Exercises in carbonate petrology

Intraclastic rocks and calcilithite

Folk proposed the term intraclast as a descriptive word to include grains of micrite and/or cemented aggregates of other allochems formed within the basin of deposition. He differentiated intraclasts from pellets on the basis of grain size. pellets were below 0.2 mm grain size. I remove this grain size differentiation and suggest use of shape. Therefore for the purposes of this course, intraclasts will be considered as irregularly shaped grains of either micrite or cemented agregates of other allochems.

Many intraclasts represent fragments of carbonate sediment that were ripped up and redeposited. These are good indicators of depositional setting and may be composed of a variety of allochems. Sources range from penecontemperaneous cementation in oolite shoals to supratidal muds. intraclasts are also formed by micritization of fossil fragments (Peloids). Calclithites are litharenites composed of a predominance of carbonate rock fragments. If the fragments become rounded they can be difficult to distinguish from intraclastic rocks. Calclithites are important because of their tectonic and climatic implications.

Grapestones - Recent

l. a. (Int/1/Hol/Bah-27716) is a crust from an area of grapestone production North of Andros and Joulters Cay. Note dark micrite cement binding pellets, grapestones and micritized oolites. Compare to (Int/2/Hol/Bah-27717) What are cements?

b. (Int/3/Hol/Abud-25612) is a supratidal crust from the Khor Al Bazam, Abu Dhabi. It is easy to see how crusts like this which also come from the intertidal and subtidal are sources of grapestones. What is cement?

c. (Int/4/Hol/Bah) is a beach rock from the Berry Islands in the Bahamas. It is full of grapestones but cemented by low Mg sparry calcite. Can this be the grapestone source?

2. Grapestone sediment from the Berry Islands of the Little Bahama Bank (Int/6/Hol/Bah-25606). Can you identify botryoidal grains? Sketch one. (Int/16/Hol/G.S.L. Ut-EWB) is from Great Salt Lake. Are these grains different to marine grapestones?

Ancient intraclasts

l. ??(Int/9/-1743) contains Illings (1954) grapestones and micritized bioclasts. Sketch some. There seem to be more micritized grains. Note the well- developed syntaxial overgrowths. Depositional setting?

2. (Int/19/Miss/Colo-572) contains some beautiful biomicrite intraclasts.

3. (Int/13/Miss/Ten-DCK) (Bangor Lst) Note eroded and rounded intraclasts. Note micritized oolites. List fossils present. Note green algal plates. Note how overgrowths on echinoderms are limited by micrite envelopes. Environment of deposition?

4. (Int/15/K/Tx-BZG) (Edwards formation) note peloids being formed by micritization of foraminifera and other bioclastic material. Name this rock.

5. (Int/23/Pm/NMx-BGF) (Seven Rivers formation) note the orientation of the elongate voids. Sketch some. These are known variously as "birdseyes", "fenestra" and "lopherite". You might as well call this rock a laminar fenestral intramicrite or pelmicrite. This rock is characteristic of intertidal and supratidal settings.

6. (Int/24/Pm/NMx-BGH) (Seven Rivers formation). Name this rock. What is it's environment of deposition? Which way is up? Sketch remnant fibrous marine cement fill of fenestrae.

7. (0/35/Jur/Tei-L3441) identify some of the extraclasts. Name the rock type for the extraclasts and identify the fossils within them.

Friday, December 21, 2012
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