shale - teaching hand specimen of soft light tan diatomaceous shale from the Upper Miocene Modelo Formation of California
This Upper Miocene shale is from the Modelo Formation of Southern California and is equivalent to the younger units of the of the Monterey Shale that underlies much of California. These specimens are fairly soft and light weight, being composed largely of diatoms, plankton that forms a siliceous exoskeleton, as well as fine grained sediments deposited in a deltaic environment. Some specimens show evidence of turbidity flow, splitting on a surface that appears hashed. Shale from this location contains occasional fossils of terrestrial leaves, seaweeds and fish, though these specimens are not fossiliferous other than with diatoms.
Diatoms contain a droplet of oil when living. When they die and rain to the deep sea floor and are buried, the oil eventually becomes petroleum or natural gas and the siliceous skeletons become diatomaceous earth. The Monterey shale contains a considerable amount of petroleum, more of which recently has been unlocked by the improved extraction techniques that have made the U.S a net energy exporter for the first time since 1958.
Students should know the basic sedimentary rocks in order of increasing grain size, from mudstone and shale, siltstone, sandstone to conglomerate and coquina. Shales differ from mudstones in that they are more compressed, form flat plates, and are less likely to disappear in a poof of dust when dropped by a student. This shale is soft and will suffer somewhat from rough student examinations.
Some shales closely resemble slate, metamorphosed from shale. To distinguish a shale from a slate, the test is to lick it. The shale will smell muddy. The slate won't. In this case, there is no confusing the Modelo shale with a slate.
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