pisolites and oncolites

Blue-green algae form concentric structures about a nucleus. These are known as oncolites. If the concentric grain is of questionable origin it's called a pisolite. In the following slides give average size of the oncolites or pisolites, note nucleus and thickness of concentric layers.

1. (Pis/2-Cam/Tx-CAN) (Wilburns Formation near Johnson City) is full of oncolites or pisolites. Note poor concentric layering. Note trilobites.

2. (Pis/3-Ter/Ut-BZE)(Green River Formation) note the desiccation features in oncolites or pisolites. Any algae filaments.

3. (Pis/4-Pa/Tx-BTU) , (Marble Falls Formation) note the small heads within concentric layers. Why do heads point in? Isn't this a beautiful slide?

4. (Pel/28/Cret/Texas) contains pisolites and foraminifera. Depositional setting?

5. (Pis/5-Pm/NMx-BEO) (Yates Formation, Dark Canyon, Guadalupe Mts.) sketch pisolites. Note radial fabric. Dubious origin? Soil, algal, diagenetic? Which do you favor?

6. (Pis/6/Pm/NMx) note fenestral fabric and ghosts of pisolites in matrix suggesting in situ growth? Depositional setting?

7. (Pis/7-Pm/NMx-BWZ) (Tansill Formation Walnut Canyon) note overgrowths on pisolites, and broken pisolites as nuclei.

8. (Pis/9/Jur/Pak-6287) contains abraided pisolites or are these rounded intraclasts?

9. (Pis/12/Dev/WAust-CB9), (Pis/13/Dev/WAust-OB9) and (Pis/14/Dev/WAust-OB9) are from the Sadler Formation in the Canning Basin. Note the variability in size of these pisolites and how some become fixed in position and grow stromatolytic heads. Compare to (B/101/Pa/Kan-5-18-13). Why are these elongated?

10.(0/1) Compare these grains to algal pisolites and list differences which help distinguish between them.


Wednesday, March 06, 2013
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