Poznámky k faciálnímu vývoji letenského souvrství (svrchní ordovik, sandbian) v severní části pražské pánve

Abstract

About 100–150 m thick intercalation of dark micaceous siltstone to black clayey shale can be distinguished in the middle part of the Letná Formation (Upper Ordovician, Lower Sandbian) at several localities in the NW and NE parts of the Prague Basin. Sandstone and greywacke, dominant elsewhere in this formation, form only a subordinate part of the sequence. In the NW belt of the Letná Formation, N of the Prague Fault (see Fig. 1C), this thick intercalation has been previously known from Bulovka – Bílá Skála Rock and from Rudolfova Štola Gallery (Röhlich 1956, 1960; Mikuláš 1994). A comparable, slightly thinner, and locally discontinuous intercalation of dark siltstone and black clayey shale was described also in the middle belt of the Letná Formation, S. of the Prague Fault (see Fig. 1C) by Röhlich (1956, 1960); such lithology is known from the Praha – Žižkov, Kyje and Malešice areas. An equivalent of this interlayer was documented by Budil (1996) in the middle part of the Letná Formation from the Černý Most area. No such interlayer of dark, fine-grained sediments has been reported from the southern belt of the Letná Formation (Fig. 1C), most probably because of its much poorer exploration. New borehole documentation at Praha – Střešovice extends our knowledge on the above – discussed dark siltstone and claystone intercalation in the NW belt of the Letná Formation. In two boreholes (HJ 11 and HJ 16), more than 60 m thick sequence of dark, fine-grained sediments has been documented in the middle part of the Letná Formation. The presence of these sediments was confirmed also by the study of archive data – previous geotechnical documentation of this area. The archive data documented the presence of a second, thinner and rather lens- like body of dark, fine-grained sediments which can be traced in the lower third of the Letná Formation in the Střešovice area. It is suggested that the sedimentation of relatively thick sequences of dark siltstones and shales reflects short-time periods of decrease in the input of clastic material. Alternatively, it could correspond to a partial, synsedimentary tectonic-related deepening of respective parts of the sedimentary basin. Locally bioturbated black siltstone and shale sequence is barren of benthos and contains only rare remains of indeterminable linguliformean brachiopods. It is supposed that such association corresponds to the Paterula Com- munity of Havlíček (1982), so far only poorly documented from the Letná Formation (Van Roy et al. 2021)