Cave geology/formation questions

[khakipuce]

I have two geology related questions but there doesn't seem to be a specific topic for this type of question so I'm posting in the general area.

[list type=decimal]
[*]Have any studies been done on the rate of passage development that is due to mechanical erosion rather than chemical (acidic) erosion in limestone?
[*]In a number of caves in Barbondale (e.g. Crystal Cave) and Ribblehead (e.g. Black Reef) there are veins of what I guess is secondary mineralisation which seem to occur in the joints. The infill is dark brown/black and would seem to be insoluble or far less soluble than the surround limestone. Does anyone know what this is? Given the proximity of the Barbondale caves to the Dent fault, I guess the mineralisation there could be associated with the fault.
[/list]

 

[Smithers]

• • • Have any studies been done on the rate of passage development that is due to mechanical erosion rather than chemical (acidic) erosion in limestone?

    
    In response to your first question, I don't know of any specific journal articles or research, but I'm sure if you have a look on the IJS website (http://www.ijs.speleo.it/) you'll be able to dig up some appropriate journal articles.  You may also want to have look at Google Scholar or Acta Carsologica (http://carsologica.zrc-sazu.si/).
    
    

    • In a number of caves in Barbondale (e.g. Crystal Cave) and Ribblehead (e.g. Black Reef) there are veins of what I guess is secondary mineralisation which seem to occur in the joints. The infill is dark brown/black and would seem to be insoluble or far less soluble than the surround limestone. Does anyone know what this is? Given the proximity of the Barbondale caves to the Dent fault, I guess the mineralisation there could be associated with the fault.

    
    
    About the black/brown mineral deposits, do these jut out of the limestone in little erratic bands?  If they're what I'm thinking of, I believe that they're deposited organic matter, however I could be completely wrong.  In terms of how they got there, I imagine the material was either deposited from solution as water percolates through the limestone and reaches a fault or micro-fracture in the rock.  Atmospheric fallout, upon which more limestone has formed could also be a contender. 
    
    However these are completely uneducated guesses, as I'm not a geologist (only an ex-geographer).

 

[gus horsley]

The rate of mechanical erosion in caves varies hugely even within one cave and is therefore very difficult to measure.  Obviously it's a much higher rate when streams are in flood, when it's assisted by suspended cobbles and sand in the water.

The mineralisation you noted may be chert, a siliceous material similar to flint, which occurs in lenses and nodules in the bedding of the limestone and is more resistant to erosion than limestone.  It doesn't occur in veins though.  However, sometimes you get small amounts of brownish carbonates such as siderite, dolomite or ankerite filling joints - these could occur in the vicinity of faults but I'm not aware of such mineralisation in the areas you mentioned.

 

[Peter Burgess]

The rate of mechanical erosion in caves varies hugely even within one cave and is therefore very difficult to measure.  Obviously it's a much higher rate when streams are in flood, when it's assisted by suspended cobbles and sand in the water.

Also much higher if the stream carries lots of abrasive sediment when in flood which might be dependent on where the stream originates i.e. the sort of ground it covers before it enters the cave?

 

[graham]

It's relatively easy to do work on limestone solution 'cos you can measure discharge and the dissolved carbonate. Measuring solid load moved out of a rising is a sight harder. Also bear in mind that - in limestone caves if not in granite ones - much of the material that is removed mechanically from the cave walls will subsequently be dissolved and carried out in solution. So would that material have been removed mechanically, chemically, or both.

S'not easy this stuff.

 

[langcliffe]

Also bear in mind that - in limestone caves if not in granite ones - much of the material that is removed mechanically from the cave walls will subsequently be dissolved and carried out in solution

And granite to a lesser extent. The feldspars chemically weather to form kaolin plus sodium and potassium ions, and the biotites and amphiboles weather to produce clays and soluble oxides. Much of the solid residue of granite weathering is clay and so is easily transported - it's the quartz that's the bugger.

 

[Brains]

As mentioned a difficult subject, BUT mechanical wear is generally considered to be much more significant in vadose development, but less so in phreatic passage - the water would have to be very turbulent to maintain an all around scour from suspended sediment.

