Shallow Surface Valleys Over Cave Passage

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Shallow Surface Valleys Over Cave Passage

Postby krstahl » Dec 18, 2011 7:03 pm

I am not a geologist but I have had a theory for the past 30 years about the passages in a large local cave system where the major cave passages follow underneath juniper-filled coulees on the surface. Pretty much where ever there is a shallow, juniper-filled coulee there is often a large cave passage underneath that runs parallel to it. I think that the junipers grow in the coulees because there is more water and that the coulees formed as the surface Madison limestone sagged plastically after the cave passages formed. With no solid base to support the surface only 30’ - 50' or so above the cave ceilings, I believe the surface limestone sagged and formed the coulees, the coulees provide drainage for surface water runoff, and the junipers grow where there is more water. In addition, I theorize that when a shallow coulee on the surface breaks into a walled canyon at its down-dip extent, the passage underneath ends at a rock collapse. I believe this is because the surface could no longer be supported by the void underneath. Over the millennia, the collapsed canyon’s soil and rock was washed into the nearby canyon gorge because there is more water flow on the surface than in the caves. The larger cave passages are about 100’ wide and are several hundred feet above the canyon gorge. The surface coulees are approximately the same width as the cave passages and best estimate are 4’ – 6’ feet below the surface plane.

Several of the old timers (some have passed on) think my theory is sound but we never had proof because no one took the time to do a surface survey. Now, with map digitizing and GE I can do the overlays with fair accuracy. I may still not have scientific proof, but I think I have pretty convincing anecdotal evidence.

Anyway, if you look at cave surveys overlaid on the surface my theory holds up pretty well even for smaller connections between large passages. One other thing I have noticed by using GE over the past several years is that quite often there are large round clumps of junipers, not always at a low spot. I have found overlay evidence that some of the round juniper clumps coincide with buddhas inside the caves similar to trees in the sinkholes in Minnesota.

Is there any scientific evidence or knowledge within the geology-trained group of cavers in the NSS that supports/disproves my theory? I have some of my cave friends that believe I am correct while others think I am not.
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Re: Shallow Surface Valleys Over Cave Passage

Postby Anonymous_Coward » Dec 20, 2011 2:33 pm

Care to let us in on where you are talking about? A map or even a location might help us out. It is all very vague otherwise.

Let's see, referring to "coulees" makes me think you are from the Northwest. (No one talks like that 'round here)

Madison limestone and "buddhas" sounds like you could be in Wyoming, Idaho, or Montana. Am I close?
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Re: Shallow Surface Valleys Over Cave Passage

Postby Anonymous_Coward » Dec 20, 2011 2:34 pm

Without knowing anything else, I'd say that if the limestone was sagging in the valleys you should be able to measure the local dip in the cave passages to confirm or disprove your hypothesis.
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Re: Shallow Surface Valleys Over Cave Passage

Postby krstahl » Dec 20, 2011 7:33 pm

Mr. Armstrong,
Your assumptions are correct. I knew "coulees" was colloquial, therefore the post title was shallow valleys to make it more clear. I could have used the even more colloquial "draws" to describe the shallow valleys or made everyone think I was from the southeast and called them hollows. The cave area(s) are in Montana. I will reserve sending out an attachment to the forum that shows the cave overlay, wanting rather to have a geologist-caver(s) PM me. I'm a little reluctant to bare it all on the forum in case I am so far off base that I am embarrassed. So, any geologist trained cavers out there that care to PM me, feel free to do so. My screen shots of the overlays don't transmit (the transparency goes to full opaque) so I will have to send GE coordinates, jpgs, and scale kmls so that you can replicate my overlays.

As to measuring dip inside the cave, I'm not sure how we would do that because, theoretically, the dip I want to measure (sagging ceiling) would be perpendicular to the passage walls. Of course the cave dips with the dip of the Madison layer, but I'm not sure how to measure perp. dip.
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Re: Shallow Surface Valleys Over Cave Passage

Postby Anonymous_Coward » Dec 21, 2011 12:56 pm

Assuming you have visible bedding planes in the passage, you should be able to compare elevation of each plane from one side of the passage to another. This could be done with the usual cave survey gear. You should be able to measure this local dip independent of the regional dip, that guides the cave passage. Check out chapter 14 "Geologic studies of caves" in Art Palmer's Cave Geology book. It describes many methods for underground geologic mapping. If you do not see dip in these beds toward the coulee then it probably does not exist.
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Re: Shallow Surface Valleys Over Cave Passage

Postby krstahl » Dec 21, 2011 2:35 pm

OK, assuming I do a strat survey in the passage and the strat on one side of the passage is higher than the other, how does that indicate (not prove) roof surface sagging? I would think that it would only show that there is general dip in the entire strat from the high side to the low side. The only way I can think of to check "sag dip" from inside the passage is if there is an exposed right angle fracture feature in which the rock has fallen away from at least two sides and that at least one face would have to be perp to the passage.

Aha! If I can find two surface coulees perp to each other, and I suspect that each overlays an independent passage, then that could give me an area to perform a "sag dip" strat survey. Example: If all the instances showed a lower strat on say, the south side of the passages then that would indicate a general strat dip. BUT, if the strat shifted from high to low and from the north to south side of passage as different points were taken, then that might indicate a "sag dip" but probably still not prove it. However, if similar results were obtained from east-west passages, then that would strengthen the theory. Of course, the more data points, the more conclusive, either for or against.

Any other ideas/theorys would be welcome and considered.
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Re: Shallow Surface Valleys Over Cave Passage

Postby BrianC » Dec 21, 2011 3:46 pm

I'm not sure where you are going with this, but I remember that in my old Ky. days we would look for cedar trees because they seem to like the calcium rich soil that caves are often formed in. As far as finding caves simply by looking for the dips, it would only be relevant in very refined areas that had somewhat of a slow cure in the formation of the limestone allowing the so called dip. Many caves are formed in rock showing no dip, possibly even in rising mountains. I am very interested though in what you may find!
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Re: Shallow Surface Valleys Over Cave Passage

Postby Anonymous_Coward » Dec 21, 2011 3:59 pm

krstahl wrote:Aha! If I can find two surface coulees perp to each other, and I suspect that each overlays an independent passage, then that could give me an area to perform a "sag dip" strat survey. Example: If all the instances showed a lower strat on say, the south side of the passages then that would indicate a general strat dip. BUT, if the strat shifted from high to low and from the north to south side of passage as different points were taken, then that might indicate a "sag dip" but probably still not prove it. However, if similar results were obtained from east-west passages, then that would strengthen the theory. Of course, the more data points, the more conclusive, either for or against.


Sorry if I wasn't clear, but this is exactly what I meant. Take many (as many as possible) measurements in many different caves in the area. You should be able to measure dip down the passage and across the passage. Compare your measurements to the known local dip. You can even measure this yourself on the surface if neccessary. You should be able to produce a pretty interesting stratigraphic map from these readings. Even if you do not find proof for your hypothesis, you will no doubt learn something about the area's geology that you didn't already know. More than likely it will be a new question!

If nothing else, it sounds like a good excuse to get some people together and go caving. :kewl:
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