skeletal and biogenic grains

Biogenic material constitutes the dominant fraction of the composition of most carbonate rocks. These fossil remains can be considered as grains, and are subject to sorting, rounding and winnowing. Fossils are good indicators of depositional setting, even when you don't know much about them. For instance, a carbonate rock with large numbers of fossils of one or two species is usually from a stressed environment like a restricted saline lagoon while a rock with a large number of genera or species is from an unstressed open marine setting. Give the Folk name for each slide using the dominant fossil as a modifier of the main rock name. Sketch the fossils you identify and try to interpret the depositional setting for each slide you look at.

1. (B/34-Pm/NMX-BNP) is a good example of a foraminiferal Biosparite. Does the fact that the fusilinids are well rounded tell you anything about the depositional environment? Note the orientation of the spar cement in relation to the fossil grains. Sketch a fusilinid and include the surrounding fibrous cement. What mineral coats many of the fusulinids?

2. (I/18-Miss/Ind. - CZZ)contains good examples of benthonic foraminifera. Sketch several, show the symmetry of the chambers, the structure of the tests and the nature of the infilling material.

3. (B/35-Ord/Aust-CIA) contains a complete bryozoan section. Note dissepiments. Sketch it showing details of symmetry. Note the partial replacement by silica. This slide also contains good examples of echinoderm, trilobite, brachiopod, and calcareous green algae remains. What can you say about the depositional setting. Why?

4. (B/36-Or/Oh-BAH) includes excellent examples of bryozoan in thin section. Sketch several in various orientations showing details of wall structure and zoothecia.

5. (B/37-Miss/NMx-BPJ) exhibits thin shelled brachiopods with good internal (geopetal) sediment. Sketch several showing the orientation of the shell during deposition. Describe the diagenetic features seen in this slide. Describe the matrix. Note overgrowths on brachiopods. Was the depositional environment stressed?

6. (B/38-Ord/Ky.-CEO). contains good examples of trilobites. Sketch several in different orientations showing carapace shape, microstructure, and extinction under crossed Nicols. Describe the matrix or cement surrounding grains. Can you tell the orientation of the slide from the distribution of spar? How? Other fossil remains include bryozoa, brachiopods, mollusks and echinoderms.

7. (B/39-Sil/Aust-CHW) contains tribolites from Australia. Sketch several including details of shape, microstructure, and surrounding matrix/cement.

8. (B/40-Miss NMx-BPG) contains many ostracod fragments as well as other fossil groups. Sketch an ostracod showing general shell form. Describe the matrix and identify other fossil fragments. Is this environment stressed?

9.(B/41-Pm/Nev-AYW) (Riespespring Ls) Sketch the encrusting foramanifera.

l0. (B/43-Dev/Oh-CYJ) contains good examples of bone fragments. Describe the general optical properties of these fragments - Sketch several. Describe the diagenesis in this slide. What do you suppose the depositional setting was?

11. Name the fossil grains present in (B/44-Miss/Nmx-BOX). Do they have any depositional significance?

12. (B/78/Hol/Aust-EVR) is a Recent sediment from Hamlin Pool in Shark Bay, West Australia. Note the foraminifera and gastropods. Is this a stressed setting?

l3. (B/80/Plie/Berm-27595)and (B/5/HOL/BAH-27677) are examples of worm tubes and/or vermicular gastropods from Bermuda and the Bahamas respectively. These gastropods form small bioherms.

14. (B/107/Plei/Fla) are from a reef rock in Florida. Notice recrystallized worm tubes(B/127/Cret/Tx-26134) contains a worm tube. Note the unrecrystallized fabric. (B/81/? -306) is a fine example of encrusting foraminifera. Compare with (B/82/Plei/Berm-27588).There are also some encrusting foraminifera and some serpulids or gastropods. Which are they?

15. (B/83/Hol/Abud-EXA) is a Holocene sediment from Abu Dhabi containing Bryozoan. Compare to (B/84/Carb/W Aust) from the Fairfield fm of the Canning basin which also contains Bryozoans.

16. (B/85/Hol/Khar-EWH)is from the Iranian oil terminal island of Kharg. Can you identify Molluscan coral and red algae grains. Compare to (B/86/Hol/Bah).Are these both open marine depositional settings?

