Strangers in a Strange Land: Not finding what you're not looking for...

Perception is a funny thing. Our preconceived biases exert a powerful influence on our interpretation of our observations. We understand what we know, and we can't really conceive of that which we don't know. Maybe Donald Rumsfeld was onto something when he famously said "There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know." Maybe he was expressing a deep philosophical truth. Or maybe he was a prevaricating politician. But there is a truth revealed here that applies to science, and geology in particular. We are very good at not finding what we are not looking for.

Charles Walcott, a respected paleontologist, discovered the incredible Burgess Shale fossil assemblage in 1909. Although he collected tens of thousands of specimens of exquisitely preserved creatures from the Cambrian period, he didn't completely realize their full potential as a window into the evolution of the earliest complex life forms. He tended to shoehorn odd specimens into known phyla and classes without exploring the possibility that these animals could be entirely different organisms from phyla that are long extinct.

Often scientists will see something new, and despite their insight, they don't fully follow through. Alfred Wegener was a geological visionary who collected extensive evidence that the continents of the world had once been collected together in a huge supercontinent that he called "Urkontinent" (German word meaning "origin of the continents"). The story is told in all geology classes about how his ideas were not at first accepted by the geological community (there were some good reasons for the geologists to be skeptical, and but other objections were rather unfair). What strikes me is a rarely noted factoid: he thought of Pangea (Urkontinent) as the starting point, that continents began as a single landmass. I may be wrong about this (and I am open to correction), but he seems to never have realized that Pangea may have been constructed from the collision of earlier continents. Yet he used mountain ranges that seem to cross from one continent to another (such as the Appalachian Mountains in the U.S. and similar mountains in Scotland and Scandinavia) as his evidence for the existence of Pangea. Mountains like these were the suture zones where older continental fragments collided and melded together. There may have been as many as four "urkontinents" in the last two billion years! Would a fuller understanding of these earlier landmasses changed Wegener's approach to dealing with his hypothesis? It's impossible to say. He died in 1930 at the age of only 50. 

Which brings us to Death Valley and our recent trip. Our "strangers in a strange land" had just learned at an earlier stop that there are four kinds of faults: normal, reverse, right lateral and left lateral. Low angle reverse faults, thrusts, are well-known from pioneering studies in the Alps and Appalachian Mountains. This was pretty much the status of fault classification in the 1940s when the first pioneering studies were being done in the Death Valley region.

Levi Noble was one of the first geologists to map the geology in Death Valley, and the project was daunting. He was up to the task, and his maps have stood the test of time. But some of his interpretations were colored by his expectation of finding the kinds of faults he had seen before. He knew that low angle faults were thrusts, because no one had seen anything different. And thrusts are caused by compression, which generally causes older rock to be pushed up and over younger rocks. Noble found plenty of thrust faults exposed in the mountains around Death Valley.

When he mapped in the Black Mountains on the east side of the valley, he found some real oddities. In some places, faults seemed to be everywhere, to the extent that the crust simply seemed to be chopped up into small bits that were only a few hundred feet across. Layers that were hundreds of feet thick elsewhere where only a few tens of feet in these outcrops, including the roadcut in the picture above, which has been called Exclamation Rock or Exclamation Point.  Noble called these unique sequences, somewhat logically, chaos. And underneath the chaos he mapped a low angle fault. It probably looked much like the fault seen in the picture at the top of this post. He called it, quite reasonably the Amargosa Thrust. But there was something odd about it.

The person in the picture above is walking in a canyon that has been carved through one of the low angle faults like the one mapped by Noble. Above the fault plane is a sequence of Neogene volcanic rocks (a few million years old). The rocks in the gorge are Proterozoic gneiss and schist...more than a billion years old.

What's wrong with this picture??

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...

A Convergence of Wonders, Day 8: Of Time, Pressure and the Plain Truth

Day 8 of our journey through the Pacific Northwest found us in a time of transition...we weren't exactly in the northwest anymore, for we had left the Rocky Mountains and were traveling southward across the Great Plains. With an important exception, the landscape had become more subtle and its geologic secrets were well-hidden. In a historical note, we were just a few miles south of the northernmost point reached by the Lewis and Clark expedition in 1805.

