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Mudrocks

© Geoff Habiger

Mudrocks have always been one of the most difficult types of rocks to study. They are hard to describe and it is usually impossible to get an accurate description in the field. But advances in geology in the past few decades have made the study of mudrocks easier. So why is it important to study mudrocks? If they are so similar and so hard to tell them apart, why bother? To begin with mudrocks are the most abundant type of sedimentary rock on the planet, making up about 65% of the stratigraphic column. Most mudrocks are also the source rock for petroleum, natural gas, and metals that make our civilized world run. The study of mudrocks is also helping to solve other problems such as atmospheric circulation and plate motion.

Mudrocks include rocks commonly referred to as shale, mudstone, siltstone, and claystone. However, the naming and description of mudrocks takes more than calling a rock specimen shale. An adequate description of mudrocks in the field should include information about the rocks texture, structure, and mineral composition. Due to the fine-grained nature of the rocks, this is more difficult to accomplish for mudrocks than for limestones or sandstones.

The initial classification of a specimen of a mudrock is based on texture. Texture is the size, shape, and arrangement of the grains or crystals in a rock. Individual grains are not always seen in a mudrock with only a hand lens, so how is it possible to determine texture when in the field? The simplest, and most effective method is to taste the rock. Nibble on a bit of the rock between your teeth to determine if the rock is gritty or not. (Just don't swallow!). Since mudrocks are composed almost entirely of clay and quartz you will be able to tell the texture fairly easily by this method since quartz is gritty and clay is slimy. If you feel no abrasion on your teeth then the rock contains greater than two-thirds clay minerals. If you get a gritty feeling then the rock contains between two-thirds and one-third clay minerals. If there are less than one-third clay minerals, then enough silt-sized particles should be present to be seen with a hand lens.

Mudrocks usually have two types of structure present, fissility and lamination. Fissility is the property of a mudrock that causes it to break along thinly spaced planes parallel to bedding and the orientation of the sheetlike clay flakes. Mudrocks with similar percentages of clay minerals can vary greatly in fissility due to many factors. When the original clay minerals are deposited, differences in their orientation can cause them to clump up in a process called flocculation. Flocculation is caused by small particles that adhere to each after colliding during transport. Bioturbation, the disruption of the depositional fabric by bottom-dwelling organisms can also affect the development of fissility. As the animals burrow through the sediment, they ingest mud and excrete pellets, and if enough mixing occurs, then the sediment can become completely homogenized. Certain diagenetic effects, such as increased temperatures and pressure can cause the clay minerals to recrystallize and may result in the development of fissility.

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