092320

 

I'm not sure yet what the large circular object in the middle is, but inside of it is a piece of a fenestrate bryozoan (the dark circle with the darker ovoid patches that look like giant alien eyes). The larger object has a cross-lamellar microstructure, which suggests that it's a brachiopod spine, but I've never seen one anywhere near this large. Also, it has suffered significant erosion before finally being deposited in this carbonate sand (otherwise known as skeletal grainstone). The rock is Mississippian and the image is 1 mm wide.

 

040120b

Micaceous very fine sandstone, thin section photomicrograph, scale bar 100 µm, crossed polarizers, equant particles mostly quartz, curved sheet-like particles muscovite mica, Rodessa Formation, Cretaceous, Citronelle Field, 11,500 feet below the surface, South Alabama.

112219c

Intraclasts. Particles ripped loose from the seafloor or wherever they might be, and redeposited in essentially the same unit where they first formed.

Mixed particle grainstone, dominated by crinoid debris, bryozoan fragments, intraclasts, and mud. It is not clear whether the mud was deposited after the particles were already in place, consists almost entirely of parts of intraclasts, and/or may have been deposited under the influence of microbes. Thin-section photomicrograph, 2.5 mm wide.

Same field of view, with three arrows pointing to three intraclasts. One intraclasts contains a large crinoid columnal and the other two contain other kinds of particles. It could be that the one pointed to by the white arrow is itself part of a larger and younger intraclast containing many fossil fragments.

Tropical shallow-marine platform environment. Relatively slow deposition and rapid lithification on the seafloor produced abundant intraclasts. Mississippian Bangor Limestone, Blount County, Alabama.

083019b

Calcite cement (pale pink) dissolving feldspar and quartz in a very fine sandstone, Rodessa Formation, Citronelle oil Field, Alabama, about 11,000 feet below the surface, thin section, scale bar 100 micrometers, crossed polarizers.

082919c

Thin section photomicrograph of silty sandstone containing mica (bright particle in the center). Rodessa Formation in Citronelle oil Field, Alabama, scale bar 25 micrometers. Crossed polarizers. About 11,000 feet below the surface.

060719d

Twinning in plagioclase feldspar sand particle, crossed polarizers, Rodessa Formation, Citronelle Field, Alabama, scale bar 25 micrometers (25 1000's of a millimeter)

052419b

Overcompacted bryozoan crinoid packstone, calcite stained pink with alizarin red S, Mississippian Bangor Limestone, Alabama, thin section, 2.5 mm wide.

052419

Crinoid ooid bryozoan packstone, Mississippian Bangor Limestone, Alabama, thin section photomicrograph, 2.5 mm wide.

051419b

Mixed-particle grainstone cemented by well-developed pore-rimming calcite cement (probably altered from original marine aragonite cement). Deposited in a high-energy shallow marine environment. Jurassic Smackover Formation, Alabama. Stained with alizarin red S. Thin section photomicrograph. Field of view 2.5 mm wide.

042619

Ooid grainstone (limestone) with prismatic pore-rimming calcite cement, probably marine. The centers of larger pores are open (white area in the upper part of the image). Jurassic Smackover Formation, Alabama, from nearly 3 miles below the surface, thin section. A tropical shallow-marine deposit.

042519c

Small particles in limestone with a complex history. Ovoid particles, probably small pellets, were dissolved away, suggesting they consisted of aragonite (a chemically unstable carbonate mineral common in modern deposits but missing from many ancient ones). Before they dissolved the pallets were coated with a dark microcrystalline rind, often called a "micrite envelope," and formed by boring and encrusting microbes (widespread in modern tropical shallow-water environments). This material is commonly calcite, and it was preserved here. Later, the open space within the rinds was filled with calcite cement, a lot like that which filled in the spaces among the particles (light pink). This deposit, a peloidal grainstone, is typical of shallow water low to moderate energy tropical environments. Jurassic Smackover Formation, subsurface of Alabama, thin section, image 1 mm wide. Calcite stained pink with alizarin red S.

041919c

A ripple made of mica and quartz, bright colors are mica and quartz is gray, thin-section photomicrograph, crossed polarizers, Rodessa Formation, about 2 miles below the surface, Citronelle Field, Alabama.

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

041119b

Clotted thrombolite with large cylindrical fecal pellets of burrowing shrimp, Jurassic smackover formation, Alabama, thin-section photomicrograph.

040819c

Micaceous silty sandstone from more than 11,000 feet down in Citronelle Field, Alabama. Cross-polarized light.

033019

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

032919c

Ostracodes, some articulated and filled with calcite cement, in lines of the Knox Group, Mississippi. Thin-section photomicrograph.

032719b

Skeletal packstone dominated by bryozoans and crinoids, matrix partially dolomitized, fossils partially silicified, Mississippian Bangor Limestone, Alabama, thin-section photomicrograph.

031719

Ooid grainstone, a high-energy tropical shallow-water sand in which interparticle pores have been entirely filled with cement. Jurassic Smackover Formation, subsurface of South Alabama, thin-section photomicrograph, stained with Alizarin Red S, which stains calcite pink, blue areas are open pores filled with blue epoxy in this thin section preparation.