Biogenic iron oxide precipitates in red cherts from Döhlen basin, Saxony
deutsche Version

Large iron ore deposits known as banded iron formations were formed about 2·109 years ago as a result of photosynthetic oxygen production by cyanobacteria. This has been elaborated in [1], where it is stated that "banded iron formations are laminated units containing iron oxides, sometimes in very fine laminae, separated by silica-rich layers". This can be vividly illustrated by much younger terrestrial formations as preserved in Lower Permian cherts (Fig.1).
Iron oxide deposition by cyanobacteria
Fig.1: Iron oxides precipitated probably by cyanobacteria grown in very fine laminae before all turned into chert by silicification of the watery habitat. Width of the picture 17.5mm.

Several fossiliferous chert varieties have been found as pebbles and boulders in (glacial ?) fluviatile deposits but never in-situ in the small Döhlen basin, Upper Saxony, Germany. Nevertheless they are thought to be of Lower Permian age like the basin itself because a few of them have retained more or less sharp edges, indicating short transport distances, and one variety resembling the one described here was found as a red chert layer of about 10cm thickness in the coal mine "Marienschacht" [2] and as sharp-edged fragments on a nearby field. (Several other types of chert, probably formed by silicifation in the depth and virtually without fossils, were found in mines, as mentioned in [3,4].)
The finely laminated iron oxide precipitates found in some of the red and yellow cherts from Döhlen basin look so similar to those from the Proterozoic banded iron formation, which are thought to be brought about by the physiological activity of cyanobacteria, that one can well assume that they, too, had originated in that way, although the involvement of cyanobacteria in the cherts considered here has never been proven directly.
The present sample is a fragment of a chert layer of at least 23cm thickness. Judging from the presence of foliage fragments and air-filled roots of the tree fern Scolecopteris, the colourful structures must have formed in a shallow swamp near the surface, since the cyanobacteria need water to live in and light for photosynthesis. This is compatible with the observation that the laminated precipitates are always disturbed as a result of some commotion in the swamp, possibly caused by strong winds, animals, falling fern fronds ...
The disturbances occurred during different stages of silicification, as can be concluded from Fig.1: Obviously the stack of layers had been torn into pieces while all was still fluid, and the individual limpid sheets separated themselves at the end of the piece seen here. Later, when the deformed stack fragment had obtained a jelly consistence, a crack ran across but no further. This indicates that silicification combined with shrinkage was faster in the layer stack than in its vicinity. 
Iron oxide deposition by cyanobacteria
Fig.2: Chert with iron oxides precipitated on microbial formations grown among plant debris, same sample as in Fig.1, fungus chlamydospores faintly seen as transparent hollow spheres.
Width of the picture 5.5mm.

Microbes in water come more often as a multitude of floccules than as a stack of thin sheets. It is not known whether or not the sheets and floccules in Fig.2 (below left and above right) represent the same microbe species. Occasionally seen smooth transitions between the two forms seem to indicate that they do.
There are more features worth mentioning in Fig.2. A former cavity, probably a swamp gas bubble, is now filled with coarse quartz. A similar cavity with a level fill (not shown here) reveals the orientation of the sample during silicification so that it can be stated that the orientation of Figs.1,2 is the original one.
Small bubble-like objects, 0.17 ... 0.27mm across, faintly seen randomly arranged along the inclined strip of decayed plant matter in Fig.2, are most probably "chlamydospores" of some fungus. They and the related hyphae are rare in the cherts of the Döhlen basin but abundant in the Rhynie chert. Beside the cyanobacteria or blue-green algae, their presence serves as additional evidence that this type of chert, like lots of others, was not formed by silicification of some sediment layer but by swamp water turning into silica gel which finally became chert. This has to be emphasized in order to contradict views adopted from outdated textbooks according to which cherts are generally formed by silicification of strata in the depth [5].

Sample: Found in 2001 among the excavated matter of a construction pit at Hänichen, Käferberg-Strasse 3. Kept in the own collection under the label H/333.1 .

H.-J. Weiss      2012

[1]  T.N. Taylor et al.: Paleobotany. Elsevier 2009
[2]  J.T. Sterzel: Die Flora des Rothliegenden im Plauenschen Grunde bei Dresden.
    Abh. math.-phys. Cl. Kgl. Sächs. Ges. Wiss. 19(1893), 1-172.
[3]  K. Thalheim, W. Reichel, T. Witzke: Die Minerale des Döhlener Beckens.
    Schr. Staatl. Mus. Mineral. Geol. Dresden 3(1991), 1-131.
[4]  W. Reichel, J.-M. Lange: Cherts from the Döhlen Basin near Dresden.
    Geologica Saxonica 52/53(2007), 117-128.
[5] R. Rößler, T. Zierold, F. Spindler, F. Rudolph : Strandsteine ...
     Veröff. Mus. Naturkunde Chemnitz 30(2007), 5-24.
Scolecopteris pinnule cross-section, Sardinia Permian Chert News8

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