The archaeologists were interested in the fact that we had arrived in the ancestral lands of the Plains Indians, and indeed since we had left Glacier National Park, we had been on the Blackfeet Indian Reservation. The billboard that greeted us at the edge of town was an acknowledgement that we were in the home of a different culture.

Our first stop was Museum of the Plains Indians in Browning. The museum (which did not allow photography) was filled with exhibits of Native American arts, and is well worth a visit.

At Browning, the landscape was irregular, with numerous small hills and isolated ponds. The hummocky topography is the result of the passage of continental glaciers moving south out of Canada. As we drove south, the shape of the land became more "normal" (for us Californians, anyway), with gullies and valleys carved by stream erosion. The Rocky Mountains remained in sight, off to the west.

At the small village of Choteau, we paid a visit to their modest museum, and had a look at some dinosaur specimens, including the display below on dino eggs. The region around Choteau is remarkable for two reasons. Excavations started in the 1970s revealed numerous dinosaur nesting sites, which were used repeatedly for tens, maybe hundreds of thousands of years. Not only were the eggs themselves important, but also the behaviour of the dinosaurs that laid them. Work by Jack Horner and others demonstrated that the Maiasaurs took care of their young, and that they moved in herds.We didn't have the time to arrange a trip to nearby Egg Mountain, so we settled for the displays.

Outside of the town of Augusta, we headed west towards Sun Canyon for a look at some remarkable Rocky Mountain geology. That glaciers had once emerged from the canyon onto the plains was immediately apparent, as the road traversed the rocky slopes of the terminal moraines.

Soft Cretaceous sediments were apparent in the streamcuts. In the picture below one can see a sweet little anticline, an upward pointing fold. The mountains loomed higher and higher as we continued west. And something was odd about them...

As can be seen in the picture below, there are two sets of prominent cliffs at different levels. They are made of the same rocks! The geology was repeating itself. It wasn't just once. As we drove up the canyon, the same carbonate rocks were repeated at least five times.

What we were seeing was another manifestation of the extreme compressional forces that resulted from the convergence of the North American continent with the oceanic crust of the Pacific Ocean basin. In latest Cretaceous and early Cenozoic time (70-50 million years ago), the crust was disconnecting from the underlying basement rocks in a series of blocks that were sliding up and over each other in a series of overthrusts. At least twenty such thrusts occurred in this part of the Rocky Mountains, and we drove past five of them.

 The outcrops and the scenery were incredible...

An additional point of interest for us...the carbonate rocks formed in a shallow sea in Devonian and Mississipian time, and were full of fossils of marine life, like crinoids, brachiopods, corals, and bryozoans. The students spent some time searching for the evidence of past life.

As we drove back down the canyon, we were reminded why creeks and rivers were flooding all over Montana and the Great Plains. There was still a tremendous, even unprecedented amount of snow left in the high country, and small reservoirs, like Diversion Lake, seen below, were filled to overflowing. 

 Going down canyon, we were given a different perspective on the repeating structural blocks.

We made one more stop in the lower canyon, looking for some ammonites. We instead found these strange burrowing structures in the Cretaceous rocks. They were identified by my Twitter colleagues as Thalassinoides burrows, something akin to burrowing crayfish or shrimp. They were huge! That's a quarter at the top of the sample.

 Back on the plains, we were treated to a view of a curious pronghorn.

The day was getting late, and we were delayed by a radiator leak, but we eventually made it to camp in Bozeman, where I had a chance to meet up with a fellow geoblogger, Callan Bentley of Mountain Beltway. He's normally at Northern Virgina Community College near Washington, D.C., but was on the road getting ready for a field class he was teaching in Montana. It was a nice to finally meet!

For whatever monotony there was on the long road across the plains, it would be made up for in our next day's journey...we were headed for Yellowstone via the Beartooth Highway.