pellets and peloids

Folk's original use of the term "pellet" implied a fecal origin for spherical to rodlike clasts ranging in size from 0.03 to 0.20 mm in diameter and containing little or no organized internal structure. While many pellets may indeed be fecal material produced by invertebrates, others are formed by the micritization of bioclastic material (termed by some "peloids"), oolites etc.; by agglutination of lime muds, or by recrystallization of once homogeneous ooze. Vaguely defined pellets are sometimes called "grumuleuse". Thus you should use the term pellet descriptively and include all spherical to rodlike clasts irrespective of size, provided they have poorly organized internal structure. Pelsparites are more common than pelmicrites, although both classes of rocks show well sorted texture, frequently with excellent cross-bedding.

1. a. Modern pelleted sands from Persian Gulf (Pel/2/Hol/Per.Gulf-l63) some of the pellets are probably of fecal origin but most can be directly traced to micritization of bioclastic material. Sketch an example of calcareous algae being micritized and pelleted. Do the same for a foraminifera. What is crystal size in pellet micrite.

1. b. Modern pelleted "mud" from the Bahamas (Pel/7/Hol/Bah-27670), (Pel/8/Hol/Bah-EXD). Note exceedingly little bioclastic material. Is it being micritized? </p> <p>c. What are some of pellet like grains in (B10/1/Hol/Bah)? </p> <p>2. (Pel/10/Jur/Eng-CKP) (Fossil Forest, Dorset) This is composed of <a href=" #" onclick="=" "return OpenTerm('pellets');">pellets of fecal origin. Give Folkian name. What is difference of these pellets from previous slides if any? What can you say about depositional setting?

3. (Pel/11/Pm/NMx-BMBP) Capitan Lst. Note pellets, intraclasts and pisolites. What do you think mineralogy of matrix is. Note fibrous replacement. Depositional setting? Is it stressed?

4. (Pel/12-Pm/NMx-BXJ) Carlsbad Group Guadalupe Mts. This sediment represents geopetal fill along a bedding plane parting. Note graded cycles. What are these composed of?

5. (Pel/14/Eo/Ut-CMU) Green River formation. Note fecal pellets merging with micrite.

6. oolites from Arabian Gulf almost completely micritized (Pel/1/Hol/EWI). Could you give any criteria for recognizing micritized oolites? (Pel/22/Hol/Bah-5448) is from Bahamas. The relict oolites can only be recognized from internal fabric.

7. (Pel/l7/Pm/NMx-BDN) Seven Rivers formation of Guadalupe Mts. Give Folkian code for rock. Sketch the junction between three grains. What is crystal size of pelleted material? These pellets appear as "ghosts" due to diagenesis. Depositional setting?

8. (Pel/18/Pm/NMx-BEW) Yates formation of Guadalupe Mts. What is happening in this slide and have you seen a similar fabric in (0/8/Plei/Bah- 27703)?

9. Compare (Pel/20-19917) with (Pel/21-3645). Can you see how softened pellets begin to merge with matrix, see (Pel/17/Pm/NMx - BDN)? The end member is (Mic/1). By blinking your eyes, wracking the field slightly out of focus, and using your imagination, you can still see very faint ghosts of pellets.

10. (Pel/24-1187) is a rock composed of micritized bioclasts or very large excreted pellets. Which do you favor? Sketch your evidence. Could some of these grains be intraclasts?

11. (Pel/31/Jur/Saudi Arabia) Fecal pellet of the crustacean Favreina, a common constituent in Jurassic and Cretaceous limestones (see Memoir #27, p.111). Sketch a pellet carefully taking note of the internal structure. What might be responsible for this fabric?

Wednesday, March 06, 2013
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