Exercises in carbonate petrology
Micrite and micritization
Relatively few rocks are classed as microcrystalline limestones because in the Folk classification they must contain less than 10% allochems. The micrites grade into microcrystalline allochemical rocks thus the 10% allochem boundaformry is a purely descriptive and arbitrary subdivision. The textural varieties to be found in micrites are numerous because of depositional or post-depositional processes or disturbance and the inclusion of numerous allochems.
Sketch a field of view for each thin section and give depositional setting.
l) (Mi/5/? PG3l34) is a micrite with no fossils.
2) (Mi/4/? 2928) is a micrite with sponge spicules and calcispheres. Depositional setting?
3) (B/106/? 822) is a calcisphere biomicrite.
4) (Mi/3/Ple/Fla-CBC). Describe the texture of this slide. What would you call this rock? Depositional setting?
5) (Mic/6/Pm/Tx-BJX) a typical micritic rock. Estimate % fossils and % micrite. Look at the micrite under high power and sketch the grains. What is the grain size?
6) (Mic/7/Pm/Tx-?) Spiculitic limestone composed of monaxon (Rod- shaped) sponge spicules and dark, organic-rich micrite. What is the mineralogy of the spicules? Describe the micrite crystals. Suggest a probable depositional environment.
7) (Mic/8/K/Tx-BZF) is of a "Birdseye" limestone. Give depositional setting. Explain the origin of the birdseye fabric (Look it up). Examine these birdseyes. Do you agree with their proposed origin? If not, explain their formation.
Blue-green algae and bacteria invest all carbonate with mucilaginous envelopes. Through photosynthesis and respiration they make the environment within the envelope acid during release of CO2 respiration and basic during release of oxygen photosynthesis. This progressively dissolves and reprecipitates the carbonate fragment and changes its internal fabric and mineralogy to micrite (sometimes aragonite and sometimes high magnesium calcite). This can be seen rather well in a series of slides starting with (B/118/HOL/BAH-11777) which like the others is from the Bahamas. In this slide the Halimeda is largely fresh and unsalted. In slide (B/119/HOL/BAH-11769) you can still recognize the Halimeda flakes by the few pores that remain open. In slide (B/120/HOL/ BAH-11848) a very few scattered open pores suggest these grains were once Halimeda, they look so much like fecal pellets it's hard to convince oneself they are not. Micritization and micrite envelopes suggest shallow water. I/26/MISS/BA Name this rock, identifying the constituents. What is the depositional setting? Are these intraclasts or extraclasts?
Partial micritization of grain surfaces provides a template which allows carbonate grains to retain their shape through diagenesis and dissolution.
1) In slide (B/121/Hol/Bah-11775) one can see the micritization of the surface of some bioclasts.
2) In slides (B/21/Plei/Eni-26202), (B/22Plei/Eni-26194) and(B/23/Plei/Eni-26233) one can see how envelopes resist early dissolution.
3) In (B/122/Pal/Libya-38486A) from about 6000 ft. in the subsurface and (B/123/Cret/Miss-27507) from Edwards at 11,641 ft. and (B/124/Cret/Miss-27508) at 11,651 ft., Envelopes are still empty. Why?
4) (B/126/?-11138) shows the envelopes completely filled. How come? Depositional setting?