Carbonate Petrography (Allochems)

Carbonate rocks are volumetrically made up depositional products and diagenetic products. The depositional products are those features of the rock, which resulted from the depositional event, whereas the diagenetic products are those features, which came about due to some post depositional process. The depositional products include carbonate allochems, non-carbonate allochems, terrigenous detritus, micrite, and primary porosity. The allochems are descrete and organized carbonate aggregates that serve as the coarser framework in limestone; to paraphrase the Glossary of Geology. Included in such are bioclasts, ooids, pelloids, and intraclasts. Bioclasts are the calcite or aragonite secretions of plants and animals of all types. Excluded are non-carbonate fossil remains. Different organisms produce different skeletal material with differing microtextures. Each type of organism has distinguishing features that allow for one to identify them in thin section with out too much difficulty; well maybe a little!

Your ability to recognize the various carbonate bioclasts is a direct function of how much experience you have. The more photomicrographs that you examine in the geologic literature the more proficient you will become at identifying these. Likewise the more thin sections that you examine the better you will get. Below are a few photomicrographs and brief characterizations. Start the learning process by examining these before you come to lab. In lab you will have samples and thin sections that you will be required to examine. If you wish to examine more thin sections please ask.

Echninderm fragments (ossicles) which are the easiest of all to identify are made of calcite. The organism produces hundreds of calcite crystals to form a 3-dimensional mesh-work within each ossicle. Each calcite crystal has the same crystallographic orientation as the next hence all will go extinct under crossed polarizers at the same time. This is the main test for echinoderm material...cross the polarizers and rotate. Sometimes you can actually see the crystal mesh-work. The individual grains will also frequently have recognizable shapes in thin section: circles, see the crystal mesh-work. The individual grains will also frequently have recognizable shapes in thin section: circles,  squares, rectangles, and the "pac-man" figure.

Brachiopods secreted calcite as microscopic rods surrounded by an organic sheath. These appear in thin section as very fine lamellae that are generally sub parallel to the exterior surface of the shell. Some brachiopods produced spines which when the spine is cut in cross section appear as a disk with the lamellae forming concentric rings around the disk.

that are generally sub parallel to the exterior surface of the shell. Some brachiopods produced spines which when the spine is cut in cross section appear as a disk with the lamellae forming concentric rings around the disk.

Mollusk which include the clams, oysters, cephalopods, etc. secreted both aragonite and calcite. Shell morphology could be confused with that of a brachiopod however the microtexture of the shell is very different. The mollusks generally produce a multi-layered shell where in the brachiopods one rarely observes more than one layer. Two types of microtexture are commonly observed; a "prismatic texture" and a "cross-lamellar texture". The aragonitic portions are usually removed by dissolution to form a moldic pore or they are replaced by the more stable calcite.

Arthropods; which include the ostracoda, trilobites, and barnacles, secreted calcite as small rods. The crystal lattice of each rod is slightly rotated relative to the adjacent rods such that when viewed under crossed polarizers one observes a sweeping extinction along the length of the shell as one rotates the stage. The ostracoda are really small and delicate. Look for the sweeping extinction and look for the delicate arch shape of the shell and in well preserved samples one can see a hook or hinge where the two halves of the shell joined. Trilobites are much larger than the ostracoda. The sweeping extinction is very pronounced. The test morphology is frequently like a "S" shape. Barnacles are much more complex in their shape and do not always have a well-developed sweeping extinction.

Ostracod	Trilobite	Barnacle

Corals present some challenges. Paleozoic forms secreted calcite whereas the Cenozoic forms produced aragonite. Start with shape recognition first; examine any paleontology book for this. Next the chamber wall is characteristic. In well preserved specimens look for crystals whose long axis is normal to the wall or septa. Be prepared to miss identify these as bryozoa and visa-versa.

Bryozoan identification can present a challenge as they could be easily confused with a coral or when fragmental with that of a brachiopod. They produced either aragonite or calcite, which formed as small rods of the same scale as that of the brachiopods. One distinguishing feature that many specimens exhibit is that the test tends to bifurcate or split like a tree trunk giving rise to new branches. A lot of variation here hence experience is the best guide in identification.

