oolitic rocks

This group of exercises links to the thin section catalog and investigates ooids and oolitic rocks. 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.

Most oolitic rocks have a sparry matrix (are oosparites) because the formation of ooids seems to require abundant agitation associated with water movement which seems to remove the bulk of microcrystalline ooze. These rocks are commonly well-sorted and cross-bedded. Occasionally ooids are washed into areas of ooze deposition or grow in such areas - although these settings are unusual, they may be of considerable environmental significance.

recent oolites

1. (O/2/Hol/Bah 25608) is a recent oolite from a shoal south east of Joulters Cay. Note concentric coats to these aragonite grains. What causes dark blotches in layers? What are nuclei. Sketch some grains.

2. (O/4/Hol/AbuD-EWK) was collected from the interior of an oolite shoal in Abu Dhabi. Compare to (Pel/1/Hol/Abud-EWJ). This is from same shoal but closer to the edge. Are these also ooids? Compare the two samples and interpret the differences.

3. (O/5/Hol/Sh.B,-EGH) is from Shark Bay, Australia. These are superficial oolites, in that they have very thin cortices.

4. (O/6/Hol/GSL.Ut-EZD) is of oolites from the Great Salt Lake, Utah. These oolites are composed of concentric aragonite and radial crystals. The blotchy grains are cut where the thin section plane does not pass through the nucleus. This fabric is common to many ancient oolites which suggest the radial fabric is primary.

5. (O/7/Hol/Bah-24941) is a cemented dune sediment from Joulters Cay. Note freshwater calcite spar cement. Sketch some contacts. Compare to (O/8/Hol/Bah-EXH). What is major difference?

6. (O/11/Plei/Bah) is a Pleistocene oolite rock from Bimini, the Bahamas. What is happening to some oolite corteses?

ancient oolites

1. O/1 has excellent oolites with combined concentric and radial structure, What nuclei are present? Note superficial oolites with one coating. This is a typical "ancient" oolite of lower Paleozoic or lower Mesozoic age. Name this rock. What is cement?

2. For all the following slides, in addition to the complete rock name list the thickness of the oolite coating, the structure of the oolite (whether radial, or concentric or both).

a. (0/19-Miss/Ky-DBR). (Newman limestone) Some nuclei are crinoid fragments, which extinguish as one unit. Sketch one of these. What is average size of cement-express as mm. Note micritization of oolites.

b. (0/20-Jur/Eng).-CKK (Osmington Oolite, Jurassic England) Note prevalence of terriginous nuclei. Are there any superficial oolites? Note presence of oolites and other grains cemented by micrite. What are these called?

c. (0/31)- Note micritization of ancient oolites.

3. (0/21/PaNMx-BRP) (Virgilian rocks of Sacremento Mts.) How would you define the allochems of this rock? What is their origin?

4. (Pis/1/Pm/NMx-BWP) (Yates formation, Guadalupe Mts.) These are pisolitic grains. The concentric, somewhat scalloped coatings are either blue-green algae encrustations or caliche concretions; make a sketch to show how their structure differs from ordinary oolites. What are nuclei? Compare grains with 0/22. Do you see an "alignment fabric" of ooids at low power? Can you explain it?

5. (0/23/Carb/Aust-CIZ) (Glenbaun Dam, New South Wales) and (0/22/Carb/Aust-CJC) (Glenbaun Dam, New South Wales). Sketch way oolites are undergoing solution pressure. Do these grains look like (0/2/Hol/Bah) or (0/6/HOL/G.S.L,Ut)?

6. (0/24/Sil/NY-CXU) (Clinton fm) What are oolites formed by? Are they primary or secondary? Does anyone know?

7. (0/39/Cret/Tex-26080) is a mudrock of unusual texture. What is the matrix composed of? Identify grapestone grains.

8. (O/27-778) are cerebroid oolites. Examine (0/32/Hol/Bah-EVA) and note algal bores.

9. (0/26/Miss/Col-482) has oolites interpenetrating. Explain fabric. Is this the same as (0/28-803)? Sketch both fabrics.

Wednesday, March 06, 2013
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