Spostrzeżenia nad utworami litoralnymi jury brunatnej na południe od Krzeszowic

Abstract

Littoral deposits of the Middle Jurassic South of Krzeszowice In the area situated to the south of Krzeszowice, the middle Jurassic lies on shales and sandstones of the Carboniferous, on porphyries and arkosic sandstones of the Permian, and on freshwater sediments of the Lower Jurassic. The bottom part of the middle Jurassic is formed by sands, sandstones and conglomerates of the Bathonian, corresponding to the zones of Opp. fusca and Opp. aspidoides. On these lies an oolitic and sandy limestone of the zone of M. macrocephalus and, partly, R. anceps. At the top the limestone comes to an end along a sharply defined limiting surface, overlaid by yellow oolitic marls of the uppermost Callovian. These are overlaid by glauconitic Oxford marls, and higher up by white ones. The thickness of the Bathonian is variable. On soft Carboniferous shales and on clays of the lower Jurassic it attains 12 metres. On the Permian porphyries of Sanka there are no sediments of the Bathonian, or else their thickness is insignificant. Immediately on top of them lie limestones of the Callovian, with rounded boulders of porphyry. In places at which the surface of the porphyries rises highest, it is immediately overlaid by marls of the uppermost Callovian. Therefore, the Jurassic beds lie in a transgressive overlap on the older rocks of the substratum. The occurrence of the littoral facies in the Callovian limestones, and their direct contact with the porphyries, compels a change of the hitherto prevailing views as to the Jurassic transgression in the Cracow district, according to which the complete submersion of this area took place in the Bathonian. Indeed, in this period the Jurassic sea did reach to the above-mentioned area, but it submerged only the lower-lying parts. Complete submersion did not take place until the Callovian, in the zone of M. macrocephalus. In the sands of the Bathonian beach deposits are preserved, exposed in the vicinity of Zalas and Tenczynek. These sands are coarsegrained and interstratified with pebbles. The latter are here mostly reworked, originating from older gravels, and therefore their rounding is not exclusively ascribable to the action of eddies. The sands are cross-bedded. It is possible to distinguish certain characteristic and recurrent types of bedding (Fig. 1). The size of these structures is small; the foreset laminae dip in all directions and there are no favoured directions. The angles of the foreset slopes attain and even exceed 30°. Most of these slopes are contained within the limits of 10 to 30°. Furthermore, the Bathonian sands in the vicinity of Krzeszowice are poorly sorted and they are interstratified with horizontally laminated and discontinuous interbedded sands. The mode of lamination and the structure of the Bathonian sands completely conform to the development of sands of Lower Foreshore Beaches, described by W. O. Thompson1. At least part of these sands could have been deposited in such a sedimentary environment. The beach sands attain in some places a thickness amounting to 12 metres. The sublittoral sediments of contemporary seas are probably analogically formed as are the sediments of the Lower Foreshore. Unfortunately, very little concerning them is known, and consequently no comparative material is hitherto available. It is possible, however, that a part of the Bathonian sands was deposited also in the sublittoral zone of the Jurassic sea. In the Callovian limestones, which overlie the porphyries of Sanka, there are also littoral formations in the form of remnants of a cliff. In one of the exposures to the north of Sanka the surface of the porphyry rises in a short distance as two steps to a height of 2 metres (Fig. 2). At the base of the steps there is detritus composed of rounded boulders of porphyry, among which preserved in an excellent state are numerous Pleurotomaria. Some specimens are attached to the porphyry, and they are in the same position today as the one occupied during life. Inasmuch as the porphyry boulders are imbedded in a cementing material of the Callovian limestone up to the very top of the bed, it follows that in the zone of M. macrocephalus and partly in that of R. anceps the sea still continued to transgress in the Cracow area. In conjunction therewith, deposited in these zones were pebbles and also conglomerates. When the sandy Callovian limestones had been deposited, an interruption took place in the sedimentation in the Cracow area, probably associated with a partial submarine erosion of the sediments. The sandy limestones are overlaid by fine-grained marls of the uppermost Callovian. In some places they also lie directly on porphyries. In connection with work which is being carried out on behalf of the Geological Survey of Poland, new, hitherto unknownex posures of the middle Jurassic have been discovered to the south of Rudawa. These formations are developed here in a manner typical of the whole area (Fig. 3). The bottom part is composed of fine-grained white sands with a bed of sandstone; they have no fauna. These sediments overlie clays of the Lower Jurassic, and on the basis of analogy to other known exposures they may be included in the Bathonian. The sands are 11 metres deep. On the sands lies a 20 cm. bed of conglomerates, cemented with limestone; this bed is overlaid by a stratum, 1,5 metres thick, of yellow oolitic limestone with fauna from the zone of M. macrocephalus. Above the limestone lie yellow marls; higher up there are glauconitic and white marls of the lower and upper Oxfordian.