Algae recognition is very important for environment of deposition determination but unfortunately this group of carbonate producers is the most difficult to deal with. This is one point where picture books really help. The blue-greens generally look like cryptocrystalline calcite or micrite (see below) except they can exhibit some sense of layering or pattern. The greens sometimes have tubules that one can see especially when they form clumps. The reds under high magnification look like stacks of bricks, actually you are seeing cell walls.

Foraminifera are best recognized by becoming familiar with their various shapes. This is where a visit to the library will pay off. Below are images of forams from the Carboniferous of Utah. 

Other carbonate allochems include the ooids also know as ooliths or ools. The root of the name comes from the word for egg for which these were originally thought to have been. They are small (medium size sand), spherical or nearly spherical (disk like in thin section), frequently have a nucleus which could be a bioclasts or mineral grain, and consists mainly of calcite or aragonite rods laid tangential fashion on the growing sphere this giving it the appearance of having concentric laminations. When they get to be 0.5 cm or greater in diameter the name pisolite is applied. Watch out for brachiopod spines.

Pelloids (pellets) are medium sand sized grains of cryptocrystalline calcium carbonate They might be of fecal origin but not exclusively. Shape can vary widely. Intraclasts are cryptocrystalline calcium carbonate grains, which are larger than medium sand in size. Think of them as big pelloids.

Non-Carbonate "allochems" are discrete non-carbonate grains which formed contemporaneously within deposition within the depositional basin and are largely due to biological activity. Included are fish teeth and bones and conodonts all of which are phosphorite; skeletal material made of opal such as sponge spicules; and glauconite pellets.

Limestone will have varying degrees of terrigenous detritus or material reworked from pre-existing strata or transported from outside the depositional basin. Included are quartz sand grains, volcanic ash, rock fragments of all types (lithoclasts) and especially the carbonate rock fragments (interclasts). Avoid using the term interclasts, use lithoclasts instead. If 50% of more of the "limestone’ is made up of terrigenous detritus then the term sandstone or conglomerate would also apply.

Between the above grains one can find pore spaces and micrite. After the depositional event there would be intergranular porosity (pore spaces between grains) or intragranular porosity (pore spaces within a grain such as the space once occupied by the snail in a snail shell); both collectively referred to as primary porosity. Micrite is cryptocrystalline carbonate that is found between grains and sometimes-filling holes within grains. A minor problem here is that though the above are depositional products there are also porosity types and micrite which are diagenetic products. The micrite in either mode of origin is basically the same so we make no distinction between the two; at least in a descriptive sense. Porosity wise we encounter moldic porosity (results from fabric selective dissolution of a grain or mineral), fracture porosity (due to mechanical breakage of the rock), and solution porosity (non-fabric selective dissolution of the rock). All three are collectively referred to as secondary porosity.

Exercise #1: Look through the grain mounts in the wooden box. These samples represent typical carbonate allochems as they would have appeared at deposition. With this type of sample you can view them in 3D. It is important that you know what an object looks like in 3D before you make any attempt to identify it in thin section, which gives you a 2D view only. And not only is it a 2D view but is also a random cut through that 3D object so you have to learn how to visualize what various 2D cuts through a 3D object will look like. Then when you get to thin sections you have to visualize how the 2D view that you seeing relates to a 3D object, thinking in 3D!!! For those of you who have not yet had paleontology this is important for you to do.

Exercise #2: Examine the various thin sections, which have key bioclasts identified and augment your laboratory exercise with some of your own drawings and written descriptions. You may if you so wish use the projecting microscope. For drawings it is important to note the thin section from which the drawing was made and the scale of the drawing. For calibration of the microscope use a micrometer slide if available or use a transparent ruler as a thin section and observe the number of markings of the eye piece reticular required to traverse between marks on the slide or ruler. If your microscope lacks a calibrated reticular then observe the width of the field of view and then estimate lengths as fractions of that field of view. Remember it will be different for each objective used. For the projecting microscope just project the image of a transparent ruler. The notes you make today will be useful in the future.

Exercise #3: Between now and next week's lab go to the library and look at the photomicrographs of carbonate allochems in the references below and any others that you might find. Especially go through back issues of the Journal of Sedimentary Research.

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