. This slide was cut across vertical fibrous calcite cements in a void.

5. (N/5/Pm/Eng-CLA) from the Magnesium limestone of Durham England was a pisolite carbonate. What happened?

6. (N/6/Hol/Ontario-BUX) (Kakabeka Falls) Spectacular example of Travertine from a cave. Amazingly like stuff you've seen before! Code? Compare to (Al/18/Hol/GSL,Ut-EYD) from an algal head in Great Salt Lake.

7. Neomorphism of grain growth - Late Diagenetic can be seen in the next thin sections where there is a tendency for crystals to increase in size to blocky equant spar. Compare (B/130/Miss/Colo-230) which has larger recrystallized crystals to (N/7/?-21968) which is a very much coarser a neomorphosed carbonate than the other two. Yet fossils still remain. Compare to (B/8/-21955) which contains no fossils.


caliche and calcrete

Caliche forms in soil profiles of semi arid regions in the capillary or vadose zone above the water table of the phreatic zone. This carbonate hardpan represents precipitation of carbonate by loss of carbon dioxide from groundwaters. It is extremely difficult to differentiate caliche laminations and pisolites from algal sediments and algal pisolites. Texturally they are extremely similar and they frequently appear in approximately the same stratigraphic position. Probably the best way to separate the two is by oxygen and carbon isotopes.

Calcrete is formed by the surface weathering of limestones. Many of the textures found in caliche occur in calcretes too. This distinction may be a quibble but I hope not.

1. (Pis/16-Pleis/WAust-CHN) (Aeolianite from Shark Bay) is a calcrete formed on the surface of a carbonate aeolianite.

2. (Pis/18/T/Tx-CAH) are soil caliche from near Oddessa, Texas. If you didn't know better could you distinguish these from algae sediments?

chemical and physical compaction

1. Shows flattening of oolites. Sketch. Compare to (O/37/Trias/Italy-FB3300) which is fractured and (0/38/?-163) which shows dissolution at points of contact.

2. In/22/Plie/Berm-275475 appears to show intrapenetration from about 7 ft. in Bermuda.

porosity

Porosity in carbonates is often highlighted by impregnating the rocks for thin sectioning with epoxy which is dyed blue. This is the approach that is taken for the carbonates shown on this site. Porosity can be formed between the grains and so is "interparticle". It can occur within grains, either as an original fabric like gastropods or leached grains and so is "intraparticle." "Vuggy" porosity is caused by dissolution which cuts across grains and cement. "Fracture" porosity is caused by fractures cutting across grains and cement. Below are a series of examples of some of these porosities.

l) Interparticle porosity, (B/138/Cret/Tx-26093) sketch, and inter and intraparticle (B/139/Cret/Miss-26505).

2) Intercrystalline and moldic particle porosity, (N/16/?-10632) Sketch.

3) Depositional interparticle porosity only partly filled by calcite in

(B/140/Pal/Libya-6012). Sketch.

4) Interparticle porosity from leached bioclasts in (B/142/Pal/Libya-38517A).

5) (B/143/Pal/Libya-5881) is a mixed intraparticle and interparticle porosity.

6) (B/144/Pal/Libya-9985), (B/145/Pal/Libya-10204) and (B/146/Cret/Texas- 26032) are fine examples of leached micrite porosity. This is unusual, why?

7) Describe and discuss origin of the porosity in (Dol/10/Jur/Saudi Arabia). Give a simple, generalized paragenetic sequence for this rock, do the same for (Dol/8/Mio/Iran).

8) (B/148/-6000) are vuggy porosity which cuts across all porosities.

9) (B/149/Pal/Libya-5958) contain fracture porosity. Sketch.

Wednesday, March 06, 2013
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