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|
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
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.