sandstone - fine-grained striped aeolian Navajo Sandstone - spectacular display specimen
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The Navajo Sandstone makes up the cross-bedded cliffs at Zion National Park in Utah. Locally it is poorly to moderately well cemented with silica cement. These specimens are moderately cemented and will stand up to student examination, though grains of sand can be rubbed off.
Sand grains in the Navajo are entirely frosted quartz dune sand, as the depositional environment of this sand was an immense inland sand sea or erg on the western edge of the supercontinent of Pangea. The erg covered much of what is now Utah and parts of Colorado, Wyoming, New Mexico and Arizona. The grains essentially sandblasted each other as they were blown by the wind. These are large enough that with a 10x hand lens, a student can see the frosted surface, a characteristic of this sandstone. Near Hurricane, Utah this sandstone is 2,400 feet thick.
The stripes are related to the movement of oxidizing or reducing solutions through the sediment or when smaller, they are due to wind-mobilized grain flows while this sand was still in the dune field. Finer grains are able to descend rapidly through coarser grainflows, forming layers on the scale of 1-2 mm. Pinstripes also form at the upper surface of grainflow strata near the toes of long slip-faces on the dunes. The stripes have low permeability and were commonly sites of early cementation, though these specimens are not well cemented and should be handled gently.
The sand grains in these specimens are larger than those of a silt and will feel like fine sand to a student. It would be instructive to compare this sandstone with one is larger grained, with one that is cemented with calcium carbonate, and with different colored sandstones, as sandstone has great variety and a student should not see only one example.
Most of the Navajo Sandstone is red or bleached white. Striped examples are far less common. This one sits flat on its base and was tilted up for its photo.
The pencil is 5 1/2" long, for scale.
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