The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on an occasional schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Wednesday, October 8, 2014

October 8. Navajo Sandstone and Rainbow Bridge




First, a quick pointer to another blog I recently encountered, called Triassica – its focus is on the Triassic and there is a lot about the Triassic-Jurassic boundary and the poorly understood extinction event at that time. The link is https://triassica.wordpress.com/

Today’s episode takes us to southwestern United States in early Jurassic time.


The arid, desert conditions of the Triassic continued into the Jurassic, at least in some areas. In what is now southwestern United States such conditions resulted in the deposition of wind-blown dunes that solidified into the thick Navajo Sandstone. It forms massive cliffs in many of the parklands of southern Utah, including in Zion, Capitol Reef, and Canyonlands National Parks. It’s thickest around Zion National Park, where the Navajo is more than 2000 feet thick in places.

Checkerboard mesa photo by J.R. Gill, USGS
The Navajo is similar to other packages of desert sands in this region, such as the Wingate Formation that we discussed last month, but the Navajo Sandstone is often not quite so red. When it is reddish, or pink, the color may come from hematite, iron oxide, in the rock itself, or it may be stained by overlying red beds like the Redwall Limestone in the Grand Canyon. In places the iron is concentrated into little deep red or purple concretions similar to the “blueberries” of hematite discovered on Mars by the Opportunity rover. But often enough, the Navajo is white in color.

Although the region must have been pretty much a broad desert sand sea like parts of the modern Sahara Desert, it wasn’t barren of life. Some vertebrate bones and tracks have been found, mostly archosaurs that were probably early examples of the bird line of dinosaurs. It’s likely that seasonal rains, a monsoon, brought precipitation to the desert.

The Navajo sands were laid down mostly in Early Jurassic time, about 175 to 185 million years ago. One of the most spectacular examples of the Navajo is the feature at Zion called Checkerboard Mesa. The cross-bedded sand layers, which are essentially the lithified sloping faces of migrating sand dunes, are cut by vertical cracks, which are called joints. The intersecting pattern of bedding planes and joints gives the outcrop a rectilinear look similar to a checkerboard.

Rainbow Bridge Photographer: en:User:BoNoMoJo,
used under Creative Commons license
Another spectacular structure cut in the Navajo Sandstone is Borohoini – the Paiute word for “the rainbow,” also known as Rainbow Bridge. This natural arch stands 290 feet above the canyon floor, making it one of the largest arches in the world.

Rainbow Bridge, Checkerboard Mesa, and all such features are basically ephemeral in terms of geologic time – they will last only a few tens of thousands to hundreds of thousands of years at most. Erosion in arid country like southern Utah is typically very slow compared to humid, rainy climates – but in arid country, when erosion does happen, it tends to be catastrophic, in flash floods, landslides, and dramatic runoff events. And collapsing arches.
—Richard I. Gibson

Checkerboard mesa photo by J.R. Gill, USGS (public domain)
Rainbow Bridge Photographer: en:User:BoNoMoJo, used under Creative Commons license

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