Agate and fossil from watery habitat (3)   ---   Enigmatic trough
deutsche Version
agate and cyanobacteriaIt is not the conspicuous agate which is most remarkable here, it is the "refuse dump" below. It is a depression in a stack of formerly horizontal layers. Apparently they consisted of water with a loose array of microbial filaments, possibly of the cyanobacterium Croftalania
[1], very dimly seen here traversing the layers but distinctly seen elsewhere in this sample. The squeezed layers below the trough indicate that they had not yet been silicified then.
The aspect of the depression in the layer stack suggests the idea that it had been caused by the heavy debris now seen at the bottom of the trough. The irregular particles do not indicate which kind of heavy matter it might be.

Image: Layers of quite differ
ent origin in Rhynie chert:
agate not affected by the large deformation of the microbial layers below. Width 4.3mm.

Enigmatic is not only the depressed layer stack but also the disruption of the upper three layers: They seem to be simply torn off at either side of the trough, and it is not seen where the parts have gone.
Pale bluish silica gel shaped like a cumulus cloud
or a cauliflower had formed in the water above the debris. The irregular pits at the top of the cloud filled themselves with spherulites, 25-40Ám, with dark liquid in between.
In a subsequent stage of deposition, yellowish debris of unknown origin floating in the water settled into a deposit with rugged horizontal surface and became silicified.
The dark horizontal layer above indicates the transient presence of silica clusters smaller and larger than a critical size for settling: The larger ones settled down but the smaller ones were kept in suspension so that they moved about in the water and eventually got stuck to the cavity wall. Hence, the 
dark horizontal layer continues right and left as a thin dark layer along the cavity wall. Subsequent slow diffusion of silica from outside into the water-filled cavity produced small clusters only so that the usual process of agate formation finally filled the cavity with chalcedony.
As it is well known, agate formation is very sensitive to tiny variations of parameters, which makes every agate a unique specimen.   
Sample: Rh2/19.2 (2001)

[1] M. Krings, H. Kerp, H. Hass, T.N. Taylor, N. Dotzler:
      A filamentous cyanobacterium showing structured colonial growth from the Early Devonian Rhynie chert.
      Rev. Palaeobot. Palyn. 146(2007), 265-276.

H.-J. Weiss     2019
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