siltstone - teaching hand specimen of red Triassic siltstone from the Moenkopi Formation
This siltstone is from the Upper Red Member of the Triassic Moenkopi Formation in Cedar Canyon, Iron County, Utah. The Moenkopi extends throughout the western two-thirds of the Colorado Plateau, with a thick sequence exposed near Cedar City, Utah.
The silt grains in these specimens are smaller than those of a sandstone and and feel gritty when rubbed. They are cemented by silica and iron oxide, so there is no reaction to a drop of dilute hydrochloric acid.
Students should know the sedimentary grain-size progression from mudstone and shale through siltstone, sandstone and conglomerate and be able to make the identifications. They should realize that fine grained sediments are deposited in a low energy environment, either far offshore or in coastal mudflats and in the riverbed and floodplain deposits of slow-moving streams.
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