Paging Dr. Balta to the white courtesy phone . . .

[Texsox]

A few questions about the geology of Big Bend National Park

 

Any idea what this is and how it formed?

 

We viewed a formation where a gray/cocoa layer, very thin, seemed to have formed over some pink (limestone?). Was this rock or perhaps a lichen?

 

There is a layer of limestone that the park rangers called Santa Elana limestone. How is limestone named and are there significant differences between limestone in various locations around the world?

 

The area was overgrazed which set up a series of events leading to the desert environment there now. There are many minor and major washes crisscrossing the desert. Were these a result of the increased run off as the area changed from grasslands to desert?

 

I may need to hire you for a week, the area is so fascinating. We wore out a couple rangers with questions.

 

 

 

 

[Balta1701]

I'm not going to say what they are at 100% certainty from a photo, but I can give you a few possibilities.

 

1. Could always be something growing on the surface

2. Could be an alteration feature, some small grain altering and giving banding around it.

3. My actual guess (first 2 are hedging my bets)...they look like Brachiopod shells, in a limestone matrix. If that's what they are, the rock has likely been slightly metamorphosed, so that it's quite a bit harder and a few of them have been slightly deforemd. There might be a crinoid or two in there also.

 

Trick when you're looking at limestones...fossils weather out of the rock differently than the rock itself, different crystallinities and different dissolution rates. When you see something on the surface like that, the next step is to break a larger chunk of the rock and look at a fresh surface.

 

I can't judge a "pink layer on top of a limestone" just from that description, need more details.

 

 

 

 

Geologic formations are named by the person that investigates them, and are typically named after something in/around the location where they are described first. That first description is generally referred to as the "Type section" or "Type locality". I note that there is a "Santa Elena Texas" in the South, close to the Rio Grande, which may be the namesake of that particular unit.

 

There are lots of variations in limestones around the world, composition, fossil content, how "lithified" (rocky) it is, etc. You can even find gradations within units, so if you look at the Santa Elana limestone in 1 spot, it doesn't always look like the same limestone in other spots.

 

This of course leads to much confusion, because a limestone that is laterall continuous over hundreds of kilometers will often be described in several locations by several people, leading to multiple formation names for single units. There are at least 5 I've counted for the sandstone that overlies the great unconformity out West, for example. Thus, formation names are always tricky and never fully consistent. There are general rules we try to follow, but that does make the literature confusing.

 

 

 

How often do the washes run and where are they sitting in the topography? If you're high up, it's often hard for washes to reset themselves, but lower down, washes can occasionally "crisscross" because different rain systems wind up hitting different areas nad starting floods in different spots. If you had a consistent pattern rain shift, I'd imagine it wouldn't take too long for dry washes in sediment to reset themselves.

 

If you're talking about washes in hard rock that crisscross, that might be a different effect, that could be either a climate shift or a tectonic organization (uplift/downdropping of some rocks relative to others)

 

[Texsox]

Thank you sir. A follow up question. What is the commonality in limestone and what would make one area's limestone different? All of the dikes we saw were rolling up and down, never of the same height. As I understand they were formed by molten rock squeezed up through openings, it is just a small sample size that caused us to only see the rolling dikes or is there some factor I am missing that causes rolling waves?

[Kyyle23]

Tex, I believe you found concrete that broke off the road.

[Balta1701]

Thank you sir. A follow up question. What is the commonality in limestone and what would make one area's limestone different? All of the dikes we saw were rolling up and down, never of the same height. As I understand they were formed by molten rock squeezed up through openings, it is just a small sample size that caused us to only see the rolling dikes or is there some factor I am missing that causes rolling waves?

I'll give you the geochemical definition first, since I am a geochemist. A Limestone, by definition, is a non-metamorphosed rock made up mostly of Calcium Carbonate (CaCO3). However, lots of other elements can substitute into that structure. Magnesium can substitute for Calcium, Strontium can go in there, you can get crazy things like uranium, and most of the rest of the periodic table. You can also get variable amounts of other minerals, most commonly quartz, mixed in. Thus, most limestones will have slightly different compositions, chemically.

