Roadtrip through the Western US – A geologists paradise Part II

In Part one I (superficially) covered the geology of the Grand Canyon, Monument Valley and the Arches National Park and mixed it up with some pictures. This part covers mainly sandstones and sandstones. So maybe a warning is appropriate: Sandstones are the most awesome rocks on earth. Fullstop. And of course this is not my personal opinion but the opinion of every sane geologist out there. Sadly there are quite a few not so sane ones that favour magmatic or even metamorphic rocks – and the totally insane ones that like to study carbonates (this is the point where I keep quiet about the fact that I study carbonates as well)! But maybe this post will help to make them disciples of the sandstone cult as well 😀

If you like sandstones (well, as just mentioned, every sane person does…) Utah IS the place to be. And Nevada, but more on that later on. Most of Utah is covered in sandstones (literally!) and you can observe wonderful bedding structures:

Cross-bedded sandstone near Escalante, Utah. Me for scale. Can you see me smiling?

Cross-bedded sandstone near Escalante, Utah. Me for scale. Can you see me smiling?

In the image above you can me standing in front/on top of cross bedded sandstones. If you look closely you can see several beds with different kind of beddings (e.g. how steep the single sandstone layers dip) and how the beds higher up in the stratigraphic column cut into the lower beds. Trained sedimentologists can use the thickness and form of the beds, the way they cross-cut each other and information on the sand particles (e.g. how big are the sand grains?; are they well rounded?) to reconstruct the depositional environment in which the sandstones formed.
The sandstones you can see here were deposited in a desert environment, the big cross bedded unit in the middle represents an ancient dune, as do most of the other units. They are part of the Navajo sandstone formation which can be found in most parts of Utah and also in Arizona and up to Colorado. It is of Jurassic age (you know when the dinosaurs were around?) and is thought to represent the largest and most extensive erg (sand sea) that ever existed on the earth!
The holes higher up were formed by erosion and are a feature that is quite common in sandstones. The layer in which the holes formed shows a feature typical for red sandstones: Bleaching. Here is an image of a particularly cool bleaching phenomenon:

Zebra Canyon near Escalante, Utah. Beautiful slot canyon in red and bleached sandstone.

Zebra Canyon near Escalante, Utah. Beautiful slot canyon in red and bleached sandstone. Note the iron concretions (round red things) within the sandstone.

Zebra Canyon got its name from the stripy red and white sandstone into which water carved its way, creating the slot canyon you can visit in an Utah near you! Slot canyons form due to erosion by small streams which are dry for most of the time but can turn into violent water masses after a strong rainfall. The streams are often called creek or wash. The water erosion leads to smooth surfaces as you can see in the picture above. But we were talking about bleaching…

Sandstone with bleaching features in the Valley of Fire State Park, Nevada.

Sandstone with bleaching features in the Valley of Fire State Park, Nevada.

The red colour of sandstones comes from iron oxides that form a very thin layer around the single sandstone grains. This coating of grains happens very early during diagenesis (the processes that are responsible for making sand to sandstone are called diagenesis) and only occurs in oxidizing environments (e.g. where oxygen is available). This means that sandstones that are formed in fluvial or desert environments often have a reddish colour. During burial different types of fluids (ground water, hydrocarbons and other gases) flow through a sandstone. Some of these fluids lead to the reduction of the ferric oxide coatings around the sandstone grains. The reduced iron is then transported out of the rock, leaving bleached (white) zones behind. On the picture above you can see that bleaching not necessarily follows grain laminae (beds of grains) but can cut across them. The exact science on where and how the preferred bleaching pathways develop is still not completely understood, so if you find this interesting (I do!) do some research on it.
Something that is out of question is that bleaching adds a spectacular feature to sandstones, making them look very pretty. If you are ever in Las Vegas you really should go to the Valley of Fire, a State Park about 1-2h north of LV. If you are there try to find “The Fire Wave”, one of the most astonishing sandstone formations I have seen so far:

The Fire Wave, Valley of Fire State Park, Nevada.

The Fire Wave, Valley of Fire State Park, Nevada.

Fire Wave II

Fire Wave II

Well, I think I will cover the West Coast (volcanoes!) in another post. This one was just for the sandstones. And how pretty they are. Hope you liked it 😉

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