The mineralisation could be many things as mentioned above, including perhaps discoloured calcite, but the prime suspect until further evidence is brought forward would have to be chert, imo...

 

[owd git]

any photo of said mineral?
description?( crystal structure/ granular? stain ie iron leaching below. shiny /dull. solid /fractured.)any other non destructive info gathering would fuel at least my geological interest.
sad owd git as i am, ta. O. G.

 

[khakipuce]

Thanks to all for the input.

The mechanical wear thing, I agree is very complex, but surely that's what makes it interesting. In the past I worked in sewerage and the inverts of certain sewers wear out due to grit being washed down - there may also have been some chemical attack, but 3" of engineering brick may have gone in a little over 100 years. In caves we see "pot holes" - i.e circular holes worn into the bed by stones being rolled around by the action of water, e.g. Dolly Tubs.

There are several things about this that interest me, one being that the pH of the water flowing though a cave must decrease as the surrounding rock is dissolved, so after some distance the contribution of acid must decrease, and for a given cave over a relatively steady state period (??? the last few hundred years ???) then is the erosion 1000m down a stream more  due to mechanical effects rather than chemical?


On the secondary mineral thing, In think I know enough geology to know that it is not chert. I'm interested in the organics idea, and I have seen black gravel in the limestone that could well be organic in origin. But the material to which I referred is in veins, it does stick out (less soluble?) and forms sharp edges. I had considered ferroan calcite but assumed this would be of similar solubility as the rest of the cave. Even the mighty Google was stumped by relative solubility of ferroan calcite (at least it returned nothing that I could access for free). It could well be that the stuff in Barbondale is of a different origin to the stuff in Ribblehead and certainly the Ribblehead stuff looked (to my not very well trained eye - should have paid more attention on those field trips) much more like the sort of secondary mineralisation that comes from geothermal processes. Whereas the Barbondale stuff could just be material that has washed into slightly open joints and then been compressed to make it a bit solid. But I'm just guessing.

I've just seen the post asking for more mineral details, I did  :spank: take a couple of loose bits out of Dog Hole. The crystals are tiny, the overall mineral is chocolate brown and the streak appears to be brown, although I am tempted to do some more streak tests to see if I can get through the surface and see what is underneath. It is fairly soft.

 

[graham]

Also bear in mind that - in limestone caves if not in granite ones - much of the material that is removed mechanically from the cave walls will subsequently be dissolved and carried out in solution

And granite to a lesser extent. The feldspars chemically weather to form kaolin plus sodium and potassium ions, and the biotites and amphiboles weather to produce clays and soluble oxides. Much of the solid residue of granite weathering is clay and so is easily transported - it's the quartz that's the bugger.

Which makes the genesis of sandstone caves interesting as well. An interesting read is: SELF, C.A. AND MULLAN, G.J. 1996. Redefining the Boundary Between Karst and Pseudokarst. Cave and Karst Science, 23. 2 pp. 63-70.

 

[owd git]

A comparative hardness (Mohs' scale) will offer as much as steak test as u have a specimen sounding poss barytes higher s. g. than calcite or flourspar would confirm.pray tell of further streak test O. G.

 

[khakipuce]

Agree that Mohr hardness would be good, but having got away with streaking it on Mrs Puce's white kitchen tiles (it's amazing what the promise of a new kitchen will do  ) I'm a bit stuck for other items of relative hardness, did just try the bottom of a cast iron pan, but I'm not sure that that is much softer than the ceramic tile (which I also re-tried) - both came out brown.

Will perhaps dust off the box-brownie in a few days and see if I can post an image.

 

[Peter Burgess]

A comparative hardness (Mohs' scale) will offer as much as steak test as u have a specimen sounding poss barytes higher s. g. than calcite or flourspar would confirm.pray tell of further streak test O. G.

Is a steak test one that tells you if a mineral is rare? Discuss.

 

[owd git]

rough Mohs
fingernail :2.5
2p piece:4
penknife (not stainless): 5.5
tool steel/ file? :6.5.
if soft enough a stroke with a smooth file gives a 'true' streak test
p s. well spotted typo' Peter.
bleme woodhatch!
O. G.

