A Modern Wonder from Ancient Dunes: The Iconic Navajo Sandstone, Part 1

Dr. Marjorie Chan is Distinguished Professor Emeritus at the University of Utah, where she served on the geology faculty for 42 years.  She has had a long career of sedimentary research in the Western U.S. with particular interest in the Jurassic Navajo Sandstone system of southern Utah, and red rock country analogs to the red planet Mars.  She has received national awards for outstanding achievements in teaching, research, service, and outreach.  Her leadership has included being past chair of the U.S. National Committee for Geological Sciences and many roles in the Geological Society of America plus numerous committees and boards.  

Sedimentary & Terrestrial Analog Research (STAR)

The University of Utah

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This is the first of a two-part series on the Navajo sandstone. Majestic, and colorful, this formation is at the center of many of the Colorado Plateau’s most beautiful places, as well as one of the most fun services you can ride your bike on.  In the first episode of our Navajo sandstone series, we talk with Marjorie or Margie Chan, retired professor of Utah State University about how the Navajo sandstone was deposited and came to look the way we see it today.

Science Moab: Can you just describe the environment that the Navajo Sandstone was deposited in?

Chan:  The Navajo sandstone is Jurassic in age, so it’s like from the time of the dinosaurs, or we could think of it as Jurassic Park. This rock unit forms many of the outcrops in a lot of our national parks and monuments of the Southwest, and it represents an ancient desert that was really vast and covered most of this.

Southern state of Utah and extended into Arizona and southern Nevada. And this vast desert was something like dry blowing sand. The environment was harsh and that it’s maybe a monotonous, homogeneous unit, but, when you start looking more closely that there’s actually a lot of stories that multiple levels from small scale all the way up to this large scale that might be very regional and cover tens to maybe hundreds of kilometers.

Science Moab: Can you talk about the interactions between wind-blown sand and the water, (ponds, lakes, springs) and water deformation of the sand that result in unique features in the Navajo sandstone we see today?

Chan: Even though the environment was an overall desert, there were times when it was wetter when you got these different types of ponds, likely fed by springs.  The springs would be places where water was coming up and sometimes building little mounds and those often had limey compositions or lime cement.

There are things that indicate water, such as a feature that we call soft sediment deformation.  There are places where you had cross bedding or this angled lamina from the dunes, but then there are places where it’s all disrupted and it’s contorted and twisted around in these angles that are very odd. And then there’s sometimes when the sandstone is actually just massive, you don’t see any structure at all.

So all of those features indicate that the original structures were disrupted and for it to reshape in such a way, water had to be present.  Some of this we believe might have happened during earthquakes, so you would have some strong ground motion that would shake the original bedding and make it deformed. It’s also possible that there was slumping that occurred from some of the dunes. 

Science Moab: Another fascinating thing about the Navajo Sandstone is the variation in color…whites, pinks, reds. Can you explain a bit what might cause that?

Chan: The Navajo sandstone is noted for some of its variation in the colors, and a lot of this has to do with how groundwater has actually moved through that formation.  of all the units on the Colorado Plateau, the Navajo sandstone is one of the most porous, or most like a sponge. Waters can easily migrate through and seep through the Navajo sandstone. And that’s one of the reasons why it has so much coloration, because fluids at different times in its history have actually moved through that rock formation.

Most of the color is controlled by the element iron. oxidizing conditions give rise to a lot of the red color and where we have reducing conditions, the iron is in a different chemical state and that’s where we can actually remove the iron and put some of the iron into solution. That’s what we call bleaching and where the formation is whiter. This has been an area that I have been studying over the last two decades…what makes the sandstones different colors and why…and a lot of it has to do with the porosity and how it’s partitioned within the formation.

Even some of the bedding has different textures and sometimes the waters can be very sensitive following along certain lamina where the flow or the porosity and the permeability might be better for fluids to move along that particular pathway. So you do see color variations on a small scale, but you also see color variations on a large scale.

Oftentimes the Navajo sandstone is more white towards the top, and it might be more red or orangeish towards the lower parts of the formation. Some of this might be related to the characteristics of a Navajo sandstone as a large reservoir. And perhaps we think some of the bleaching fluids, which could be things like hydrocarbons that are capable of doing the bleaching chemically, tend to be lighter and rise up into the top part of the formation, which is why the upper parts are often bleached.