I'm working on a project out here (Vercors), installing anchors in a fault controlled cave. Every now and again I seem to hit a patch of very hard limestone, as if it's been case hardend. It appears to be around 3 or 4 centimetres deep before returning to what I would call normal hardness - if that makes sense. Does anyone know if this effect occurs during the faulting process and has any one else noticed it?

Cheers,

Glenn

via the Speleo Vercors wifi LAN at La Jarjatte-en-Vercors

We experience a similar problem when drilling shot holes at the end of Rolling Thunder in Swildons.
In some places the rock is so hard it is almost impossible to drill into it.
The rock is in bands or layers and the hard stuff is more prevalent in one wall of the passage, thus forcing us to always work the other (left hand) wall.

If it's anything like the Peak, it's almost certainly chert & it can be bloody hard. We've recently hit a really bad spot in our dig & the only solution so far is extra drill bits (reground after each trip) & extra battery packs.
I hit a band of it recently as I collared the hole & it drew a shower of sparks & blunted a new drill bit - but that's extream.
In west Peak lime it is either finely distributed through the rock - the more chert the harder the stone, or in nodules or bands as yours seems to be. Just hope you don't find a big bit!!

Hi Glenn
Faults certainly do affect the surrounding strata. Theres three main reasons that I can think of.

One is that fault movement can cause heat which mobilises fluids and any minerals that its carrying within the surrounding rocks. This may have the effect of flushing a zone of some of its minerals or concentrating them up.

Another effect is that a fault is a line of weakness which will allow fluid from further away to pass along it. If this fluid contains minerals, they may be deposited into the pore spaces in the rocks either side of the fault or crystallise within the fault space its self.

The last theory that springs to mind is that the heat involved in the movement in the fault, may have been sufficient to metamorphose the limestone. That means that the limestone would be recrystallised to some degree. The ultimate outcome of the metamorphism of limestone is of course marble, given enough heat and pressure.

Hope that lot makes sense. There may well be more explanations that people can think of as well

Can I ask where your cave is? I was having a dig through our papers here at work and there is loads of information about mineralisation of thrust zones in that region.  If theres something relevant, Ill email it if you like.

Is it possible that the fault movement weakened some layers, i.e. the hard limestone is original, and the softer limestone is in the beds that altered? I suspect not, but worth considering maybe. I think Rhian's metamorphic explanation is best.

There are dozens of possibilities regarding faults and limestone hardness around them.  There usually wouldn't be enough heat/pressure in a normal fault to create marble but you can get significant alterations of wallrock depending on the nature of the fracturing, presence of hydrothermal or hypothermal fluids and porosity of the limestone.  Dolomite is a common example associated with mineralising fluids passing through highly faulted limestone; a good example is at the Great Orme Mines where the Carb Lst has been entirely replaced by dolomite in places.  Also a significant hardening of limestone is likely if mineralising fluids containing quartz passes through porous wallrock.  The indication here is that quartz molecules can be shunted through micro pores for a significant distance away from a fault.  Another possibility is that the limestone itself is siliceous (which often leads to concentrations of chert) and that the action of the fault creates friction which allows silica already present in the rock to recrystallize; this is commonly seen as parallel striations on the walls of the fault known as slickensides.

gus horsley said:

Another possibility is that the limestone itself is siliceous (which often leads to concentrations of chert) and that the action of the fault creates friction which allows silica already present in the rock to recrystallize; this is commonly seen as parallel striations on the walls of the fault known as slickensides.

Click to expand...

In Bagshaw last week and saw some good examples of slickensides. Shame i couldn't photograph them as well as they look...

Many thanks to all who have replied. I don't think it's chert, I'm hopefull I would have recognized that. The replies from Rhian and Gus fit more with my gut feel (for what that's worth).

Rhian, I'm reluctant to give a precise location at the moment (for hopefully obvious reasons). I don't suppose on the high plateau some where between St. Agnan and the Grand Veymont would be good enough? I can pm you a lat long reference later this week (I am just about to leave for my return to the UK)

Hopefully this problem is now passed as I have moved away from the fault and future trips should be more straightforward, assuming I don't drop anymore batteries down 80m shafts...

Cheers,

Glenn

via the Speleo Vercors wifi LAN at La Jarjatte-en-Vercors