102620d

 

 

Tiny fossil sponge, one of many embedded in a microbial reef. S = spongocoel, the hollow inside of the sponge; white arrow = needle-shaped spicules; M = fine-grained limestone mud matrix; F = fenestra, from the Latin for window, a relatively large void now filled with calcite cement crystals. Mississippian of North Alabama. Scale in the lower right.

083120d

 

Compound ooid. An ooid is a tiny sphere of calcium carbonate that grows in agitated warm water, often nucleating on some small pre-existing particle. Ooids are abundant in the tropics, in places like the Bahamas. In this case, an ooid grew on a small chunk of microcrystalline calcium carbonate. After the ooid grew for a while a blob of microcrystalline carbonate formed on one side of it, perhaps under the influence of bacteria (suggested by the two white ovals and dark patches within it). Then, the ooid was reactivated and added a couple more layers. It was finally buried by a mixture of mud, sand, and various kinds of shells. Mississippian, North Alabama, image 1 mm wide.

 

052120b

This rock has been through a lot. It began its existence in the Jurassic as a shifting sand shoal composed of tiny calcite spheres called ooids. The shoal was buried, and the ooids were cemented together with delicate crystals of calcium carbonate. The rock was now a limestone. Later, many of the ooids partially dissolved. Perhaps at about the same time, all of the limestone was turned to dolomite. The cement crystals became solid dolomite (solid bluish white curving areas in the picture), but the ooids still had a lot of holes in them (ovoid bluish or whitish areas). Also, some of the water-filled spaces among the original ooids had never been filled with cement (three dark blue areas with concave boundaries in the upper middle, middle, and lower middle of the picture). After a long time, thousands of feet of rock had buried the former shoal, and because it was so full of open spaces (dark blue) it was crushed.

030819c

Fenestrate bryozoan in dolomitic limestone, carbonate mound, Bangor Limestone, Mississippian, Alabama. Thin-section photomicrograph stayed with Alizarin red S.

Foundation of a Mississippian carbonate mound

Thin section photomicrograph (image 2.5 mm wide), part of an ancient carbonate sand shoal. Later, a small mound grew on top of the shoal. Ooid grainstone.

A Mississippian carbonate mound thin section

What is it? Suggestions include a tubular calcimicrobe or encrusting foraminifer.

Some pictures of rocks

Slide Set on Carbonate Porosity

http://www.gsa.state.al.us/online_pubs.aspx

"Educational Series 1"

A very large file, but lots of cool photos. Also, the photos are all from the Smackover Formation in Alabama.