How do calcite layers form?

[ACDigger]

A dig that SUSS has been working on has encountered numerous layers of calcite (see attached photo).

The passage appears to be a phreatic tube that is almost completely infilled with mud, with only a very small trickle of water currently passing through. The dig initially consisted mainly of mud and a lot of large galena blocks, but this has gradually changed into a sequence of calcite layers, some of which are over 3 inches thick.

Does anyone know how this sort of calcite layering forms? And as a bonus question, what might cause the calcite deposition to stop abruptly and the passage to revert to being mud-filled rather than calcite-dominated?

 

[The Old Ruminator]

Caves usually respond to climate and undergo considerable changes. The sediment usually means an active or phreatic passage whilst the calcite forms when the passage no longer contains water or carries a stream . Drip and deposition will also vary with climatic condition. On Mendip caves tended to be dryer during periglacial cycles when the ground was too frozen for water to enter the cave. During warmer interstadials or intergacials the cave becomes active again. On Mendip we see many crogenic structures where the original speleothms have been damaged by ice. The Anglian Glacial epoch was a particulary harsh glaciation but the Hoxnian interglacial was a little bit warmer than today. Phreatic caves usually have deep deposits of sediment left over from when the passages were under the water table. A bit simplistic but it might help.

 

[Andy Farrant]

There will be an inlet nearby bringing in water saturated with calcium carbonate which allowed a flowstone floor to develop. A change in environment/climate caused the calcite to stop precipitating and allowed sediment to accumulate. This might have been due to climatic fluctuations, but a change in the hydrology may also be the cause. Another layer developed when conditions became more favorable for flowstone formation. Repeat. You can see something similar in GB cave by the Bridge, where we have cold climate gravels overlain by a stalagmite layer formed 52,000 years ago during a warmer interlude during the last Devensian glaciation, followed by more cold phase gravels washed in during the peak of the Devensian glaciation (c. 23,000 years ago) when speleothem formation ceased. More stalagmites cap the sequence dated to the current Holocene warm period (the last 11,000 years). The fill should revert to just mud when you get past the flowstone generating inlet. However some flowstone floors continue for some distance; the flowstone floor of the Snowy River passage in Fort Stanton Cave is over 17 km long! It would be interesting to date the calcite to see how old it is.

 

[ACDigger]

There will be an inlet nearby bringing in water saturated with calcium carbonate which allowed a flowstone floor to develop. A change in environment/climate caused the calcite to stop precipitating and allowed sediment to accumulate. This might have been due to climatic fluctuations, but a change in the hydrology may also be the cause. Another layer developed when conditions became more favorable for flowstone formation. Repeat. You can see something similar in GB cave by the Bridge, where we have cold climate gravels overlain by a stalagmite layer formed 52,000 years ago during a warmer interlude during the last Devensian glaciation, followed by more cold phase gravels washed in during the peak of the Devensian glaciation (c. 23,000 years ago) when speleothem formation ceased. More stalagmites cap the sequence dated to the current Holocene warm period (the last 11,000 years). The fill should revert to just mud when you get past the flowstone generating inlet. However some flowstone floors continue for some distance; the flowstone floor of the Snowy River passage in Fort Stanton Cave is over 17 km long! It would be interesting to date the calcite to see how old it is.

There is a small inlet in the roof, around where the calcite layers became more prominent. However this is downstream from the rest of the dig. I know that water in caves can flow upwards, does that mean that calcite be formed upwards as well? Or should we be expecting to come across a larger inlet somewhere further along? (Hopefully not in 17km!)

 

[The Old Ruminator]

As far as phreatic water goes that indeed can flow uphill. Any scalloping in yot passage ?

 

[ACDigger]

There is a very minimal amount of scalloping, noticed some at the start of the dig (where it was open space). However not at the face (although this may be as the only solid wall we can see now is the roof), so unsure as to whether the walls are scalloped.

 

[T pot 2]

These localised calcite deposit could be aragonite which does not necessarily require water to form.