Little Mysteries in Big Canyons: The Faulty View from Yavapai Point, Grand Canyon

The Grand Canyon certainly has its faults. Loads of them actually. For a place that is famous for having relatively unbroken strata for tens of miles, the number of fault lines is kind of staggering. A stop at Yavapai Point and the Geology Museum is a nice spot to gain an appreciation for the role of faulting in the shape of the canyon.

If you look just right of center in the photo above, you can see the canyon of Bright Angel Creek and the linear pattern that it follows. Erosion worked preferentially at removing the crushed and broken rocks along the fault line, giving the canyon its unusually straight appearance. The picture below provides a closer perspective.

Faulting has influenced the region numerous times in the history of the Kaibab Plateau, including episodes of faulting in Proterozoic time 1.7 billion years ago that were primarily compressional in nature. Later, another period of mainly extensional faulting took place around 600-800 million years ago as the ancient supercontinent of Rodinia was breaking up. As can be seen in the geologic map below of the Bright Angel Creek area, the ancient rocks are riddled with faults (the entire map of the eastern part of Grand Canyon National Park can be found at http://pubs.usgs.gov/imap/i-2688/i-2688.pdf).

Source: http://geomaps.wr.usgs.gov/fed_lands/task1.html

There's an interesting little "twist" just downstream of Bright Angel Creek in the canyon of the Colorado River. Can you see it? There's another fault exposed in the Proterozoic rocks of the Granite Gorge Metamorphic Suite and the Grand Canyon Supergroup. The fault is ancient, having not offset the overlying Paleozoic rocks. It may be a trick of perspective (and I'm open to correction), but I think I'm seeing a prominent drag fold where the layers have been twisted in the direction of the fault motion. If it is a drag fold, can you see the little mystery?

I've annotated the picture below to help define the fault relationships in the outcrop. So, what strange thing happened here? When a few of you have guessed, I'll tell you what I think in the comments section!

Into the Great Unknown: The Aftermath of Chaos...Finding Beauty in the Oldest Rocks of Grand Canyon

If you've been following the story so far, you will know that we reached day nine on our rafting journey through the Grand Canyon, and that day nine was a bit of a disaster for yours truly. Our raft had flipped in Crystal Rapid, the worst rapid on the river, and I took a very long swim in frigid turbulent water before being plucked out by my guardian angels (Barry, Bev and Jeff). We made it through eight more rapids that day without incident, and made camp at a narrow strip of sand called Hotauta.

As I noted in the previous post, my camera was badly damaged (I had a spare in storage), and my trip journal was soaked. I spent the evening carefully separating and drying the pages, hoping that I could recover most of them. I also had to deal with the psychological aftermath. Being in an unexpected ride down a violent river without my boat might be an expected part of river running (and certainly no one was saying it was an easy thing to do), but I'm a desk-bound professor most of the time. This was something very new for me.

The best thing I could imagine would be to have a peaceful and serene day on the river, and the Colorado seemed to understand this: there were ten rapids in our path on day 10, but only one of them, Walthenberg (6) was greater than 5 on the rapids rating scale. And it came early in the day. We had three excursions planned in the side canyons, and they sounded like wonderful places.

It seemed a good sign to have a big Swallowtail Butterfly land on my gear as we were loading the rafts. It seemed a bit disappointed that my bag wasn't the biggest darn flower ever, but it hung around long enough to get a picture.

We went through the first two rapids, Bass and Shinumo, and pulled out at Shinumo Creek.

The walk was short, but quite interesting, as the creek filled the canyon bottom. I was absolutely sure I was going to slip on the muddy rocks and destroy my remaining camera. But it was just beautiful, and the fifteen foot waterfall was a refreshing retreat from the hot sun.

We headed back onto the river, and soon passed the creatively named 113 Mile Rock, an outcrop of schist that practically blocked the river. It also marked the halfway point in our 226 mile trip (it was day 10 of 16, so our average daily mileage was going to increase).

Since Walthenberg Rapid, we had been traveling through the oldest rocks to be found anywhere in the Grand Canyon region: the Elves Chasm Gneiss, dated at 1.84 billion years. These rocks may represent the ancient crust on which the other metamorphic rocks were emplaced many tens of millions of years later. I hate to say it, but I was distracted that day and didn't realize that we were passing through these rocks until a day or two later when I retrieved my geologic map out of the luggage. But I managed to snap a number of pictures because the rocks were intriguing to look at whether I knew their age or not.

 In many places the rocks are intruded by pink dikes and veins of granite pegmatite, a rock with exceedingly large crystals of feldspar, quartz, and muscovite mica (above). In a few places I could make out darker intrusions that looked like basalt (which is youngest in the picture below: the black or the light colored intrusions?)

