Geology of Oregon Series: Part 2: Painted Hills Unit of the John Day Fossil Beds National Monument

Sarah here– this post is a continuation in a series about a recent trip I made to Oregon, USA. Check out the first post, on the geology of hot springs, here

It can be hard to believe that the lush, gorgeous forests that I was hiking in to get to the hot springs were only a short way away from the desert of Oregon- but it is! I visited the John Day Fossil Beds National Monument, which is separated into a few different areas. Arguably, the most famous of the areas is the Painted Hills Unit, which contains amazing sedimentary rocks and fossils. 

The Painted Hills are aptly named- the large hills of clays that preserve magnificent and diverse bands of colors (Figure 1). These hills are remnants of volcanic explosions just over 30 million years ago, when nearby volcanoes were erupting. The ash and fine-grained material from these volcanic eruptions were carried by wind and deposited in this area- over long periods of volcanism, it added up to quite a lot of ash! High volumes of volcanic ash can spread a long distance over wind- we know this, because we have witnessed this in human history. For example, when Mount Saint Helens erupted in 1980, ash from this event was found hundreds of miles away (though most of ash, like the Painted Hills, fell within a far shorter distance of just a few miles from the eruption site). Over millions of years, the materials within ash (like glass particles and different minerals) can alter into clays, hence why we see clays today! 

A landscape of hills on a goegeous sunny day. The hills look painted- there are bands of colors running horizontally across them in shades of green, yellow, and red. The colors are just magnificent.
Figure 1. A full view of some of the colors that can be seen in the Painted Hills Unit. 

So- why the colors? Great question! Now, you might think clay is not that interesting- it’s just mud, right?! Well, I am hoping that by the time you’re doing reading this, you’ll have a brand-new perspective on clay! Clay is an incredible resource for geologists when we are trying to learn about past environments in Earth history: particular types of clay can inform us of the types of igneous and metamorphic rocks that were likely present millions of years ago, they can tell us about the levels of oxygen present in the environment when the clay was forming, and even about the climatic conditions of the area. How? There are a number of ways, but a really cool way is by identifying the color of the clay- clay turns specific colors during different conditions. For example, black-colored clay is often associated with extinctions because black clay indicates a lack of free oxygen in the ocean, which leads to death for living creatures. 

At the Painted Hills, we see reds, greens, purples, yellows, and a whole rainbow of shades in between. As the clays were being formed, it recorded the changing climate. Yellow and reddish-colored sediment (Figure 2) that we see today indicates a time on Earth where the area was cooler and drier- this is because of certain elements in the minerals leeching out. Brighter reds are an indicator of more humid, rainy, tropical times in the area (Figure 3). Iron- rich volcanic ash falling from nearby eruptions settled down in layers, and over time, the iron reacted to the presence of warm, wet conditions, causing the red color (think of what happens when you leave your bike in the rain and it rusts- the iron in the bike is reacting to the oxygen present through the moisture-this reaction makes what we call rust). The warmer the climate, often, the faster this reaction can happen- so in more humid climates, we’ll see more of that really red color. The iron is present in the ash in cooler times, too, but because those times were drier, less iron oxidation was occurring. 

two large hills of clays- the one on the right is distinctly yellow-green in hue and the one on the left is predominantly yellow-ish, with red toward the top.
Figure 2. The hill on the right, especially, shows the yellow-hue of drier and cooler climates 
A wooden walkway over red hills. These red hills are a rich, vibrant red. Some close by show mud cracks indicating that the areas is currently dry.
Figure 3. This is a great example of how iron-rich ash, when deposited, creates stunning, vibrant reds through the process of the iron oxidizing (rusting). Note the sedimentary structures, mud cracks, at the bottom right of the image- clay does this when it becomes drier.

Other colors, like a purple-ish hue, are indicative of a type of volcanic rock called rhyolite, which is a pink rock. This type of rock is formed from volcanoes that have more granitic magma (granite can be thought of as our continental crust), as opposed to an oceanic one, which would be basaltic magma, or oceanic crust). In this case, in the volcanic eruptions forming these hills, the continental plate was moving toward an oceanic plate, the latter of which was being subducted (the oceanic plate is far denser, so it is the one that subducts). Volcanic eruptions are incredibly common results of when two plates are colliding (which is called a convergent plate boundary). The rhyolite here weathered over millions of years, which we see now in the form of this lovely colored clay in Figure 4. 

A purple-ish, gray-ish hued hill (quite small compared to the others, which were tens to hundreds of feet high)
Figure 4. Rhyolite can weather into the purple- ish hue shown above 

One of the other spectacular remnants of this region, as I mentioned above, is the fossil preservation. There are animal fossils present, but I wasn’t able to see any of them. However, I saw magnificent plant fossils- primarily, I saw petrified wood (or permineralized wood). This type of fossil is created by the wood itself being completely replaced by silica-rich materials- in this case, the wood was buried in silica-rich volcanic ash (continental volcanoes, like ones producing rhyolite) are richer in silica than oceanic ones). Over millions of years, the wood becomes permineralized. I found some gorgeous pieces of petrified wood there (Figure 5). 

My foot on a large piece of petrified wood (~2 feet long). the structure of the bark is visible, but altered heavily and the colors are muted shades of red and brown
Figure 5. A chunk of petrified wood- this used to be a tree trunk before it was covered in silica-rich ash. Over time, the wood is altered heavily and what is left is the structure of the wood (though how much of the structure remains can vary), but chemically, it is no longer the same. Colors of petrified wood are extremely variable, because silica-rich minerals have variable colors (e.g., rose quartz , tiger’s eye, and amethyst gemstones are the same mineral- quartz!) 

Stay tuned for more on the Geology of Oregon series! 

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