Mikroskamieniałości z osadów dewonu z otworu wiertniczego Jamno IG 1

Abstract

DEVONIAN MICROFOSSILS IN BORE HOLE JAMNO IG 1

Summary

Microfossils discussed in this article come from the marine sediments of Devonian age, and have been encountered in bore hole Jamno IG-I (North-west Poland). The carbonate-arenaceous deposits of the lowermost members of the Upper Devonian and Middle Devonian rest here on the arenaceous-siltstone deposits of the Old-Red type, referred to the Lower Devonian. The first microfossils have been found at a depth of 1968 m. These are ostracods of the species Bairdia zigulensis P o l. and foraminifers - Nanicella porrecta Bykova and Spathognathodus aculeatus(Br. et Mehl) which are abundant in the Frasnian deposits. Thus we may suppose that at this depth we have to do also with the sediments of this stage. At a depth of 1836-1910.6 a very rich fauna assemblage has been encountered, mainly ostracods and scolecodonts, accompanied by conodonts, charas, foraminifers and fish scales. Based on the stratigraphical range of the ostracods, we may refer the deposits of this interval to the Givetian. This conclusion might also be proved by the scolecodonts, since two among the three species prevailing in the assemblage under consideration belong to those known only from the Givetian. This contradicts, however, the presence of other microfossils,the stratigraphical ranges of which are wider, and comprise the Frasnian deposits too. Thus we have to do here with a microfauna assemblage characteristic of both Upper Givetian and Lower Frasnian deposits. In consequence of this a precise boundary between the deposits of these two stages cannot at present be drawn on the basis of the so far encountered microfauna. Of particular importance is here the presence of the representatives of Characeae. These fossils point to a fact that the sediments were laid down along the shore of the sea basin to which fresh water materials were brought, among them chara representatives, or that this basin was shallow, characterized by a low salt content that allowed the chara individuals to live in this environment. However, since abundant corals occur in these deposits, we may suppose that the chara representatives were brought here from inland waters.