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Cave sedimentation model for steep slopes

PostPosted: Jun 30, 2011 5:55 am
by stefan
In one caving trip some time ago we stumbled upon a small cave that was (and still is :) ) filled with sediments. The gallery shape indicates that it was an exurgence. The gallery slope is around 35 degrees and when we first found it, it was filled to the ceiling after the first 5 meters in length and around 2 meters in depth. After a few digging trips we managed to move forward about another 5 meters and approx. 2 meters in depth. The sediments removed were: a thick layer of dry clay followed by some gravel and small stones mixed with a moister clay. The gallery slope is constant and seems that it will go on like this for a while.

I tried to figure out how did this cave get filled up with sediments but I didn't come to any conclusion. The presence of the small gravel could be explained by the fact that the cave was an exurgence and it transported this material but it seems to be quite high on the slope. Moving on to the thick layer of dry clay that fills the rest of the gallery up to the ceiling, I can't explain how it got there. I don't think it could have been transported by water because the lower parts of the gallery would become filled to the ceiling first thus obstructing the transport from that point up. I first thought that the filling came from outside the cave but I also ruled out this option since the deposits were to compact to me organic material from the surface (we found no air pockets). Another possibility that the clay could have infiltrated from the surface with rain water in the course of many thousands of years.

I am not an expert in cave sedimentation and until now I found no relevant material to fit this specific case. So if anyone has an idea on how sedimentation occurs in this kind of settings or can point me to a helpful resource it would be great.

Here is also a picture of the cave entrance to give an idea on how steep the slope is: http://www.speotimis.ro/galerie/Cheile-Garlistei-oct-2009/noua-pestera.jpg

Thanks in advance!

Re: Cave sedimentation model for steep slopes

PostPosted: Jun 30, 2011 8:06 am
by Spike
My interpretation would be that during an earlier stage of development velocities of the discharging water during high flow events was such that gravel of the size fraction you observerd was beign transported along and up the conduit such that a angle of repose of ~ 35 degrees was sustainable. As long as the velocity of the fluid in the conduit and conduit cross-section stays consistent, the gravel can be transported up the conduit, irespective of height. When the velocity drops the material can slump back down the conduit, or it may stay locked in place, also as the velocity drops fines can settle out and fill in void space around the grains, locking them in place. With time the sytem can evolve to where there is not sufficient velocity to transport gravels through the conduit, but finer material may be transported up and onto the slope forming your clay layer. It does not have to fill up from the bottom. You aren't looking at a basin or an old channell being filled with sediment, but a pipe that pipe that can generate enough velocity/pressure to move material through it. There are several caveats of course, but this is the basics. Sediments like this can tell you a lot about he paleo-hydrology of the system, so it's worth studing in more detail.

Re: Cave sedimentation model for steep slopes

PostPosted: Jun 30, 2011 10:02 am
by stefan
Thanks for your nice explanation. So if I understand correctly that gravel can be quite high on the slope and the gallery could continue to descend for a long distance, meaning that there will be lots of digging to do ? :grin: I was hoping that we're somewhere at the end of the downwards slope.

Spike wrote:You aren't looking at a basin or an old channell being filled with sediment, but a pipe that pipe that can generate enough velocity/pressure to move material through it.

Can you give me some references (books, scientific articles...) on the subject so that I can read more info ?

Spike wrote:Sediments like this can tell you a lot about he paleo-hydrology of the system, so it's worth studing in more detail.

I would also like to make such a study but I don't know what would this imply. Can you tell me more about this? Could this be done by amateur cavers with interests in the science of caves or it requires more experience and/or special devices?

Re: Cave sedimentation model for steep slopes

PostPosted: Jun 30, 2011 12:35 pm
by rlboyce
How do you know it is a resurgence and not an insurgence? Is imbrication present? Is the layering parallel with the passage floor or is it level? Do you think different densities (rock vs. clay) could partially or fully explain the layering you have found?

Re: Cave sedimentation model for steep slopes

PostPosted: Jun 30, 2011 12:50 pm
by Spike
First I'd look at Palmer's book Cave Geology for a general understanding and for more references.

