Nová interpretace fosilního záznamu v metamorfovaných horninách u Koberov (krkonošsko-jizerské krystalinikum, geopark UNESCO Český ráj, Česká republika)

Abstract

The belt of dark carbonaceous rocks (shale and metamorphic limestone) near Koberovy (Krkonoše-Jizerské Mts. Crystalline Complex) is historically considered a record of the Silurian marine environment, due to the findings of poorly preserved but abundant fossils. Prantl (1948) considered them to be graptolites and, based on existing knowledge, classified the rocks as Silurian. Chlupáč (e.g. 1989) was of a similar opinion. However, later, the same author (Chlupáč 1993) re-interpreted all the remnants of the fauna found as traces of the activity of organisms, thus extending a possible stratigraphic interpretation to virtually entire Phanerozoic. Two different types of Koberovy fossils (two samples) were donated to me by prof. Chlupáč shortly before his death. These are two plates of graphitic calcareous shale, the metamorphism of which is revealed on foliation planes (parallel to bedding) for example by the presence of metamorphic minerals such as white mica (muscovite) and chlorite. The first of these has almost straight to slightly curved “strings” on the surface – probably the original bedding plane – in the left part (oriented according to Fig. 1A), which are dark under normal light; however, surface minerals have a pronounced luster, therefore under a particular illumination, the “strings” appear as light lines. The width of the lines is almost constant, about 0.6 mm. Another group of lines occurs in the centre of the sample. These show a “loop” pattern which can be compared to the double “e” in cursive writing. The width of these lines is around 0.6 mm again, with random fluctuations to lower values. A similar and final biogenic structure is in the top right of the Figure 1A. These are three semi-arcs, the estimated half of which is missing on the sample, probably representing an original 3-D spiral-like structure. Width of this structure is on average slightly larger than the previous two structures: it reaches about 0.8–1.0 mm. The second finding is preserved similarly: on the photograph (Fig. 1B), reflected light is used to highlight the dark fossil on the gray carbonaceous slate background. This sample contains four crescent-shaped segments, two in the “D” position and the remaining two in the “C” position. The next segment has an irregular oval shape and the last is a dashed line. The size of the structures is about 30 mm. Chlupáč (1993) withdrew from the original opinion that these poorly-preserved fossils are graptolites (Prantl 1948, Chlupáč 1953). Instead, he offered an interpretation that these were trace fossils. However, we cannot agree with this view for following reasons: 1. Ichnofossils, which have a similar contour as the spiral structures of the first finding, belong in particular to the ichnogenera Gordia Emmons, 1844 and Spirodesmos Andree, 1920. These fossils are surface grazing traces preserved in hyporelief or epirelief. Such ichnofossils have a small conservation potential during regional metamorphism, with movements and chemical transformations on the foliation planes. No rich relief-ichnofauna has been found in regional metamorphic rocks worldwide. 2. On the other hand, ichnofossils, which have the character of full reliefs, can be preserved even in high-grade metamorphic rocks (staurolite isograd in Jeseníky Mts., Chlupáč 1987a). However, they do not appear on the foliation planes, but are preserved within the rigid beds (quartzite) and are usually best observed in vertical fractures of the rock. The preservation of Gordia ichnofossil in a sequence of rhythmically alternating layers of black calcareous shales and graphitic crystalline limestones (sometimes dolomitic) is hard to imagine. For instance, even in completely unmetamorphosed rocks, surface ichnofossils and trace fossils of shallow tiers do not tend to persist; the limestone environment is specific and largely restrictive for the preservation of ichnofossils; see Savrda et al. (2012).