Waterways swallet, Staffs

[Tangent_tracker]

Did this excellent cave last weekend, and when looking at these features underground, we pondered....

Is this magma, and was it generated underwater before the limestone formed, or has it found it's way through fractures and cracks sometime later?

 

[huwg]

Excellent question, and good imagination!

I've never had the pleasure of visiting Mendip, but I think it is more likely to be chert. This is often found in limestones, and is silica based, usually forming at much lower temperatures than you would get with magma, as fluids flow through the rock.

When magma gets to the surface, it is generally called lava, but you do also get intrusions where the magma has exploited weaknesses in the rocks. I'm not sure there is much of this in UK caves, but you can see examples around, and a top tip for identifying them is that they are often coarser grained (bigger crystals and grains) in the middle, with finer on the outside.

Great picture by the way!

(I'm sure someone more knowledgeable will come and correct me!)

 

[Tangent_tracker]

Thanks for the prompt reply. This is in fact just south of Derbyshire in Staffordshire. Fantastic place with some of the best geology I have seen in these parts!!

 

[wellyjen]

There is lava above the limestone further north in Castleton. I'm not a geologist, so can't say more. See https://ukcaving.com/board/index.php?threads/cavedale-lava-samples.30838/

 

[Andy Farrant]

Yup, almost certainly chert. Both the Ecton Limestone Formation and the Milldale Limestone Formation contain chert (silica nodules similar to flint). It's not soluble, hence it sticks out from the cave wall.

 

[Tangent_tracker]

Yup, almost certainly chert. Both the Ecton Limestone Formation and the Milldale Limestone Formation contain chert (silica nodules similar to flint). It's not soluble, hence it sticks out from the cave wall.

ok thanks. So has the CHert squeezed its way through after the limestone formed? During?

 

[Brains]

Chert - Wikipedia https://share.google/IIkEg6EDN9IGdMITE
"Most chert nodules have textures suggesting they were formed by diagenetic replacement, where silica was deposited in place of calcium carbonate or clay minerals."
My understanding is that silica from radilorians and glass sponges, and other silica rich detritus, when present in a high lime environment (chalk, limestone) can become mobile in the sediments. As oxidation and reduction fronts progress through the material, silica is mobilised into pools and blobs. These will be in poorly formed layers that are NOT coincident with bedding. Fossils and trace fossils (eg burrows) may form a focus for the deposits. The process is diagenetic, ie is the earliest stages of sediment becoming rock and then metamorphic rocks.
Oh, and magic, lots of magic

 

[Brains]

This is a simple image of how chemical diffusion through a material can generate Liesegang Rings. The silica will diffuse into a layer, while the lime and other sediments will diffuse out. The resultant chert (or flint if it occurs in one specific bed of the chalk!) is a relatively pure deposit and is crypto crystalline SiO2. It is not fine grained quartz. Under an electron microscope it appears as minute spheres of material.

 

[Andy Farrant]

The chert would most likely form quite early after the limestone was deposited, so formed slowly in-situ, replacing the calcium carbonate with silica, which is why the fossils also get replaced by silica. The silica comes from sponges and other organisms. The precipitation is due to redox and pH changes at the boundary between oxygenated sediment and anoxic sediment (reducing conditions) within the carbonate sediment as it was deposited. It often precipitates around organic material such as mucus-lined burrows within the carbonate sediment.