Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone and shale. Most of the particles of which the stone is composed are less than 1⁄16 mm (0.0625 mm; 0.00246 in) and are too small to study readily in the field. At first sight, the rock types appear quite similar; however, there are important differences in composition and nomenclature. There has been a great deal of disagreement involving the classification of mudrocks. A few important hurdles to their classification include the following:
Mudrocks are the least understood and among the most understudied sedimentary rocks to date. Studying mudrock constituents is difficult due to their diminutive size and susceptibility to weathering on outcrops. And most importantly, scientists accept more than one classification scheme. Mudrocks make up 50% of the sedimentary rocks in the geologic record and are easily the most widespread deposits on Earth. Fine sediment is the most abundant product of erosion, and these sediments contribute to the overall omnipresence of mudrocks. With increased pressure over time, the platey clay minerals may become aligned, with the appearance of parallel layering (fissility). This finely bedded material that splits readily into thin layers is called shale, as distinct from mudstone. The lack of fissility or layering in mudstone may be due either to the original texture, such as the presence of non-platy grains which disrupt the fabric during consolidation, or to the disruption of layering by burrowing organisms in the sediment prior to lithification. From the beginning of civilization, when pottery and mudbricks were made by hand, to now, mudrocks have been important. The first book on mudrocks, Geologie des Argils by Millot, was not published until 1964; however, scientists, engineers, and oil producers have understood the significance of mudrocks since the discovery of the Burgess Shale and the relatedness of mudrocks and oil. Literature on this omnipresent rock-type has been increasing in recent years, and technology continues to allow for better analysis.
Nomenclature Mudrocks, by definition, consist of at least fifty percent mud-sized particles. Specifically, mud is composed of silt-sized particles that are between 1/16 – 1/256 ((1/16)2) of a millimeter in diameter, and clay-sized particles which are less than 1/256 millimeter. Mudrocks contain mostly clay minerals, and quartz and feldspars. They can also contain the following particles at less than 63 micrometres: calcite, dolomite, siderite, pyrite, marcasite, heavy minerals, and even organic carbon. There are various synonyms for fine-grained siliciclastic rocks containing fifty percent or more of its constituents less than 1/256 of a millimeter. Mudstones, shales, lutites, and argillites are common qualifiers, or umbrella terms; however, the term mudrock has increasingly become the terminology of choice by sedimentary geologists and authors. The term "mudrock" allows for further subdivisions of siltstone, claystone, mudstone, argilite and shale. For example, a siltstone would be made of more than 50-percent grains that equate to 1/16 - 1/256 of a millimeter. "Shale" denotes fissility, which implies an ability to part easily or break parallel to stratification. Siltstone, mudstone, and claystone implies lithified, or hardened, detritus without fissility. Overall, "mudrocks" may be the most useful qualifying term, because it allows for rocks to be divided by its greatest portion of contributing grains and their respective grain size, whether silt, clay, or mud.
Claystone A claystone is a lithified and non-cleavable mudrock. In order for a rock to be considered a claystone, it must consist of at least fifty percent clay (phyllosilicates), whose particle measures less than 1/256 of a millimeter in size. Clay minerals are integral to mudrocks, and represent the first or second most abundant constituent by volume. They make muds cohesive and plastic, or able to flow. Clay minerals are usually very finely grained and represent the smallest particles recognized in mudrocks. However, quartz, feldspar, iron oxides, and carbonates can also weather to the sizes of typical clay mineral grains. For a size comparison, a clay-sized particle is 1/1000 the size of a sand grain. This means a clay particle will travel 1000 times further at constant water velocity, thus requiring quieter conditions for settlement. The formation of clay is well understood, and can come from soil, volcanic ash, and glaciation. Ancient mudrocks are another source, because they weather and disintegrate easily. Feldspar, amphiboles, pyroxenes, and volcanic glass are the principle donors of clay minerals.
Mudstone
A mudstone is a siliciclastic sedimentary rock that contains a mixture of silt- and clay-sized particles (at least 1/3 of each). The terminology of "mudstone" is not to be confused with the Dunham classification scheme for limestones. In Dunham's classification, a mudstone is any limestone containing less than ten percent carbonate grains. Note, a siliciclastic mudstone does not deal with carbonate grains. Friedman, Sanders, and Kopaska-Merkel (1992) suggest the use of "lime mudstone" to avoid confusion with siliciclastic rocks.
Siltstone
A siltstone is a lithified, non-cleavable mudrock. In order for a rock to be named a siltstone, it must contain over fifty percent silt-sized material. Silt is any particle smaller than sand, 1/16 of a millimeter, and larger than clay, 1/256 of millimeter. Silt is believed to be the product of physical weathering, which can involve freezing and thawing, thermal expansion, and release of pressure. Physical weathering does not involve any chemical changes in the rock, and it may be best summarised as the physical breaking apart of a rock. One of the highest proportions of silt found on Earth is in the Himalayas, where phyllites are exposed to rainfall of up to five to ten meters (16 to 33 feet) a year. Quartz and feldspar are the biggest contributors to the silt realm, and silt tends to be non-cohesive, non-plastic, but can liquefy easily. There is a simple test that can be done in the field to determine whether a rock is a siltstone or not, and that is to put the rock to one's teeth. If the rock feels "gritty" against one's teeth, then it is a siltstone.
Shale
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