claystone - teaching hand specimen of diatomaceous claystone from the Sisquoc Formation, Santa Barbara County, Calif.
Claystone and mudstone are similar, and the Sisquoc has both. A claystone is defined as a nonfissle mudrock consisting of at least 50% clay. Clay measures 1/256 of a millimeter in diameter. A field identification is easier. Claystone feels smooth when nibbled or when rubbed against a tooth, while mudstone, which has a higher percentage of silt-sized particles, will feel gritty. This claystone passed the nibble test. It is Late Miocene to early Pliocene in age.
Shale and mudstone are mudrocks composed of particles of the same size, but shale is fissile and weathers into flat chips, while mudstone is blocky when weathered.
A clay-sized particle is 1/1000 the size of a sand grain, so it will travel 1000 times further than sand at a constant water velocity. If you find a claystone, it formed in very quiet water.
Mudstones and shales are at the small-grained end of the sedimentary rock spectrum. Mudstone and claystone both contain clay and in this case, diatoms. Shales are harder but have a similar grain size. Students ought to be able to give the sedimentary rocks in order of grain size, from mudstone and shale, siltstone, sandstone and conglomerate. Generally, in mudstones and shales, the clasts (sediment fragments) are not visible to the naked eye. Students should also note the clayey feel of the surface of these specimens. The light color is due to the diatoms.
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