17. (B/87/Miss/NDak-2163) contains crinoids, bryozoan, brachiopods (identify types), trilobites and other grains. Sketch some. Depositional setting?

18. (O/35/Jur/Tex-L 344l) contains oyster fragments and inoceramus fragments. Can you identify any other bioclasts? Note the extraclasts.

19. (In/21/Miss/Wyo-TOR3737) contains intraclasts and what bioclastic grains? Compare to coral in (B/88/Pa/Wr-ll).

20. (B/89/?-4A) is a crinoidal limestone. What other bioclasts can you identify?

21. (B/90/Der/W Can-l3) contains a stromatoporid from the Devonian of the Miette Reef in western Canada.

22. (B/91/Dev/W Aust-CBS) contains pelecypods and calcispheres from Canning basin. This environment of deposition was stressed by the depth of water.

23. (B/92/Dev/W Aust-CB20) is from a sponge algal build up in the Canning basin. Identify the bryozoans, the coral and the sponges? The depositional setting was stressed by the depth of water.

24. (B/93/Dev/WA-CB23) is from the Pillara Fm of Windjana gorge. This backreef facies of the Devonian of the Canning basin contains of fenestral fabric with Renalcis and Stromatoporids. Depositional setting?

25. (B/94/Pm/Texas-341) contains the remains of the organism tubiphytes and some fusilinids. Draw a tubiphytes. What do you think is the depositional setting?

26. (B/95/Jun/-1074) contain echinoderms, inoceramous, and oysters. Also green algae Pentactin?

27. (B/96/Eoc/Saip-l3l) contain benthic foramifera including Nummulites.

28. (B/97/Pa/Ut-17) contains the coral Syringopora.

29. (B/98/Miss/ Col-545) is identified as ostracod limestone. Do you agree? Why or why not. How about (B/99/Pa/Kan-533)? Depositional setting?

30. (B/102/Dev/Car-2690) contains stromatoporids and red algae Solenopera.

31 (B/103/Dev/Ca-2692) contains stromatoporid Parachaetes. Compare to (B/104/Dev/Ca-2801)and (B/105/Dev/Ca-2905). Depositional setting?

32. (B/128/Cret/Texas-26112)contains Dictyoconus, a foramifera characteristic of the Cretaceous, which looks like a Chinese cooley hat.

33. (B/160/K/6A 1060-2A) Name the two rock types present as laminations in this rock. Depositional setting? Note the Manganese oxide crust on the exterior surface-imBedded with encrusting foraminifera. This feature develops on sub-marine outcrops where normal sedimentation is absent (Blake escarpment-depth = -1500 m). What is the source of the Mn for the precipitate?


Introduction to Fossil and Recent Calcareous Algae
Text from P. Braithwaite and R. Folk, Modified by C. G. St. C. Kendall

General Statement

Fossil algae occur in bioherms, biostromes, horizontally Bedded marine strata or stromatolytic mats and fresh water deposits. In modern reefs they frequently play the part of binding the loose sediment together and at the same time, strengthening the coral framework. In many cases they form quantitatively the major part of these modern reefs as well as the frame builder. Algae are also found preserved in normally Bedded units.

General Ecology

As algae are photosynthetic, they most abundaformntly grow within the euphotic zone of the water body. It is evident, therefore, that they are generally restricted to relatively shallow water conditions; the depth varying with the depth of light penetration. In clear tropical waters they have been recorded to a maximum of 900 feet, particularly the red algae. However, the vast majority grow within the first 100 feet below mean tide level, the number increasing toward a maximum just beneath sea level. Turbid waters place further restrictions on growth.

The red algae grow in tropical to cold seas, but many genera are steno-thermal and restricted in range (see Mme. P. Lemoine). As a group, they prefer agitated clear water, and are usually found on the windward, exposed side of reefs. The calcareous green algae, on the other hand, are restricted to tropical areas and generally prefer quiet water typified by back reef and lagoonal settings. The calcareous blue-green algae grow best with a high light intensity and hot water conditions, where the water is saturated with calcium carbonate.


Fossil algae come mainly from three phyla: Rhodophyta (red), Chlorophyta (green), and Cyanophyta (blue-green). calcite is precipitated within the walls of the cells in the case of the red algae. It is extra-cellular aragonite, precipitated around the filaments, in the other two phyla. As a result of this difference, the detailed structure of the red algae are much better preserved. The greens retain only molds of tubes and fruiting structure, and the blue-greens show only the gross structure of the colony.