 

[Pitlamp]

For a paper on the black fragile mineral in Black Reef Cave see BSA Cave Science 4(32) 1962 pages 360 - 361. It's nice to think that you keyboard tappers can't get absolutely everything you want to know from the interweb - it's still well worth browsing through a good caving library!

If vadose canyon incision is largely the result of physical erosion due to sediment load then why are the floors of vadose canyons scalloped? (Others who have indicated above that the situation is far from simple are absolutely right.) Also, never underestimate the role of physical erosion within the phreatic zone - see a separate thread on this topic. Overall rates of incision of vadose canyons can be estimated from speleothem dating. (I seem to remember that a typical rate of incision in the Dales is about 20 mm per thousand years but I can't remember where I read that as I type - it'll almost certainly be in something published by Tony Waltham.) Also this is very much an average value which will change constantly due to umpteen variables.

Are you trying to calculate how much bigger a squeeze will get before the end of your caving career?

 

[khakipuce]

Ptlamp, thanks for that, not having BSA Cave Science 4(32) 1962 to hand, I don't suppose there is a chance you could give us a brief synopsis?

I seem to see a fair few stream beds that aren't scalloped but I guess that this may suggest that no scalloping ever formed, rather than it formed and then was worn away.

Unfortunately my girth seems to be increasing a little more than 20mm per thousnad beers so I will preumbably have to give up caving some time after a week on Tuesday :-\

 

[graham]

For a paper on the black fragile mineral in Black Reef Cave see BSA Cave Science 4(32) 1962 pages 360 - 361. It's nice to think that you keyboard tappers can't get absolutely everything you want to know from the interweb - it's still well worth browsing through a good caving library!

I've scanned BSA publications for people before I'm sure I'll do it again (but not today, got a meeting to go to).

If vadose canyon incision is largely the result of physical erosion due to sediment load then why are the floors of vadose canyons scalloped? (Others who have indicated above that the situation is far from simple are absolutely right.) Also, never underestimate the role of physical erosion within the phreatic zone - see a separate thread on this topic. Overall rates of incision of vadose canyons can be estimated from speleothem dating. (I seem to remember that a typical rate of incision in the Dales is about 20 mm per thousand years but I can't remember where I read that as I type - it'll almost certainly be in something published by Tony Waltham.) Also this is very much an average value which will change constantly due to umpteen variables.

I'd doubt that figure - I think it is Tony's yes - is typical. There is evidence from Co. Clare of far more rapid downcutting of canyons.

Are you trying to calculate how much bigger a squeeze will get before the end of your caving career?

Not big enough is the answer

 

[Pitlamp]

Graham - figures may well be different for Co.Clare - remember all those variables - the degree of valley lowering by glaciations will vary (which causes rejuvenation of passages and thus influences incision rates considerably). I guess there's a lot of work to be done in this area.


I don't have a copy of that ref to hand but as I remember when I last looked at it the mineral was "bog iron ore" (a hydroxide of iron IIRC). If you talk really nicely to Graham and you don't want it immediately - he may well copy it for you. He has been very helpful to me in a similar way in the past.

Note also a reference to a similar looking manganese mineral in Psilomelane Pot (see Northern Caves Volume 2, 1991, page 72).

 

[khakipuce]

Graham - I would love to see a copy of the BSA article, if/when you have the opportunity.

Pitlamp - I recon you have it with the bog iron, the bit I have looks pretty much like http://en.wikipedia.org/wiki/File:Limonite_bog_iron_cm02.jpg. The problem, of course with asking these questions is that the answers lead to more questions, so the next question is how did it get there? Seems that bog iron forms due the oxidation of iron rich groundwater so my first guess would be secondary mineralization (as it is unlikely there would be much free oxygen on the ocean floor as the limestone formed.

Thanks all.

 

[Cave_Troll]

i would expect that the actual amount of chemical dissolving by water is not that great.has the water going in to Giants actually dissolved enough limestone to radically change its pH by the time it gets to the bottom?

Granite caves are presumed to soley be formed by mechanical means i beleive