Mudstone is the preferred general term for the large group of fine-grained siliciclastic rocks, composed primarily of particles 63 μm in size. Mudstones (mudstones) are the most abundant sedimentary rocks, constituting at least 50% of the sedimentary rocks in the stratigraphic record.
However, due to their grain size and their subjectivity to weathering and landslides, they present a greater challenge in the field and have been relatively understudied.
Common synonyms in use by other authors include shale, argillite, siltstone, slate, although these have slightly more restricted meanings in this classification. Shale occurs in block and is not fissile, while shale is fissile.
The mudstones can be subdivided based on grain size (siltstone and argillite), sediment fabric (mudstone and shale) and composition.
Because metamorphic mudstones are better preserved and more easily studied than their non-metamorphic counterparts, the terms argillite, slate, and phyllite are also shown in Table 1 of genetic types mudstone types.
| Types of mudstones | subtypes | nature and origin |
| Siliciclastic mudstone (native or clastic mud rock) | Claystone (clay dominant) Siltstone (silt dominant) | Finer-grained and coarse-grained mudstones (respectively), the former with more clay minerals, the latter with more silt-quartz. |
| Mudstone (non-fissil) Shale (fissil) | Subtypes characterized by the type of factory; shales tend to break up into thin sheets along a strongly aligned fabric; Mudstones as such are not fissile with a block or massive fabric. | |
| Argillite Slate Phyllite | Subtypes based on increasing metamorphic grade: slate (low) to phyllite (medium); structures typically well preserved in low and medium grades. | |
| Siliciclastic mudstone mixture – biogenic | Loam | Biogenic calcareous mudstone to muddy chalk or limestone. (Not always biogenic.) |
| Sarl | Biogenic siliceous mudstone to impure chert. | |
| smarl | Mixture of biogenic calcareous and siliceous mudstone | |
| Pebble mudstone/diamictite | glacial diamictite | Chaotic, massive unit, without stratification, with characteristics that indicate a glaciomarine, glacio-lacustrine, or subglacial moraine deposit. |
| Debrite (olistostrome, lahar) | Chaotic to poorly structured, massive to stratified unit, with characteristics indicative of subaerial or underwater debris flow deposition. | |
| Melange (includes mud-rich and gravel-rich varieties) | Chaotic, massive unit, without stratification, with exotic and large clasts in a fine-grained matrix, tectonic cut +/- penetrating. Associated with major faults, emplacement of ophiolites, mud volcanoes, etc. | |
| Other types | black shale | Fine-grained sediment rich in organic carbon (1% TOC) |
| red mudstone | Continental (fluvial) deposit | |
| volcanoclastic mudstone |
Biogenic-siliciclastic mixed mudstones are very common lithologies and the term marl (or marlstone) has long been used for calcareous shales and chalk or muddy limestones.
More recently, the terms sarl, for siliceous biogenic shales, and smarl, for sediments of mixed composition, have been introduced.
Black shale is a widely used term for sediments that are generally fine-grained and rich in organic matter; these become oil shales with increasing organic carbon content.
Volcanic shales are known as fine-grained tuffs or volcanic dust deposits.
Mudstone Sedimentary Characteristics
Thickness: highly variable, from very thick apparently thin units to very finely rolled sections.
Shape: includes regular, tabular, extensive units (thin and thick), but also abrupt endings due to erosive cutting; Individual sheets show a wide range of flat, lenticular, and twisted forms.
Limits: sharp or gradational; the upper part of the bed is subject to erosive and bioturbational marks; Compaction and diagenesis often sharpen the boundaries of mudrocks so that they appear more abrupt than when first deposited.
Structure
The effects of weathering and erosion can seriously obscure structures in mudstones, but in other cases a close inspection will be very rewarding.
Microstructures commonly observed in thin sheets of silt within finer-grained shales include: parallel, cross, and convoluted lamination; sorting and graduated laminated units; routes, loads and characteristics of escape of water.
Very close inspection of the outcrop, including the use of a magnifying glass, is necessary to determine if the lamination is truly parallel or cross microlaminated.