A poke around the Karst Information Portal turned up an interesting paper on springs and sediments at max discharge by White et al in 2008 "Threshold events in spring discharge: Evidence from sediment and continuous water level measurement" It would be worth reading as well.

I would also take some time to read about ebb and flow springs, as sediment blockage rather than siphons likly control the changes in discharge in many cases. Cave divers often report gravels, and cobbles deposited on the floors of rise tubes and in some cases divers have been traped by disturbing sediment that was deposited at high flow that slumped back into the lower conduit. in one case the diver was able to dig himself out :yikes:

Sorry I don't have better references on hand, but with a little searching you should find what you are looking for. Relationships between grain size sorting, and velocity are well understood in sedimentary geology liturature, and velocity is a function of conduit cross-section and discharge, which is just intuitive.

Re: Cave sedimentation model for steep slopes

PostPosted: Jun 30, 2011 3:54 pm
by stefan
rlboyce wrote:How do you know it is a resurgence and not an insurgence? Is imbrication present? Is the layering parallel with the passage floor or is it level?

Here is how the gallery looks from the entrance to the point where it was filled to the ceiling.
Image
This shape is similar to a cave situated about 30 meters lower. It also has a steep gallery and it is a current resurgence. Thus we concluded that this was an older resurgence from the time the valley level was higher. I don't understand what you mean by imbrication (maybe it's because my English is not that good). The layering of the gravel and moist clay is, as much as we could see, parallel to the floor. We dug our way following the ceiling just enough to allow us to move around and dig. Only now, 10 meters from the entrance, we also found the solid rock floor so this is why I said "as much as we could see". The dry clay just fills the rest of the gallery to the ceiling.
rlboyce wrote:Do you think different densities (rock vs. clay) could partially or fully explain the layering you have found?

First I want to add that the gravel we excavated contained granule-sized fragments up to 1-2 cm or better said pebbles. We found just a few larger rocks from the size of a fist to one slightly larger than a helmet. If by this question you refer to the weights of the materials then I could say yes, the heavier materials were at the bottom.

@Spike:Thanks for the references and the story about the divers. My main problem was that I'm not quite familiar with the terminology and my search o internet was not that fruitful. But now with your tips and explanation I might just have all I need.

Re: Cave sedimentation model for steep slopes

PostPosted: Jun 30, 2011 5:01 pm
by rlboyce
Imbrication, as far as I understand it in the sense of rocks and water flow, is simply the natural overlapping of rocks in a particular manner. If water flow is not present and imbrication is obvious, one can tell which way the water used to flow. If you can find signs of imbrication, interpreting the pattern may help you determine how that sediment got there. If the pattern suggests the flow went down-slope, then the sediment was probably deposited from an outside source, not from old resurgence flows.

Image
(taken from http://www.umt.edu/geosciences/faculty/ ... 0/L6B.html)

I am not familiar with the cave, but as a suggestion I would not completely rule out the possibility that this is fill-in material which came in from the entrance. The material would not have to be organic, as heavy rains can carry dirt with them. Looking at the picture of the entrance, I see a nice slope above it and it seems entirely possible to me that over the years dirt from this slope has found its way into to cave. There are many such instances where this has occurred in my stomping grounds, although I'm not saying this is the case with your cave.

Also, I recommend that you take one more look at the layering, both local layering and the overall trend of the layering. Especially where it meets the ceiling (signs of pooling?). Try not to disturb the remaining sediment as much as possible so the layering will be easier to see.

Good luck with the digging!

Re: Cave sedimentation model for steep slopes

PostPosted: Jul 4, 2011 2:20 am
by stefan
Now I understand what imbrication is. I wasn't familiar with this term so thanks for the explanation. I did not notice any clear imbrication in the gravel layer but I did not pay that much attention to this so I'll have to take a closer look next time I'll go digging.

We also thought at what you said about rain water carrying dirt from above but the color of the dirt inside the cave is different than the outside dirt (outside - black, inside-reddish).