Red Permian cherts with freshwater
stromatolites
Silicification of watery habitats often produces cherts with
details due to microbial activity. The red Permian cherts from the
Doehlen Basin (Saxony, Germany) are rich in such detail, which comes
with quite different aspects, such as conspicuous stacks of thin sheets
or a confusing multitude of irregular-shaped lumps and spongy
structures.
Obviously the thin sheets in Fig.1 had been
fused into a solid isotropic block so that the cracks did not feel the
texture but propagated along curved paths for reasons unknown.
Generally, the
results of consecutive deposition of sheets under the influence of
microbes, mostly cyanobacteria, and subsequent mineralisation are known as
stromatolites from saline environments. Less well known are recent stromatolites in fresh water [1]. The
layered structures in the red
Permian cherts seem to be freshwater stromatolites.
Fig.1: Thoroughly solidified microbial sheet stack with
cracks not related to the texture.
Image width 5.5mm.
Fig.2: Stack of microbial sheets with
enigmatic connections to irregular-shaped lumps
nearby.
Image width 9.2mm.
Fossil structures
like that one in Fig.1 resemble petrified wood. Close inspection can
provide arguments favouring an interpretation as stromatolites: 
The apparent absence of tissue, also the connection of the
sheets to irregular-shaped microbial lumps
(as in Fig.2) would not be compatible with an interpretation as wood
fragments.
Fig.3: Non-typical stack of microbial sheets: deformed before
silicification.
Image width 7mm.
An uncommon formation which does not fit
into the stack of sheets in Fig.3 is the row of small red "flames".
As
a highly
peculiar and apparently incomprehensible fact, the Permian freshwater stromatolites serve
as favourite substrates for the growth of enigmatic "eerie shapes", see
Permian
Chert News
18, 27, 28, 36, 37, 38, 41.
Samples: Figs.1,2: W/8.1,2 found at Wilmsdorf about 1995; Fig.3:
H/333.1 found at Hänichen in 2001.
H.-J. Weiss 2022
[1]
P. Freytet, E. Verrecchia: Freshwater organisms that build stromatolites:
A synopsis ... Sedimentology (1998), 45, 535-563.
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