Note the paleo-current directions.
Irregular traces of bioturbation, as well as diagenetic nodules or concretions (calcite, dolomite, siderite, pyrite), are common in many rock successions.
Factory
Fissile lamination is a fine-scale, sub-parallel, irregular separation fabric, particularly common in black shales and many compacted hemipelagic mudstones.
It is probably mainly due to the alignment parallel to the stratum or the flat packing of the laminated grains aided, in the case of the black shales, by the presence of organic matter.
Texture
Mudstones have grains in the sizes of silt and clay (ie, 63 μm).
Within this caveat, there is a wide range of types including pebble and sandy mudstones, granular siltstones, fine cohesive shales and clays, and fissile shales.
Only sizes coarser than silt can be described in the field with the aid of a magnifying glass.
Otherwise, for semi-consolidated rocks, silts have a gritty feel between fingers or when chewed, while clays are soft and pasty.
Colors can also be indicative of grain size.
For one mudstone siliciclastic, silt sheets are generally light in color, while a finer-grained clay mudstone is dark in color.
Microfossils are typically some of the coarsest particles present and are visible.
Grain grading and morphology can only be determined by laboratory analysis.
Composition
Clay minerals, fine micas, quartz, and feldspars are the most abundant components of a siliciclastic mudstone.
Fine-grained detritus and biogenic carbonate (and silica) are equally important in the marl-sarl family.
A variety of other grains occur in smaller amounts, or in some cases more abundantly.
These include volcanic ash, zeolites, iron oxides and sulfides, heavy minerals, sulfates, and organic matter (kerogen). Microfossils, macrofossils, and diagenetic nodules may also be common.
There is a close relationship between grain size and certain components in a mudstone. However, for the most part, the composition must be determined in the laboratory using X-ray diffraction, electron microscopy, or a variety of geochemical techniques.
Color
Color can be an indication of the composition in a mudstone. They occur in a variety of colors ranging from red, purple, brown, yellow, green, and gray to black.
Various shades of light, medium, and dark gray are most common. The darker colors are generally due to the preservation of organic carbon, becoming very dark gray or black in black shales and oil shales.
The paler colors are due to a smoother texture (with significant amounts of quartz and feldspar), or a higher biogenic content (especially calcareous material). The red to green spectrum generally depends on the oxidation state of the iron present: fully oxidized terrestrial mudstones are typically red, with smaller areas greenish.
Classification of mudstones
Two main schemes are used in the classification of a mudstone, but neither is very easy to apply in the field.
The first is based on relative proportions of the three grain size components: clay, silt, and sand.
The second is based on relative proportions of the three mineral components: mud, biogenic carbonate, and biogenic silica.
The most readily applied field terms are shale (nonfissile), shale (fissile), siltstone (granular but not sandy), and marl (calcareous).
Black shale is the general term applied to all fine-grained, dark-colored sediments (including mud rocks, marls, and microspheres) with relatively high amounts of organic carbon.
These sediments are potential hydrocarbon source rocks, depending on the type and amount of organic matter present.
Good source rocks can contain from a few percent to 20% organic carbon, especially that derived from plankton and bacteria.
However, organic carbon is mainly present as kerogen, a complex in the progressive diagenesis of organic matter during burial and heating.
It is too fine and scattered to be visible in the field, even with a handheld lens.
Oil shale is a very organic-rich black shale, having more than 20% organic carbon, mainly as kerogen and possibly with some bitumen present.
The kerogen in many oil shales has been derived from algae material, while any liquid heavy oil can be found oozing from cracks and joints in the shale.
Oil shales can act as hydrocarbon source rocks, but also provide a direct source of fossil fuel.
Oil can be extracted by direct heating (distillation) of the shale, producing between 10 and 150 gallons per ton of rock.
Oil shales are soft and even malleable; they can be cut with a knife into fine shavings that curl like wood shavings.
Pebble mudstones and cobble mudstones are typical chaotic sedimentary facies formed as glacial diamictites, subaerial and underwater debris (olistostromas), and tectonic melanges.