Nove nalezy tzv. fosilnich seizmogramu z kosorskych vapencu spodniho devonu Barrandienu (stupen lochkov)

The so-called fossil seizmograms represent interesting microstructural phenomena which are, in the Barrandian area, bound to fault polishes and slickensides occurring along the bedding planes of Devonian limestones. Slickensides were created by mutual slip of limestone and shale layers during fold deformation. “Seizmograms” were originally described from the Lower Devonian of this area by E. Suess (1895), later also by other authors, especially in the late nineteenth and early twentieth centuries.
As “fossil seizmograms” were called fine, in shape and direction consistently oriented but curved (hook-shaped) striations incised on smooth surfaces of fault polishes (Figs 1, 2). Their findsites are mainly bound to Kosoř Limestone facies of the Lochkov Formation (Lochkovian stage). In combination with tectonics, the lithological character of Kosoř Limestone (e.g. only a thin layer of shales among the beds of bioclastic limestones) is apparently the most convenient factor for their origin (Figs 3, 4). The type locality is the historically mentioned valley “Kosořské údolí near Radotín” which is nowadays called Černá rokle near Kosoř, in the SW part of Prague. This paper describes a new locality - quarry called Cikánka near Praha-Slivenec (Fig. 5) where “fossil seizmograms” were newly found (Vorel 2001). In total, the previous papers mention and describe only about ten different samples of these interesting microstructural phenomena; this paper adds another two samples (Figs 6, 7).
Shape of “seizmograms” is highly variable, but it seems that sharp shapes are not so common and most of the striations have round or tortuous shape.
Size of the previously mentioned “seizmograms” ranged from a few mm to several cm. Purkyně (1917) describes circumscribed paths of engraving particles approximately 4 cm long. Size of striations of the newly described samples is about 2 cm.
Engraving particles could be either primary, the more solid rock fragments, or secondary formed small crystals and concretions of common diagenetic origin. Also the mineral micro-objects, formed during younger processes (e.g. by precipitation in joints or along discontinuities) can play a significant role in the process. According to the author’s last lithological study of these rocks (Vorel, 2001), the following authigenic minerals can be taken into consideration: quartz, pyrite, possibly dolomite.
Origin of “seizmograms” is the most disputable question. From observations it is evident that “seizmograms” could have been formed already during folding, e.g. by mutual slipping of layers, however the origin of this phenomenon may be far younger, associated either with subsequent extension phase or even with neotectonics.
On the newly found locality at quarry Cikánka near Praha Slivenec, the occurrences of "“seizmograms” are bound to slightly dipping flank of anticline (approx. 20° to the SE), whose axis lies several hundred meters NW. On the old locality Černá rokle near Kosoř, there is a similar situation, only in this case it is a slightly dipping syncline. In these parts of fold flanks, which were located relatively close to the fold axis (several hundreds of meters), there were probably the most favorable conditions for the origin of slickensides and these interesting striations, historically described as “fossil seizmograms”.

References

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