Dajki neptuniczne w Rezerwacie Przyrody Nieożywionej Bonarka w Krakowie - świadectwo późnokredowych ruchów tektonicznych na Wyżynie Krakowskiej – dyskusja

Stanisław Dżułyński

Abstrakt


NEPTUNIAN DYKES OF BONARKA - A TESTIMONY OF THE LATE CRETACEOUS TECTONIC MOVEMENTS IN THE CRACOW UPLAND - DISCUSION

Summary
In an earlier issue of the Przegląd Geologiczny (nr 12, 1994), Wieczorek et al. [3] presented arguments in favor of allegedly Santonian age of faults disrupting the abrasion surface in Upper Jurassic limestones at the base of Upper Cretaceous marls in the geological reserve of Bonarka in Kraków. The premise, from which such conclusion has been drawn, was the horizontal arrangement of Cretaceous marls in the upper part of the section exposed. Nowadays, the contact between the Cretaceous marls and Jurassic limestones is covered with rock debris and is not exposed. It was, however, exposed by the end of the World War 11 and the faults dissecting Jurassic limestones were seen to continue in the overlying marls. At a distance of ca. 3 m, above the base of marls (corresponding to vertical displacement of faults in limestones), the disjunctive disturbances were passing into flexures and these, still higher up, into undisturbed and horizontally disposed marls (see [1, 2]). Consequently, the faults observed correspond to the upper Miocene tectonic activity which resulted in the "horst and grab en" morphology of the present foreland of the Carpathians. The "clastic dykes" described by Wieczorek et al. are tectonic fissures, filled with brecciated and partly crushed Jurassic and Cretaceous rocks. Shortly before the faulting, the boundary surface between the Jurassic and Cretaceous rocks was penetrated by silica-bearing solutions, rising by means of joint fissures. Prevented from working their way upward by impermeable marls, these solutions spread horizontally along the Jurassic/Cretaceous boundary, producing remarkable siliceous incrustations on the abrasion surface. These incrustations, which are not synsedimentary but secondary, post-Cretaceous phenomena, are displaced by faults. This is in agreement with post-Cretaceous age of the faults in question.