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.

031020c

Microcoprolites made by callianassid shrimp, Parafavreina ziczac, Jurassic Smackover Formation, south Alabama, thin-section photomicrographs, some of which are stained with Alizarin red S, which stains calcite pink, but does not stain dolomite. Samples from between 2 and 3 miles below the surface

Pellet grainstone within thrombolite, well 2769, 12,306.3 feet, 2.5 mm wide

Pellet packstone, well 1571, 14,688 feet, 2.5 mm wide

 
Pellet packstone, well 10,255, 13,457 feet, 2.5 mm wide



Pellet in mixed particle grainstone, pellet containing burrow in lower right (circular area), well 5432, 12,935 feet, 1 mm wide

 
Intraclast containing pellet in partially dolomitized mixed particle grainstone, well 3988 14,432 feet, 1 mm wide.

 Pellets in partially dolomitized mixed particle grainstone, well 4557, 12,820.5 feet, 2.5 mm wide.

 Partially dolomitized pellet packstone in which some pellets are preferentially dolomitized, well 1903, 11,770.9 feet.2 .5 mm wide

Pelmoldic pellet packstone (pores filled with light blue to very dark blue epoxy), well 1878, 11,789 feet, 2.5 mm wide.

010920c

Stylolite (jagged black line) is an irregular three-dimensional pressure-dissolution surface in limestone. This one separates silty peloid packstone (pink, upper right) from mostly dolomitized limestone (left and bottom). Impermeable stylolite prevented dolomitizing fluids from reaching the upper-right portion of this thin-section. Black material along the stylolite is insoluble residue of limestone that has dissolved away. Jurassic Smackover Formation, subsurface of Alabama. Thin-section photomicrograph, 2.5 mm wide.

052319d

Dolomitized limestone (pink=calcite stained with alizarin red S; white and gray= dolomite), early dolomite formed under exposure surface at top of upward-shoaling cycle, Mississippian (Serpukhovian, Chesterian) Bangor Limestone, a few meters above a carbonate mound, near Moulton Alabama, thin-section photomicrograph,

042719

Dolomitic limestone (gray area in the upper right) separated from limestone (red area in the lower left) by a stylolite, or pressure dissolution seam (black). Jurassic Smackover Formation, Alabama, thin section, 2.5 mm wide.

041919

Two examples of dedolomite (dolomite that has been dissolved) from the Jurassic Smackover Formation in Alabama. In the first image, some dolomite crystals are hollow. Blue indicates open pore space. In the second image, hollow dolomite crystals have been filled by calcite cement (pink, stained by Alizarin red S). These samples come from rocks buried several miles below the surface. Thin section photomicrographs, 1 mm across.

041719d

Dolomite (gray) partially replacing calcite (pink) in a mixed-particle grainstone cemented with marine cement crystals (light pink). Particle in the center is a shrimp fecal pellet. 13,000 feet below the surface, Jurassic Smackover Formation, thin section photomicrograph, field of view 2.5 mm wide

041219b

Bimodal pellet dolograinstone. Dolomitized pellets (gray) in matrix of altered calcite cement (red) with fine porosity within and among pellets (blue), Large pellet in the center of image (arrow) is Parafavreina, a shrimp fecal pellet. Red color is the result of staining with Alizarin red S. Jurassic Smackover Formation, Alabama, thin-section photomicrograph.

033119

Dolomite (gray and white); this rock was once very porous, but all the pore space has been filled with calcite cement (stained pink with Alizarin red S), note partial replacement of rhombic dolomite crystals with calcite; dolostone of Knox Group, Mississippi, thin-section photomicrograph.

033019

Baroque dolomite growing into void within dolostone of the Knox Group, Mississippi, thin-section photomicrograph.

032819b

Dolomite (white and gray) with a small void lined with dead oil (bitumen; black) and filled with calcite cement, stained pink with Alizarin red S. Cambrian-Ordovician Knox Group, Maben Field, Mississippi. Thin-section photomicrograph.

032519

Calcite (bright red) and partially dolomitized limestone (dark red with light-colored specks of dolomite), partially silicified (very bright white and gray areas), Mississippian Bangor Limestone, Haletown Tennessee, thin-section photomicrograph.

032419

Cross-laminated oolitic dolomitized grainstone, Jurassic, Smackover Formation, porous oil reservoir, North Choctaw Ridge Field, Alabama, five-centavo piece for scale.

032319

Limestone (calcite, stained red with alizarin red S), partially replaced by dolomite (white and gray), which has in turn been partially replaced by calcite, indicated by the worm-eaten outlines of the dolomite crystals. Thin-section photomicrograph. Jurassic Smackover Formation, West Appleton Field, Alabama.

031919d

Dolomite crystals (white and gray) that have replaced a pre-existing limestone fabric. Calcite cement crystals (red) have filled in all the open spaces left by the dolomite, and partially dissolved some of the dolomite crystals. Jurassic Smackover Formation, Alabama, thin-section photomicrograph.

031919

Partially dolomitized ooid grainstone. Some ooid laminae, and some cement generations, are still calcite. Bright white objects are quartz sand particles. Jurassic Smackover Formation, Alabama, thin-section photomicrograph, stained with Alizarin Red S.

031219

Crinoid columnal (center of image) in partially dolomitized skeletal packstone, Bangor Limestone, Mississippian, Alabama. Thin-section photomicrograph stained with Alizarin Red S, which stains calcite pink.

031119b

Fragment of rugose coral embedded in dolomitized lime-mud matrix. Bangor Limestone, Mississippian, Alabama. Thin section photomicrograph stained with Alizarin Red S, which stains calcite pink.

030819c

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

030619b

Three views of rocks from miles below the surface of the earth.

Anhydrite replacing limestone and filling fracture in Smackover Formation, south Alabama. Thin section photomicrograph. Image about 1 mm across.

Light-colored dolomite, which partially replaced red-stained limestone, is being in turn replaced by limestone. Smackover Formation, south Alabama. Thin section photomicrograph. Image about 1 mm across.

Wormy-looking sponge in the upper part of image, Smackover Formation, south Alabama Slabbed and polished core. Core 4 inches wide.