Nalez pravdepodobnych pseudomorfoz ledovych krystalu v paseckych bridlicich (kambrium, pribramsko-jinecka panev)

The formation of ice crystals that leave imprints in the underlying substrate is a well-known phenomenon from current observation and laboratory experiments (Häntzschel 1935, Reineck 1955, Van Loon 1990). According to these studies, ice crystal imprints form at subfreezing temperatures at the sediment-air interface in a water-saturated fine-grained substrate. Observation of ice crystal imprints is particularly relevant to tidally affected sea beaches, riverbanks and rainwater ponds. Both natural and artificial ice crystal imprints show a wide variety of shapes. There is no difference between structures from marine and freshwater environments on a macroscopic scale. As far as is known, the morphology of recent ice crystal imprints depends mainly on the grain size of the host substrate, its water saturation and the freezing temperature or the number of repeated freeze-thaw cycles (Allan 1926, Mark 1932, Häntzschel 1935, Mikuláš 2010). It is evident that imprints of ice crystal imprints (usually their pseudomorphs, to be more precise) can pass into the fossil record with a similar probability as, for example, traces of the activity of organisms. Nevertheless, adequately described findings of pseudomorphs after ice in the geologic record are extremely rare; recently it is mainly the work of Voigt et al. (2021).

During a routine inspection of the Medalův Mlýn site (Fig. 1), known as the best and easily accessible outcrop of the so-called Paseky Shale (Cambrian Series 2; see below), I found structures that I tentatively identified as imprints of ice crystals. The imprints (Fig. 2A-B) were found on a single shale fragment (imprint and counterprint) taken from the disintegrating surface of the outcrop within the layer marked on the profile (Fig. 1). The shale fragment is siltstone, with grains hardly visible to the naked eye, with a small proportion of mica, light brown in color. The thickness of the split shale plate was 12 mm. Twelve isosceles triangular formations are visible on the plate, interpreted as pseudomorphs after ice crystals. The shape of the isosceles triangles is characterized by the apex angle, which ranges from 20 to 28° in the pseudomorphs from the Paseky Shale. The length of the triangles is 3–9 mm, the width is 2.5-3.7 mm. All triangles are arranged in a 38 mm long cluster. Six of them form a star-like cluster.. Four are situated next to one another and facing a similar direction (± 20°). The remaining two appear to be randomly placed and oriented; one is in an area densely covered by the other pseudomorphs, the other is somewhat apart. Conspicuous circular imprints with a diameter of 1.2 or 1.5 mm occur at the tips of one of the clusters and one solitary triangle. About 50 cm higher in the profile, a layer with abundant microbial coatings (microbially-induced sedimentary structures, MISS) occurs (Fig. 4).

Evidence of subfreezing temperatures at the sediment-air interface in the water-saturated fine-grained substrate in the upper part of the Paseky Shale indicates a very shallow-water origin of the deposits, including the occasional emergence of the bottom. Combined with the occurrence of MISS, it is clear that microbial communities were occasionally exposed to a terrestrial environment, which may have affected their ability to survive in a wet, but only exceptionally flooded, environment for a long time. It is remarkable as well that the arthropod community of the Paseky Shale could have been exposed to frost (including possible freezing of the water surface). In the future, all this knowledge acquired individually can represent a valuable basis for considerations about the development of the settlement of brackish and fresh waters and about the evolutionary adaptations of the groups of biota present at that time.

References

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