It has not been possible to classify all the fossil algae in terms of the present day taxonomy. In particular, the group known as the Porostromata (Pia) have representatives which could be either green or blue algae. Also, the position of the Solenoporacea within the red algae is questioned by some botanists. The classification of the blue-greens is the most difficult question of all. Algae change their morphology with the environmental conditions, and this group can only be classed in terms of gross morphology. J.H. Johnson's practice of assigning genus and species name to these was dangerous, and W. Anderson's non-biologic approach is probably preferable at this stage.

Many of the genera in the green and red algae are defined on the basis of the structure of the fruiting parts. If these are not present, no generic classification can be made.


Calcareous Algae in Thin Section
Samples from C. G. St. C. Kendall and H. Harlan Johnson's Collections

Chlorophyta (Green algae)

In life, the green algae form complex plants with elaborate branching stems and filaments. The living parts become encrusted with aragonite, which dissolve upon fossilization. Hence, all we see in ancient rocks are the molds which are usually filled with sparry calcite.

1. a. (B/53/Hol/Bah-11784)and (B/54/Hol/Bah-11779) show Halimeda flakes.

b. (B/15/Plei/Eni-26153) shows Codiacean algae Halimeda (perforated plates) in various degrees of recrystallization. Compare with modern Halimeda leaves of (a).

2. Dasyclad algae are represented by (B-56-PmNMx-BXZ) (Tansill formation) show perforated plates which formed segments of leaves and stem. (B/58/Pm/Tx-15) is of Mizzia from the Apache Mts. of Texas and has a good fringing needle segments.

3. (N/1-Pm/NMx-BZB)shows an "Umbrella" effect of sedimentation caused by shelter of plates of algae Ivanovia. These slides are from green algae reefs of the Sacremento mountains. Note the curving green-algae strips now filled by spar. Sketch this texture and include the fibrous crystal growth. (Al/7/Penn Ariz) shows same algae but with tubes better preserved. Hold thin sections up to light to see gross fabric.

4. (B-61-Pen/Tx-BUD) (Marble Falls) is Archeolithophyllum. (B/62-Pm/NMx-BQP)from Sacremento Mts. show common type of Pennsylvanian and Permian green algae-strips of spar like those above, but these are frequently fragmented into chunks of spar and are very angular.

5. (Pis/21-Pen/Tx-BUG) (Marble Falls formation) is also called "algae biscuit". Are these oncolites or caliche or what?

6. a. (B/65/Miss-JA271) is a bioclastic limestone containing the algae Ortonella.
b. (B/66/Ord/Norway-14) contains Hedstromia. Sketch.
c. (B/67/Ord/Norway-JA5279)contains Palaeoporella. Sketch.
d. (B/69/Ord/Norway-1805) and (B/70/Ord/Norway-12) contain celosphaeridium. Sketch.

7. (Al/7/Pen/Ariz-3022)contains beautiful algal plates, once aragonitic, now altered to calcite spar. Note the micrite filled algal tubes. Compare to Halimeda.

8. (B/76/Pm/NMx) contains beautiful dasyclad algae.

9. (Al/15/Jur/Colo-18) contains the stems of the algae Chara.

Cyanophyta (Blue-green Algae)

These are associated with aragonite and high magnesian calcite in the Holocene.


Girvanella algae are characterized by minute interwoven tubes, like a ball of earthworms, forming encrusting or nodular masses. Some workers assign them to green algae, others to the blue-green.

1. (0-34-Carb Aust/NSW-CJY) (Glenbaun Dam) shows spectacular Girvanella: often the structure is obliterated by recrystallization, but these are well preserved. Identify Girvanella in (B/71/Miss/Kan).

2. (B/72-Pm/NMx-BMC) (Capitan limestone) Shows encrusting micrite probably formed by a consortium of blue-green algae and encrusting foraminifera, but there appears to be some evidence of Girvanella too. Note the hydrozoan and the beautiful and spectacular bryozoan fronds. Also note the fibrous cement.