 

There are also differences in what the limestone is actually made up of. You can have a fossiliferous limestone, made up of shell parts entirely, a micritic limestone, made up of "micrite" which is very fine grained carbonate, and you can have gradations between them. Your sample, for example, coudl be between them...shells in a micritic matrix. Beyond that, different limestones from different eras will be made up of different types of fossils...you won't find trilobite rich fossils in the last 250 million years, because they've been extinct, so a 65 million year old limestone will be different in fossils from a 500 million year old limestone.

 

Can you redescribe the dike question? Not quite sure howt o answer that one from what is written.

[Balta1701]

Tex, I believe you found concrete that broke off the road.

We call that "Anthropomorphite".

[Texsox]

LOL, the nearest road was over three miles away. Funny you should mention a road, on that trail there is a portion of a concrete road that was abandoned when Big Bend was created. It is really weird, in the middle of a desert, to see a 30 or 40 yard section of an old one lane road. At first I assumed it was an old driveway, but through later research I learned the old road was routed closer to the Dead Horse Mountains.

 

During the 1850s the US Army conducted two expeditions through that area to test the usefulness of camels in the southwest. It was partially to map the region and also to help with the Mexican-American war efforts. The project was dropped as the Civil War broke out and later never followed up on, in part because the expeditions were conducted under the commands of Jefferson Davis and Robert Lee. A little know camel fact I learned, camel urine is paste-like and their feces is almost bone dry to conserve water.

[Texsox]

Hopefully this will not make it more confusing

 

If we looked at a topo map of the dikes they would be running across the lines, not parallel to them. All of the dikes were in the areas of rolling hills so the tops were at constantly changing elevations. I would have thought at some point there would be a "flat top" section. Erosion has taken away the other material from them at an almost constant level. So from the ground to the top of the dike that distance is fairly consistent, but the terrain is always through rolling hills never straight across a mesa.

 

 

[Texsox]

Adding another one, and thank you . . .

 

I've identified a couple of the ranges as being part of the Appalachians and some as part of the Rockies, how are ranges classified? If they both come so close together, why isn't this one large "U" shaped range? Age? Type of rock?

[Balta1701]

Adding another one, and thank you . . .

 

I've identified a couple of the ranges as being part of the Appalachians and some as part of the Rockies, how are ranges classified? If they both come so close together, why isn't this one large "U" shaped range? Age? Type of rock?

Generally, this one is going to happen based on what is driving the uplift. If the rocks are being uplifted by compression from the East, they probably fit into the Appalachians; compression from the West would fit into the Rockies. There are age constraints here also; the uplifts happened at different times as well, so you can probably distinguish which mountain range things fit into based on age, but really, the dividing line is "one is driven by Europe and Africa smashing into North America", "One is driven by subduction along the west coast and various smaller things smashing in".

[Balta1701]

Hopefully this will not make it more confusing

 

If we looked at a topo map of the dikes they would be running across the lines, not parallel to them. All of the dikes were in the areas of rolling hills so the tops were at constantly changing elevations. I would have thought at some point there would be a "flat top" section. Erosion has taken away the other material from them at an almost constant level. So from the ground to the top of the dike that distance is fairly consistent, but the terrain is always through rolling hills never straight across a mesa.

Still not getting a good idea on the meaning here.

[Texsox]

If we were to take a walk on top of the dikes we would be constantly walking up and down hill, I did not see a spot where they were flat across.

[Balta1701]

Did someone actually call them "Dikes"? The technical term for a "planar magmatic intrusion" which is also "Horizontal" is a "Sill".

 

If they are actually "Dikes" and they're now horizontal, that would imply substantial rotation of the features, which could easily have folded them up.

[Texsox]

Generally, this one is going to happen based on what is driving the uplift. If the rocks are being uplifted by compression from the East, they probably fit into the Appalachians; compression from the West would fit into the Rockies. There are age constraints here also; the uplifts happened at different times as well, so you can probably distinguish which mountain range things fit into based on age, but really, the dividing line is "one is driven by Europe and Africa smashing into North America", "One is driven by subduction along the west coast and various smaller things smashing in".

 

Is that kind of like Occidental versus Oriental mountain ranges?

