Monday, January 17, 2011

What a Geologist Sees - Part 28 [Original Post Date 9/13/09]

Shales. Mudstone. Claystone. Clay. They are the most common sedimentary rocks or sediments.

Yeah, sometimes they can be a bit boring if we only look at them from one dimension. While on vacation, I searched this Tulsa, OK outcrop for more than a half-hour and found nary a fossil. As I was pressed for time and saw no other outcrops nearby, I kept looking.

In other shale outcrops, splitting apart the layers can bring nothing or it can bring to light a fossil leaf, a fossil seed, a fossil shell, a trace fossil,...

In the case of the specimen at left, this is a sample of shale that has been baked by an underground "coal seam fire" sometime in the past. The heat baked the shale to a natural ceramic and preserved the Cretaceous-aged fossil leaves within. (I regret not having collected more samples from this site 30 years ago.)

Shale is a clastic sedimentary rock that consists of compressed clay. It differs from claystone or mudstone by being "fissile" as shown in the upper photo. Fissile refers to the property of splitting into thin layers, which is caused by the alignment of the microscopic, flat, hexagonal clay plates (visible only to scanning electron microscopes). In the fissile shales, bedding planes are often observed, presenting the planes of weakness that allow the splitting.

Claystone or mudstone, while being hard, do not have the same alignment of clay plates, thus they fracture in a more "massive" fashion with no visible bedding planes, often leaving a curving "concoidinal" fracture, as seen below in the kaolin sample.

When younger or less-compacted (and therefor softer), as on the Gulf or Atlantic Coastal Plains, we refer to it as "clay".

As defined by the Wikipedia entry, "Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and other cations."

There are five groups (and other subgroups) of clay minerals, including some 14 minerals (one of which is not always considered a clay) - according to the Wikipedia entry.

Clays are derived from the chemical weathering of silicate minerals and rocks. Minerals such as micas, amphiboles, pyroxenes, but especially feldspars. In continental settings, such as exposures of highly-weathered rock, reddish colors (for the famous Georgia red clay) for some clays can come from iron-staining.

Once clays have been delivered to the river systems (including swamps and lakes) and then to the ocean, their eventual colors are a function of their environment of deposition. Due to the minute nature of the clay plates (less than .004 mm) and their buoyancy, quiet water conditions are needed for the clay flakes to finally sink to the bottom of the ocean (or other water body). Sometimes slight increases in water energy can result in silt (.004 mm to .063 mm) being deposited within the clay or as separated, interbedded layers. Silt is usually made up of minute silica (quartz) grains, but may include other minerals.

In the case of the Tulsa shale outcrop, this was probably an open-marine setting, where there was relatively good water circulation (and oxygen availability for bacterial degradation of any organics), usually yielding a light- to medium-gray color. The same is true for the shales interbedded with the thin limestones of this particular facies of the Ordovician Lexington Limestone. The alternating layers suggests fluctuations in the environment, due to changes in sediment supply, water depth, or other factors.

When you see reddish-colored shales or claystones (in a sedimentary setting), those were usually deposited in a continental or transitional setting, such as a river system, delta, or a tidal flat setting. The red color is due to the subaerial oxidation of the iron within the clay sediments. Usually the preservation of ripple marks suggests a certain amount of silt in the rock.

Currently, the most popular color of shale (for geologists) is dark gray to black. The dark colors are most-often due to the preservation of organics in stagnant (anoxic) conditions, where slow water circulation fails to replenish oxygen. So when organics drop to the bottom, the bacteria that would normally be there to "eat" them, are absent. This is the case in swamps (blackwater) or in restricted marine basins, e.g., the Black Sea or the deep part of the Gulf of Mexico.

This preserved organic material becomes the interpreted source of oil and/or natural gas, depending on the type of organics and/or the temperature conditions. An increasing amount of our domestic natural gas is being produced from Paleozoic and Mesozoic dark shales, such as the Marcellus, Barnett, Woodford, Eagle Ford, and other shales, due to our ability to "frac" (fracture) the shales using hydraulic pressure to shatter the shale and proppants (sand or minute ceramic spheres) to prop open those fractures. Without this process (or natural fractures), the shale is generally too "tight" to produce much of anything. Generally, the shales with a little bit of silt-sized silica are a little more brittle and frac more easily.

Another aspect of clay is that when compressed, the alignment of the flat clay plates makes the semi-impervious (or impermeable), i.e., they don't pass water or other fluids very well. This is why we use clays in ceramics. Layers of clay or shale can serve as "confining beds" to separate layered aquifers or as "caprock" to trap hydrocarbons.

There are other characteristics of clays that make them both a bane and a boon to humans. Some clays swell when wet and this can be useful when adding clay pellets to seal the annular space in a water (or other) well, but can play havoc with heaving (and cracking) of roads and foundations with wet/dry weather cycles. The brick steps to my back porch are a testament to this characteristic of rising and falling with wetting and drying, as they have broken loose from the foundation of the porch.

Clays also can act as absorbents for pollutants, whether in your cat's litterbox or in dealing with oil or other spills. They can be used in filtration settings, as filler material, as the "binder" for Kaopectate,...

To go any further would require re-writing "War and Peace" in a geological sense. I hope you get the picture that clay is more than just hardened mud.

For other "What a Geologist Sees" posts, click on the Tag below.

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