Mikrofauna górnojurajska w otworze wiertniczym Piekary

Abstract

UPPER JURASSIC MICROFAUNA IN BORE-HOLE PIEKARY (NEAR POZNAŃ)On the basis of extend of foraminifers the author asserts that, in the section of bore-hole: Dogger sediments reach as far as to the depth of 368.6 m.; from depth 368.0 to 366.0 m., there are sediments of the Nevisian; from depth 366.0 to 269.0 m., sediments of the Argovian and Rauracian; and, finally, from depth 269.0 to 218.0 m., sediments of the Astartian (Fig. 1). The micropalaeontological boundaries determined between these substages do not correspond exactly with the boundaries established on the basis of the macrofauna.The depth of 368.5 m. has been aS8umed to be the boundary between the Dogger and the Malm, in acccordance with observations made as to both their lithology and macrofauna. It proved impossible to determine; on the basis of their micro-fauna, the sediments of the Divesian which are documented by macrofauna. In samples taken from the depth of 368.0 to 366.0 m., there already appeared a group of foraminifers characteristic for the Nevisian.Beginning with depth 364.9 m., many new species appeared, characteristic for the Argovian. As the limestones grow harder and more compact, a gradual impoverishment of the microfauna may be observed; but when the section revealed mad intercalations, the foraminifers again became more numerous. - It is impossible on the basis of the microfauna found in the, investigated section, to define the boundary between Argovian and Rauracian sediments. As a rule, there occurs an impoverishment in foraminifers in the Rauracian of Central Poland, compared with the Argovian; however, in the vertical section at Piekary this feature has not been observed.The boundary ,between the Rauracian and the Astartian bas been determined micropalaeontologically at the depth of 269.0 m., in the zone of the mass disappearance of species of foraminifers known from Rauracian sediments ,but not observed in the Astartian. Still, this boundary should be looked upon as approximate only, because on the basis of the disclosed macrofauna this boundary should rather be drawn at 252.0 m. depth. However, in view of the lack of adequate micropalaeontogical evidence, it seems inadmissible to shift the boundary to this depth.