Geochemical Study of the Ordovician/Silurian Boundary Section in Zdanow (Bardzkie Mts., Poland)

Abstract

The Zdanow section represents a part of the basinal sequence of the Bardzkie Mts. (Sudetes, SW Poland). The sequence, which includes the uppermost Ordovician to Lower Carboniferous sediments, belongs to an accretionary prism which is believed to be deposited to the north from Gondwana-derived Armorica microcontinent, located in the high southern lattitude during early Silurian. The O/S boundary interval is characterized by a transition from the flysh dominated Jodlownik Beds (upper Ashgill) into hemipelagic and pelagic Lower Graptolitic Shales (lower Llandovery) (Fig. 1). The Jodlownik Beds are represented by sandstones and siltstones laid down under oxygenated bottom conditions. Towards their top, intercalations of single layers of laminated hemipelagic dark shales appear and finally the deposition turns to black shales. The 30-cm thick black shales change abruptly to radiolarian bedded black cherts and siliceous black shales, deposited probably on a distal plain in fully pelagic setting, under anoxic bottom conditions. Within the studied interval, two layers of altered tufts, mixed with clastic material, occur. The upper part ofAkidograptus acuminatus Zone corresponds to the lowest part of radiolarian chert sequence, suggesting that the Ordovician/Silurian chronostratigraphic boundary may be located in underlying black shales (Fig. 1). The upper part of the Zdanow section, the S/D transition, was described earlier (Porebska and Sawlowicz, 1997).