Painted Desert

Painted Desert

Saturday, June 13, 2009

Geology, and things

Today, let's go back in time to visit some more photographs of interesting things.

Here we are walking back to our vans after a day of mapping at our first area. You might need to click on the picture to see the vans; they're quite far away. We were mapping along a line that was about a mile long and had about 200 feet of relief which we were constantly going up and down. I have seen geology described as the most physically demanding of the sciences; I don't know whether this is true, but the physicality of it is certainly an appealing factor. It makes supper taste better.

Here, we have headed out to a mapping area after a rain to find that the bentonitic hills are still too wet to allow passage. In the video below, our camp director struggles to walk back to us after an exploratory foray down a slope:

Bentonite forms from the weathering of volcanic ash, so its presence in these rocks indicates there were volcanic eruptions going on nearby during the Cretaceous. Besides making work impossible for geology students, some of the many amazing uses of bentonite include:

- the absorbent clay in cat litter
- an ingredient in Portland cement
- landfill sealant
- a lubricant and sealant in oil and water well drilling
- a clarification agent in the production of canola oil, beer, and honey
- an animal feed supplement
- a filler in pharmaceuticals such as calamine lotion
- an antidote for heavy metal poisoning
- an impurity remover and fabric softener in laundry detergent
- a thickening agent in paints
- a de-inker in the paper recycling process

Wow. Okay.

This is a picture of the Jurassic Morrison Formation. I took this picture because I got deja vu when I wandered into the area--it looked so much like the badlands I'd worked in last summer, down to the shape, color and abundance of hard concretions littering the surface. We walked over this hill and I said to my partner, "I'm looking for bones," and he said, "What does bone look like?" I said, "Like this," and picked up a piece from between my feet. We had found a pile of fossil bone... and unlike last summer in the Triassic rocks, where dinosaurs were extremely rare and most of the bone was from other reptiles, these seem likely to have been dino bone. Just little chunks, you couldn't tell what part of the body they were from. But it was interesting to find. I'm not sure I had any reason for thinking there should be bone there other than that it reminded me of similar places I'd found bone last summer.

Here is an interesting rock. Well, I guess it's only interesting if you know what you're looking for. The cracks in this rock intersect at angles of about 60 and 120 degrees, which means something. If the rock were being squeezed between the lower-left and the upper-right, it would be undergoing compression (squish) in those directions, and extension (pull) perpendicular to that, from the upper-left to the lower-right. Those are two kinds of stress. A third kind is shear stress, in which the particles are being neither squished nor pulled but are trying to slip past each other. The lines of maximum shear stress are exactly the cracks you see above, so the cracks provide a visual tip to where the pressure was coming from when this rock was under stress. Or at least, it would if the rock hadn't fallen off the outcrop and tumbled down to our feet.

There is a fault in this picture. Geology students learn about faults and stress and things in a class called Structure, which I have not had, because URI's structure professor retired and hasn't been replaced yet. I was very confused when I first saw this. See the dark band in the middle of the picture... that is a bed of sandstone, which forms a cliff that has made a bit of shade. And it just stops. When I ran into this in the field it took me a few minutes to even realize there was a fault, that it wasn't just a sandstone formed in a river channel that was rather small and localized.

Here is another picture of the same area, from a different angle. Take a close look and see if you can spot the fault, where a particular rock type fails to continue as it goes to the right or left. It was sort of exciting to recognize this in the field without anyone being there to tell me what it was.

Something much more complex is going on in this area. The beds on the right are flat-lying, in the middle they dip to the right, and on the left they dip to the left. It took a couple days of studying this area to figure out where the folds and faults were exactly and what they were doing.

Well, all that is "structure," in the singular. There are also sedimentary structures, which are (usually!) smaller-scale and also interesting. They can tell you a lot about the environment in which a rock was deposited.

Here are what are called trough cross-beds. To see what kind of environment forms this structure, click here. In that picture, the water would be flowing from the upper left to the lower right, and sand grains would be lifted off the long sloping backs of the ripples and would avalanche down the steep fronts, making the ripples migrate over time. The pattern they leave behind is what you see above. In a general sense, anyway. I do not know if the exact shape of ripple you see in the link would generate the exact cross beds as above, but that's the idea.

