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Mudstone

A fine-grained, dark gray sedimentary rock, formed from silt and clay and similar to shale but without laminations.

With increased pressure and time the platey clay minerals may become aligned, with the appearance of fissility or parallel layering. 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 original texture or to the disruption of layering by burrowing organisms in the sediment prior to lithification. Mud rocks, such as mudstone and shale comprise some 65% of all sedimentary rocks. Mudstone looks like hardened clay and, depending upon circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit.

It's a very fine-grained sedimentary rock containing grains less than 62.5 µm in size; called shale if it is fissile i.e. splits easilyt into thin layers. The term mudstone encompasses siltstones and claystones and those rocks containing a mixture of both clay- and silt-sized particles. Mudstone forms in a variety of environments resulting from the deposition of mud in, for example, oceans and freshwater lakes.

Mudstone is a commonly used term for a rock formed from a soft plastic silt-clay mixture with more than half of its size fraction smaller than 4 micrometers. In other words a fine-grained argillaceous rock. The term is loosely defined by geologists, and truly needs to be more clearly defined, partly because carbonate geologists also use the term mudstone to describe fine-grained carbonates.

Geologic uses:

1. Shales and mudstones are the most common source rocks for petroleum and natural gas.

2. Indicators of paleo-oxygen levels, paleoecology, and often sources of paleontology data since fossils are often well preserved in shales and mudstone.

3. Closely associated with coals.

4. Organic-rich shales are sometimes the source rocks for stratiform ores of metals.

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