Geology students learn their sedimentary rocks in neat and tidy categoriessandstone, limestone, shale. But the most widespread and ordinary-looking sedimentary rocks are intricate mixtures that sprawl across the boundaries of the geologist's concepts. They're mudrocks.
The Basics of Mud
To approach the topic of mudrocks, we must start with the class of sediment that constitutes them: mud. To the geologist, mud is sediment that's finer than sand; more precisely, it's a mixture of mineral particles smaller than 1/16th of a millimeter. The size spectrum of mud goes from silt (1/16 to 1/256 mm, 63 to 4 microns) to clay (less than 1/256 mm or 4 microns). In the hand, dry silt feels like cornmeal and leaves the fingers clean, whereas dry clay feels like cornstarch and stains everything brown. Wet silt feels like firm soil and holds together weakly. Wet clay can be slippery or sticky depending on the water content. With a little water, it holds its shape but deforms easily in a plastic waythink of modeling clay. With a lot of water, clay makes a mess. Clay is the essence of muddiness.
The silt and clay size categories roughly correspond to different minerals. Silt particles are usually quartz or feldsparminiature sand grainswhile clay is usually clay minerals. Silt particles therefore are tough and inert, like billiard balls. Even when they're eroded to clay size, they form a crumbly sediment. But clay crystals are microscopic flakes that stick strongly together with electrostatic forces, accounting for the plastic behavior of clay.
Mud that's primarily silt is called silt, and mud that's mostly clay is called clay. Either one can have a small fraction of other particle sizes, including sand and gravel.
Optional Ingredients of Muds
Complicating the picture are three other optional ingredients of mud: silica, calcium carbonate and organic matter.
Silica (noncrystalline quartz) is mainly contributed in microscopic particles, either the shells of diatoms and radiolarians or the phytoliths made inside plant stems. Mud with enough of these may be given the name of ooze, siliceous ooze.
Calcium carbonate may rain down into mud as the microscopic shells of calcareous plankton. It may also precipitate from the water in various situations. The resulting limy mud is known as marl.
Organic matter is the dead bodies of plankton and bacteria, with or without shells. It tends to make mud slimy and black. Depending on the exact setting, mud very high in organic matter may be called gyttja ("GIT-ya") or sapropel.
From Muds to Mudrocks
Mud of all kinds is laid down in lakes and the ocean, and once buried deep enough for long enough it becomes, through lithification, a mudrock.
Each of the various kinds of mud yields its own mudrock. In the field, geologists typically check mudrocks first by sight, looking for silt grains with their magnifier. Only large silt grains are visible. If they see these, it's a matter of judging the proportions of silt and clay and calling the rock either siltstone or mudstone.
Otherwise the next step, if the rocks are soft enough, is nibbling on them. (The geologist may skip this step and take the rock to the lab.) Siltstone and mudstone are gritty between the teeth, even if the silt grains are too small to be visible. Mudstone made of pure clayclaystonefirst crumbles like Necco wafers, then ends up smooth as chocolate between the teeth. Claystone that is fissile, easily splitting into thin layers, is given the name shale.
Optional Ingredients of Mudrocks
A mudrock may be too hard for nibbling. Both calcium carbonate and silica act as cements that make rocks harder. Adding calcium carbonate pulls the rock partway toward limestone, and marl lithifies into marlstone, or calcareous mudstone. Adding silica pulls the rock partway toward chert; it would be fun if ooze produced "oozestone" but instead it's just called siliceous mudstone. (A regular clayey mudstone, if you need to single it out, is called argillaceous mudstone.)
Adding organic matter pulls the rock partway toward coal, and the result is called carbonaceous mudstone. Organic matter doesn't make a mudstone harder or softer, but it does make it interesting to petroleum prospectors. Depending on an intricate set of physical and chemical conditions, a carbonaceous mudstone may produce oil or gas or perhaps just black carbon. The mudrock may release that petroleum or keep it trapped inside its pore spaces.
Complications of Shale
"Shale," like "granite," means different things to different people besides geologists.
Ordinary people gave shale its name long before the rise of geology, deriving the term from a Middle English word for a dish or shell. (It descends from an ancient Germanic root with the sense of "to cut," and its many cousins include the words shell, scalp, half, skull, caul, shield, shelf, scalpel and cutlass.) Therefore most people are content to use "shale" for any featureless rock that splits easily. Truth be told, so are most geologists when they're in the field.
Commercial stone dealers may use "shale" for any flagstone that isn't sandstone, slate or phylliteany stone that isn't sandy or shiny or possessing a nice flat cleavage. That would mean "fissile mudstone" to the geologist.
To engineers, shale basically refers to mudrockswhatever is finer grained and softer than sandstone. When it comes to planning and maintaining mines or excavations, "shale" is a substance with many different engineering properties that must be determined by testing and then dealt with.
Oil and gas drillers have long used "shale" loosely to mean whatever lies between the interesting stuff: reservoir sandstones with lots of pore spaces for oil and gas to travel through. That is, shale is tight rock, a barrier to the movement of petroleum. Oil shale is an entirely separate thing, a highly carbonaceous mudrock that is burned outright or roasted to distill out its burnable fraction as shale oil. Just a few years ago, shale oil was the latest thing in cutting-edge fossil fuels. But since then the explosion of oil and especially gas production from tight "shale" by hydraulic fracturing (fracking) has had petroleum engineers relearning their trade. The old shale barriers have become the new shale reservoirs, and "shale oil" has an old and a new meaning. The pros are learning to watch their language.
Drillers are finding that the new shale reservoirs, the rocks they used to ignore, have all the complexity that geologists find in mudrocks. The most productive sources are not true shale at all: argillaceous claystone is in fact a poor candidate for fracking because it's too soft to crack. The good rocks are the harder ones, meaning that they're marlstones and siliceous mudstones. Today, muds and the rocks they form are a new frontier of geological sophistication.