Mudstone and Shale

Introduction

Mudstones and shales are the most abundant sedimentary rocks on Earth, making up roughly half of the entire sedimentary record. Their fine grain size has historically made them difficult to study - but modern instruments including scanning electron microscopy and electron probe microanalysis have opened these rocks to the same rigorous investigation applied to sandstones and carbonates. Understanding mudstones and shales is critical to petroleum geology, paleoenvironmental reconstruction, and understanding the geochemical cycling of elements at Earth’s surface.

Mudstones and shales are fine-grained, siliciclastic sedimentary rocks. They contain more than 50 percent siliciclastic grains less than 0.062 (1/256) mm in size and are composed mainly of silt-size (1/16-1/256 mm) and clay-size (<1/256 mm) particles. ^[1]

Mudstones and shales make up roughly 50 percent of all sedimentary rocks in the geologic record. Historically they have been understudied because their fine grain size makes them difficult to examine with an ordinary petrographic microscope. This is changing with the development of instruments such as the scanning electron microscope and electron probe microanalyzer that allow study of fine-size grains at high magnification. ^[1]

Terminology

The naming of fine-grained siliciclastic rocks has accumulated significant inconsistency. Two major usages coexist. In the first, “shale” serves as the class name for all fine-grained siliciclastic sedimentary rocks, subdivided by clay content and lamination into mudstones, mudshales, claystones, and clayshales. In the second, “mudrock” replaces shale as the general term, with shale and mudstone as its laminated and nonlaminated subdivisions. ^[1]

A third usage, now widely adopted, restricts shale to fine-grained argillaceous rocks that display lamination or fissility (the ability to split easily into thin layers), while nonlaminated rocks are termed mudstones. This is the usage followed here. ^[1]

Despite the terminological messiness, there is general agreement on the core distinction: laminated fine-grained argillaceous rocks are shales; nonlaminated ones are mudstones. ^[1]

Mineralogy

The principal minerals of mudstones and shales are clay minerals and fine-size quartz and feldspars. They also contain varying amounts of carbonate minerals (calcite, dolomite, siderite), sulfides (pyrite, marcasite), iron oxides (goethite), and heavy minerals, as well as a small amount of organic carbon. ^[1]

The principal clay mineral groups are kaolinite, illite, smectite, and chlorite. Their relative proportions change systematically with burial temperature and rock age. In rocks older than the Mesozoic, the proportion of illite and chlorite increases at the expense of kaolinite and smectite - a trend attributed to diagenetic alteration of kaolinite and smectite to form illite and chlorite. ^[1]

No discernible trend of overall mineralogy versus age is apparent from age-sorted compositional data, except possibly a slight decrease in feldspar content with increasing age. Many factors affect shale composition, including tectonic setting and provenance, depositional environment, grain size, and burial diagenesis. ^[1]

Some minerals - carbonate minerals and sulfides - form within shales during burial as cements or replacement minerals rather than being detrital. Quartz, feldspars, and clay minerals are mainly detrital (terrigenous) in origin, though some fraction may form during burial diagenesis. ^[1]

Chemical Composition

The chemical composition of shales and mudstones is a direct function of their mineral composition. SiO₂ is the most abundant chemical constituent, ranging from 57-68 percent in published analyses, followed by Al₂O₃ at 16-19 percent. ^[1]

• SiO₂: Contributed by all silicate minerals present, particularly quartz. Shales contain less SiO₂ than sandstones because they have less quartz. ^[1]
• Al₂O₃: Derived mainly from clay minerals and feldspars; more abundant in shales than sandstones because of the greater clay content. ^[1]
• Fe: Supplied by iron oxide minerals (hematite, goethite), biotite, and minerals such as siderite, ankerite, and smectite clay minerals. ^[1]
• K₂O and MgO: Related mainly to clay mineral abundance; some Mg is supplied by dolomite, and K is present in some feldspars. ^[1]
• Na: Related to the presence of clay minerals (e.g., smectites) and sodium plagioclase. ^[1]
• Ca: Supplied by calcium-rich plagioclase and carbonate minerals (calcite, dolomite). ^[1]

Classification

Most classification schemes for mudstones and shales do not rely on mineral composition because the analytical techniques required - XRD, electron microscopy - are time-consuming and expensive. Most schemes instead emphasize the relative proportions of silt and clay and the presence or absence of fissile lamination. Fissility is defined as the property of a rock to split easily along thin, closely spaced, approximately parallel layers. ^[1]

The classification of Potter, Maynard, and Pryor (1980) is based on grain size (percentage of clay minerals) and the presence or absence of lamination. Under this scheme, fine-grained argillaceous rocks divide as follows: ^[1]

| Clay-size % | Bedded | Laminated | | ----------- | ---------------- | ------------------- | | 0-32 | Bedded siltstone | Laminated siltstone | | 33-65 | Mudstone | Mudshale | | 66-100 | Claystone | Clayshale | ^[1] |

Additional informal modifiers can refine the classification to record colour, cementation type (calcareous, ferruginous, siliceous), induration, mineralogy, fossil content, organic matter content, fracture style, or bedding geometry. ^[1]

Origin and Occurrence

Mudstones and shales form wherever fine sediment is abundant and water energy is low enough to allow settling of suspended silt and clay. They are particularly characteristic of marine environments below storm wave base adjacent to major continents, but also form in lakes, quiet-water reaches of rivers, and lagoonal, tidal-flat, and deltaic environments. The fine-grained products of siliciclastic weathering greatly exceed coarser particles in volume, so fine sediment is abundant throughout many sedimentary systems. ^[1]

Because fine sediment is so plentiful and can accumulate across so many quiet-water environments, mudstones and shales constitute roughly 50 percent of the total sedimentary rock record - by far the most abundant sedimentary rock type. They commonly occur interbedded with sandstones or limestones in beds ranging from a few millimetres to tens of metres thick. Nearly pure shale units hundreds of metres thick also occur, and marine shale units tend to be laterally extensive. ^[1]

Notable examples of shale units include the Cambrian Burgess Shale of western Canada (famous for preserved soft-bodied animals), the Eocene Green River oil Shale of Colorado, the Cretaceous Mancos Shale of western North America (a thick eastward-thinning wedge from New Mexico to Saskatchewan and Alberta), the Devonian-Mississippian Chattanooga Shale covering much of North America, the Silurian Gothlandian shales of western Europe, northern Africa, and the Persian Gulf region (containing a pelecypod and graptolite faunal association), and the Precambrian Figtree Formation of South Africa. ^[1]

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