Here is the same sort of structure, from a core sample we investigated today. Well, I guess they don't look that much alike, now that I look at them, but they are the same thing, both produced by migrating ripples with sinuous crests.

Above is the sort of structure that doesn't look like much until you know what to look for. This section of core shows some grey and black sand and mud grains in flat-lying beds, which are messed up in some parts. How did they get messed up? There are many ways that the natural stratification of sediment can be disturbed, from being squashed under the weight of overlying sediment to being shaken in an earthquake, but usually the disruption is caused by "bioturbation," or disruption by living organisms. In this case, a small animal, perhaps a shrimp, burrowed into the sand in the center of the picture. You can see how the burrow goes straight down then takes a jog toward the number 59. Just above and to the left of the number, you can even see how the burrow is lined with mud (which is darker). The black splotch in the lower-left is also a burrow, either the same kind from a different angle, or a different kind.

We know that these things are burrows because they have modern analogs. We can look at sediments offshore today and see the same patterns, and see what creatures are making them. Above is a modern analog for bioturbation that I captured after one of our (many) rainstorms. You can see tracks and flopping-around-marks created by earthworms that were stuck in this puddle. We can see marks very similar to this in marine rocks, and I have, though I don't have any pictures of them.

Here is a much more impressive kind of track: this one was made by a dinosaur. That bulge is the underside of the track. At some point this tilted rock was a flat-lying tidal area, and a dinosaur stepped on the sand, causing it to bulge downward into the mud beneath it. When the sand hardened into rock, the shape was preserved.

Here, we explore a track site, with many tracks across this bed of limestone that a local intermittant stream has eroded down to. These tracks were discovered just a few years ago and they upended some ideas about the formation they are in. The Jurassic Sundance Formation was thought to be wholly marine, but since dinosaurs didn't walk on the bottom of the ocean, we now know that at least part of the formation was deposited right at the shore or even above the water.

My foot next to a three-toed dino track.

Anyway, today we worked on the core:

Describing its lithology and deciding what kind of environment all the bits of it must have been deposited in. Some things are much easier to see in a core, because the rock was taken from deep in the earth and wasn't sitting out at the surface being pitted and stained by the weather, but some things are harder to see, because the core is only a few inches wide. But it was interesting to look at. This is part of an exercise that has been run by an oil industry fellow who came out to teach us, and it included deciding where the oil was most likely to be.

Anyway, as you know, it's not all work out here. We drove to Cody to look at some other rocks one rainy day this week, and they let us stop and shop for an hour.

Here is downtown Cody. It looks awesome. It had a Dug Up Gun Museum. It's too bad I only had an hour to explore, but I did buy some jewelry and glow-in-the-dark dinosaur socks.

We had an alumnus come visit the camp with his family, and they made care packages for all 24 students and set them out for us. They had band-aids, bug spray, toys and some other things. On the left above is my roommate Kaylee. Kaylee says she would like to be mentioned in the blog. That is her. Kaylee.

We are not always the most ambitious bunch when it comes to leisure pursuits...

Here we are in the middle of a two-hour session of sitting out in the cold trying to throw pieces of gravel into beer cans. This delightful and spontaneous game is reproduced for you in the video below:

But I must say that finally, the weather has gotten a bit better out here.

Here is a storm moving off in the sky as the sun sets, throwing orange light on the orange Chugwater Formation cliffs behind the camp.

Students play frisbee in the gathering twilight.

Finally, last night was a big event. The rodeo came to Greybull, which is the closest town of any size, and most of us went down to see it.

The setting was stunning, with the folds of Sheep Mountain in the background.

There were bucking broncos...

...and small children chasing after a calf with a ribbon on its tail.

Some of our group.

And a group photo of most of us, plus a couple grad students. Yes, I am in this picture too.

We are halfway through field camp! Three more weeks of blog entries, and then I will be home.

1 comment:

Dad said...

I remember reading about bentonite being used in thixatropes for paint, but didn't know where it came from. I'll have to research that. Glad to see what kind of bottles are being used for the pebble game. Great pictures.