Przetortońska rzeźba Przedgórza Karpat między Krakowem a Dębicą oraz jej wpływ na sedymentację miocenu

Abstract

Pre-Tortonian relief of the Carpathian Foreland between Kraków and Dębica and its effect on Miocene sedimentation In the Carpathian Foreland between Kraków and Dębica (Fig. 1), the Miocene roclks of the Carpathian foredeep discordantly overlie Mesozoic sediments belonging to the SE extension of the Miechów synclinorium. The Senonian marls and marly limestones with cherts, as well as the Jurassic limestones, overlain by the Miocene sediments are for the most part intensely weathered. In drill cores several fissures have been noted; they are predominantly open, but are frequently filled with clay or with calcite. A characteristic feature is a coating of iron oxide which covers mainly the walls of fissures, sometimes impregnating the entire sediment. The coating has been traced from a depth of 2,5 m down to 100 m below the Miocene rocks. These above features of the uppermost Mesozoic sediments have been attributed to the Early Miocene (pre-Early Badenian) weathering. The surface of the Upper Cretaceous and Jurassic sediments was then in the zone of influence of air and atmospheric water, which effected oxidation of iron sulphides in the rock. Those iron compounds dissolved in underground waters were rapidly precipitated as hydroxides owing to low mobility of iron ions. This process initially occurred directly above the ground water level. Since oxidation takes place above the unconfined ground water level, the thickness of the zone of aeration has been assumed to be equal to the depth of the water table. The zone of aeration may be also recognized on the basis of interpretation of the data, yielded by borehole geophysics (Fig. 2). Table I illustrates the characteristics of geophysical well logs in this zone. The zone of aeration produces a negative PS anomaly, two or three times greater than that in the underlying rocks, when resistivity has been reduced by half and the size of borehole considerably enlarged. The gamma radiation intensity has a constant value with a possible slight tendency to increase. The intensity of secondary gamma radiation shows a slightly decreasing tendency in this zone. Geological and geophysical data served as a basis for an isopachous map of the aeration zone (Fig. 1). It is, at same time, a map of depth down to the unconfined underground water level averaged 20—40 m, then, varying from 0 to 100 m. According to Niedzielski (1970), the present-day shape of the first level of ground water on the Miechów Upland reflects the actual relief. Consequently, the depth distribution of the pre-Tortonian ground water level may represent to a certain extent relief at that time. Since the variation in thickness of the fossil aeration zone is very close to the present-day gradients of water table (Niedzielski 1970), their equality was assumed for the construction of palaeomorphologioal sections. On these assumptions relative hypsometric differences on sections (Figs 12—15) were calculated from the equation cited on p. 535. The reconstructions performed imply that the part of the Carpathian under discussion Foreland had been characterized before the Tortonian by slight hypsometric differences that did not exceed, as a rule 200 m and by the inclination of the surface ranging from 0 to 5%. An attempt has been made to find a correlation between morphology thickness variation of the Lower Tortonian sediments. Areas with similar geological structure show a correlation between the relief derivative, i. e. the thickness of the aeration zone, and the thickness of supra-anhydrite Miocene rocks (Figs. 5—10). The best correlation was obtained for the axial part of the Miechów synclinorium (r = —0,69) and for the peripheral (N of the Vistula) part of the Carpathian foredeep r = 1—0,53). The relationship between palaeo-relief and the thickness of the Lower Tortonian sediments can also be seen on sections (Figs. 12—15) and maps (Figs. 1—3). It may be inferred from the foregoing considerations that the Early Tortonian sedimentation was controlled by the existing relief. Similar observations have been already reported from the Miechów Upland by Radwański (1968). This theory seems now to hold for the greater part of the Carpathian Foreland.