[Balta1701]

Is that kind of like Occidental versus Oriental mountain ranges?

Don't know my Mexican geology all that well, but that at least sounds like a plausible comparison.

[Texsox]

I wish I could find a decent photo, I think my explanation is still inaccurate. All of the dikes were in the rolling hills, none on a flat feature. Looking across the desert to them we would see a wavy line. They are still standing horizontal like the back of a stegosaurus. And yes, the NPS rangers all referred to them as dikes, I believe volcanic dikes to be more precise.

[Texsox]

Don't know my Mexican geology all that well, but that at least sounds like a plausible comparison.

 

I am trying to learn, the problem is the books I am finding are either too simple or too advanced. It seems as if there has been so much activity there, and so jumbled up, that explanations are not very straight forward and can change dramatically in a few miles.

[NorthSideSox72]

I wish I could find a decent photo, I think my explanation is still inaccurate. All of the dikes were in the rolling hills, none on a flat feature. Looking across the desert to them we would see a wavy line. They are still standing horizontal like the back of a stegosaurus. And yes, the NPS rangers all referred to them as dikes, I believe volcanic dikes to be more precise.

Do you mean vertical? Because that is what the bolded sounds like, and that is what the other volcanic dikes I have seen look like.

 

[Texsox]

Thank you, I meant vertical.

 

Have you seen any that were along a flat section of ground?

[Balta1701]

Let me ask it this way. Hypothetically imagine that one of these dikes was running "north-south" perfectly. If you walked along the dike, you'd be moving up and down in elevation, but moving constantly in a single direction, along 1 vertically oriented plane...that part I can explain if that's the question...is it?

 

The other possible thing you could be saying, if you tried to walk along the dike, and you started off north-south, at some points you'd be traveling more westerly, at some points you'd be traveling more easterly, but you'd average out to heading north/south, just with some waviness to the direction you're traveling... Is that what you're describing?

[Texsox]

You would always be heading north-south but always up and down. You would always be climbing or descending. They seem to match the terrain. Also, as I have been thinking about it, could it be that they would be located along laccoliths which would account for the rise and fall in elevation? The laccoliths are all dome shaped.

[Balta1701]

Ok, i get it now.

 

What you are looking at are in fact volcanic dikes. Basically, they intrude along a single plane, rising up through the crust.

 

When a volcanic dike reaches the surface, you get an eruption...and that eruption, ideally, is something like a fissure eruption. Something like the image below. You get an eruption along the line, and then lava flows away from that point.

 

Now what you ought to have noticed if you're walking along just a vertical plane is that there's no lava flow anywhere. You're just walking along the feeder dike. That means, stratigraphically, the actual eruption had to happen somewhere above your head...maybe hundreds of meters above your head. Since those dikes were emplaced, there has been substantial erosion...of everything, not just the dikes, but of the rocks around them as well.

 

That means the topography itself is being controlled by the erosive processes. The dikes can influence erosion, because they're more resistant, which is why you described them as stegosaurus plates...but whatever surrounds them is also being eroded, and stream paths and other structures can focus erosion in certain areas. The waviness then is a response to the rocks aroudn the dikes being eroded in different amounts.

[Texsox]

Thank you, that makes sense. Once I learned that it is erosion, not some eruption or uplifting that creates features like below, a lot of the geology started making sense.

 

[Balta1701]

Here's a good example image. The big flat thing is a several-meter wide dike, but note how it abruptly stops right when it gets to the current valley floor. There might be a fault there or the Dike might keep going underneath the valley...but the dike itself is only influencing the topography, not controlling it. The dike wraps up over the hill and comes back down the other side. The dike itself probably used to go upwards hundreds of meters from the current level, but everything above the current surface has been eroded away. The dike stands high because it is more resistant than the stuff around it, but clearly whatever it is that formed the valley floor is now the feature dominating the topography.

 

[Texsox]

There we go. Perfect. Here is what I'm intrigued by. I am assuming that liquid rock flows and has about the same characteristics as any other liquid, chiefly it seeks it's own level. If that is true the molten rock would have been later uplifted to that shape. How would a liquid follow that contour?