Bedding Plane Formation

[TheBitterEnd]

Volcanic Bombs - that's a new one on me; but like I said there are probably more theories than actual chert nodules.

 

[bograt]

It was a theory well bandied about in the '70's, lumps of molten rock thrown out by a volcanoe and cooled rapidly by sudden immersion in water.

 

[Brains]

Sorry if this has already been described, and I missed it, or did not understand, but what are the "chert nodules" that you see in limestone.

Thanks in advance.

The silica is derived from sponge spicules and micro organisms with silica skeletons (radiolarians) which has been remobilised post sedimentaion. The chert itself is crypto crystalline silica. The nodules form layers that are not bedding related and are therefore post sedimentation. The nodules may center on fossils, burrows, or just randomly. The shapes can also form pseudo fossils.
In some volcanic terrains springs have been reported with such an extreme chemical composition that on contact with "fresh" water a colloidal silica gel is precipitated (cf black smokers)

 

[TheBitterEnd]

Hopelessly off topic but interesting non-the-less, what quantity of sponge is required to deposit say a 500g chert nodule?




BTW, What is the air-speed velocity of an unladen swallow?

 

[Brains]

500g of silica sponge spicules...

 

[owd git]

african or European?

 

[TheBitterEnd]

The African swallow's not migratory


... so presumably is unlikely to drop a chert nodule in the sea 

 

[TheBitterEnd]

500g of silica sponge spicules...

I started out with some bedtime reading and ended up looking a sponge taxonomy. Anyway the spicules are presumably only a fraction (10%, 20%?) of the whole sponge.

What about diatoms? They have silica cell walls.

 

[JasonC]

The silica is derived from sponge spicules and micro organisms with silica skeletons (radiolarians) which has been remobilised post sedimentaion. The chert itself is crypto crystalline silica. The nodules form layers that are not bedding related and are therefore post sedimentation. The nodules may center on fossils, burrows, or just randomly. The shapes can also form pseudo fossils...

This was (more or less) what I was once told by a stonemason re small chert inclusions in Portland limestone (potentially a major irritant for the carver/mason).
But I can't see this being the origin of larger masses of chert you see eg in Goyden

 

[gus horsley]

Flint and presumably chert are probably largely formed after the original deposits had turned into rock.  Evidence of this is found in fossil sponges, echinoids, etc in chalk and the flint has used the fossil as a nucleus, outgrown it and, in some cases, almost totally obliterated it.  This could only occur post-deposition.  One theory is that both flint and chert could accumulate by a process similar to ionic replacement where molecules of silica are "attracted" to each other and coalesce to form a sort of silica gel which then solidifies.  This model has also been applied to the concentration of minerals in the weathered zones of lodes (zones of secondary enrichment).  Bedding planes, under suitable conditions, would allow this process to occur and that's presumably why chert and flint occurs generally in layers.

 

[chriscastle46]

This has been a fascinating thread. I (very) occasionally get answered intelligent questions where I work at Cheddar, one of which is how do bedding planes form? I'd asked the various boffins who come to Cheddar from time to time who gave me an answer which equates to Punctuated Aggradation, but it is most interesting to see the answer in detail, although rather difficult to understand if you don't have the background knowledge.

 

[NigelG]

Just to make things even more fun, chert can also form sheets, filling bedding-planes or joints over large areas. There are fine examples of this puzzle in the Upper Jurassic limestone of Portland.

It may not require much change in sedimentation to produce a bedding-plane in otherwise apparently related rock. In the same formation (Portland) the break from the Whit Bed to the overlying Roach is usually a clear bedding-plane, but some lateral facies variation or other means we also see a more diffuse change within a single bed in places. Yet the physical character of these two members differs considerably!

The environment of deposition was relatively benign, tropical to sub-tropical, shelf sea rich in life but gradually shallowing to lagoonal conditions, and no volcanoes around.