3. (B/73-Pen/NMx-BQP) (Virgilian) Can you see any Girvanella? Note encrusting foraminifera and possible Dasyclad (?) algae plates? Examine (Pis/11/Ord/Ohio-#6) pisolites of Girvanella? Foraminfera?

Stromatolytes and other Cyanobacterial or blue-green algae fabric

Blue-green algae are tiny filamentous cyanobacterial plants which secrete mucilagenous sheaths. Their slimy filaments trap and bind sediments and also precipitate calcium carbonate through their photosynthetic processes forming mounds, nodules, or columnar structures called stromatolytes. With the exception of some cherts the filaments are not visible in the fossilized structure because they soon decay: all we see is the sediment and precipitated carbonate enmeshed in the mat. The reason algal stromatolytes contain so few allochemical grains and are dominated by micrite is micritization caused by photosynthesis is a common occurrence in these sediments. Often the only way the structure can be identified as algal is from the presence of crudely scalloped laminations of differing calcite crystal size. Unfortunately this same texture is duplicated in soil caliche and weathered calcrete surfaces of limestones.

1. (B/74-Pm/NMx-BMK) (Capitan limestones) contains a piece of what may have been cemented algal mat. Look for crinkled laminae in this stromatolyte.

2. (Al/5-Jur/Eng-CKR) (Fossil Forest, Lulworth Cove, Dorset). Almost certainly an algae head. Paleontologists have named the algae species etc. Looks like Renalcis. Can you see algae threads? Note gypsum pseudomorphs.

3. (B/77/Dev/W.Aust-CBZ-3) is an example of a stromatoporid encrusted with Renalcis (an algal-like growth) and then radiaxial cement. Look for dark clotted micrite.

4. Algal heads grow in the Great Salt Lake, Utah. (Al/8/HOL/GSL Ut-EYV) is an example. Note fibrous aragonite cement. Some of the heads are almost 80% fibrous cement as in (Al/9/HOL/GSL,Ut-EYC) Note perched sediment.

5. (Al/10/Eo/Wy-11) is an example of algal sediment from the Green River. Note same radial fabric in cement but which seems recrystallized. Compare to (Al/11/Eo/Wyo-10) which is also Green River.

6. (Al/12/Dev/WAust-CB20A) is an example of stromatolytic heads from deep water in the Canning basin of West Australia. Note the quartz-rich silt on top.

7. (Al/13/Hol/Bah-24957) is an example of the algae Scytonemia cemented in a supratidal crust on the west side of Joulters Cay in the Bahamas. The algal tissue is represented by dark rings with blobs below.

Rhodophyta (Red or coralline algae)

The red algae have external forms similar to corals and may occur in either upright branching forms or as encrusting masses. They secrete calcite within the cell walls so that the original cellular structure is preserved-they rarely recrystallize since they are composed of high magnesium calcite and so resist dissolution. In hand specimen they appear as very white specks and chunks.

1. a. (B/52/Ple/WAust-CHM) includes excellent examples of red algae. What can you tell about the environment these algae lived in from the textural maturity of the slide? Sketch one or two individuals.

1. b. (B/13/Plei/Eni-26275) contains red algae from Eniwetok.

2. (Al/1/Eoc/Peru)contains Lithoporella, an encrusting red algae. Compare to (B/47/Ter/Saip-178). Sketch.

3. (B/45/Eoc/Saip)contains Lithothamnium, the common encrusting red algae from modern reefs. Compare to (B/48/Eo/Rijuk-41) or (B/49/Eo/Peru) and sketch.

4. (B/46/Ter/Pana) contains encrusting foraminifera and red algae. Depositional setting?

5. (B/50/Mio/Saip) is Corallina from Miocene of Saipan.

6. (Al/2/Hol/Paliau) is Goniolithon from the Pacific. Depositional setting?

7. (Al/3/Mio/Eni) is Lithophyllum. Compare to (B/51/Mio/Sai-111)

8. (Al/4/Rec/Biki-13) is Porolithon from Bikini Atoll in the Marshall Islands.

9. (B/75/Hol/Bah-11835)is of Goniolithon and Halimeda and other bioclastic grains. Goniolithon is common to sea grass mounds.

10. (B/79/Plei/Ber-27587)is a fine example of corals encrusted with red algae as (Al/14/Plei/Ber-27591) is a beautiful encrusting red algae.

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