 Before Glen Canyon dam was built the Colorado River carried an incredible amount of silt and mud. According to some sources, when the river used to run at 100,000 cubic feet per second, half of what flowed down the river was sediment (Death in the Grand Canyon by Ghiglieri and Myers). The sediment gives the river the tools needed to sculpt the incredibly hard rock, and we passed numerous beautiful exposures of intricately shaped gneiss, schist and granite.

We were still in the Granite Gorge, but we noticed that the inner canyon wasn't as deep as it had been, and we started to see Tapeats Sandstone in the cliffs not too far above us.

As we neared Elves Chasm, the Tapeats was at river level, and we were treated to an exposure of the Monument Monocline in the sandstone layers. A monocline is a fold in the rocks that looks like a carpet thrown over a step: level horizontal rock, then a flexure as the rock bends downward, and then horizontal layers again. They tend to occur when a fault fractures harder rocks at depth, but only bends the sedimentary layers above.

The folds were an unexpected sight. So were the travertine deposits that were exposed along three miles of the river starting at Mile 116. Travertine is generally composed of calcium carbonate (the mineral calcite) which was leached out of the overlying Redwall, Temple Butte, and Muav formations and deposited by springs in the Bright Angel Shale. The travertine completely covered the Tapeats in places, including the area around Elves Chasm.

What is this "Elves Chasm" that I keep mentioning? It was our next stop, but that will be in the next post!

Strangers in a Strange Land: Confront your faults, it's good for your sole

The Strangers in a Strange Land continued their journey through Death Valley National Park last month. We had spent some time observing and interpreting a unique outcrop east of Shoshone, and after a short break in the urban nightmare of Shoshone itself (one gas station, one coffee shop, an RV park or two) we headed through the Black Mountains over Jubilee Pass into the south end of Death Valley itself. Although the road is paved, the south end sees few of the tourist buses and casual visitors who spend most of their time at Badwater and the resort at Furnace Creek. But the geology is wonderful. Especially if you are interested in faults...

Source: National Park Service

As can be seen in the diagram above, there are four basic faults type, normal (caused by extensional force), reverse (compressional forces), and strike-slip (the fault above is a left-lateral strike-slip; the blocks would move the opposite direction if they were along a right lateral fault). Strike slip faults are caused by shearing motion.

Normal and reverse faults can be distinguished by observing the relative motion of the headwall and footwall (as shown above). Extension causes the headwall move down relative to the footwall, making a normal fault. Compression forces the headwall upward relative to the footwall, forming a reverse fault.

Our students had just learned about these four basic fault types at the Charlie Brown outcrop, but they were looking at fault planes in a roadcut. As we entered Death Valley, we started seeing the effects of recently active faults on the landscape. We stopped along the road near a couple of odd features that don't really make sense on a valley floor where deposition should be the dominant process. It was a particularly instructive spot, as we could see evidence of movement along two kinds of faults from one viewpoint.

First is the terrace at the top of the post (also seen in the Google Earth image above). The gravelly sediments in the photo are from an alluvial fan along the base of the Black Mountains. The surface was once a smooth gentle slope, but fault motions lifted the rocks into the terrace, forming a fault scarp. The black rock is intriguing...it is basalt, which apparently rose through the crust along the weakened rock in the fault zone.

The second fault is less obvious from the valley floor where we were standing, but if you look carefully you can see that the eroded cinder cone has been split in two, and the portion of the cone on the far side of the fault has moved to the observer's right. It is a right lateral strike-slip fault crossing the valley floor. The offset is clearer when seen from above, as in the Google Earth image below.

This juxtaposition of two kinds of faults raises questions. Two different forces are clearly at work here, shearing and extension. Are they both presently active, or has the stress regime changed in recent time from extension to shearing or vice versa?

At this point we are below sea level on the floor of Death Valley, a 100+ mile-long fault trough. Mountains rise high on both sides of the valley, with a total relief of more than 11,000 feet (few places on the continent can claim such extreme elevation changes over so short a distance). Such fault valleys are termed grabens (the German word for grave or trench), while the mountains are termed horsts (German for eagle's nest or aerie).

Although we could not see an example from where we were standing, the Death Valley region also has examples of reverse or thrust faults (thrusts have a fault plane angle of less than 45 degrees). They have a tendency to push older rocks over younger, as can be seen below along the Keystone Thrust west of Las Vegas. The gray layered rocks are Paleozoic limestone formations (400-500 million years old) which have been pushed over the bright yellow and orange rocks of Mesozoic sandstone formation (around 200 million years or so). Check out Georney's on the ground visit of the Aztec Sandstone at Red Rock Canyon here. These faults are not currently active.

 Our students were treated to examples of most of the fault types within the course of a day. A nice simple explanation for the existence of Death Valley. The land stretched and grabens developed. Oh that it could be so easy. We rounded a corner and had our first view of one of Death Valley's turtleback faults. The story was about to get complicated...