Dolny torton w Benczynie koło Wadowic

Abstract

The Lower Tortonian at Benczyn near Wadowice In 1947 during the mapping of the Wadowice sheet for the State Geological Survey a rich Miocene fauna was found by the second writer. In the present paper the geological position of the fauna is described by the second writer, while the first author has determined the fossils. The fauna occurs in a stream - bed at the village Benczyn, situated half - way between Skawina and Zator, south - west of Cracow, at the border of the Carpathians (fig. 1) It occurs in clays in which stratification is only slightly marked. The clays of green - grey colour, calcareous and plastic, dip towards SE (up to 40°), plunging under the Ciężkowice sandstones and Menilitic shales (Eocene). These beds form a small slice overthrust on the Miocene clays and dipping southwards, under the frontal part of the Silesian nappe; this is composed in this sector of two units (fig. 2). The bottom of the Miocene clays is not exposed, but. from a near bore - hole one may infer that in this area the- Miocene reposes directly on the Upper Jurassic limestones. At the bore-hole the boundary between the Miocene and Ju rassic lies at a depth of 89 m. On the western slope of a hill, marked on the map as p. 301 and undercut by the stream, loams and slumps mask the subsurface deposits but at places gravels composed of quartz and Carpathian sandstones may be observed. Probably those gravels contain Ostrea digitalina, loose fragments of which may be found on the slope. Also in the recent gravels, of the stream shells of Ostrea digitalina are occuring. On the opposite side of the valley, near the point 331 sands with no fauna are covering transgressively the Cretaceous Lgota beds of the Silesian nappe. The gravels and sands occuring on both sides of the valley lie at a height of 270—330 m. At the same height at Bacharowice, 4 km from Benczyn, gravels and sands repose discordantly on the Cretaceous of the Silesien nappe; they contain a Tortonian fauna discovered by the second writer in 1930 and determined by prof. Friedberg1). Ostrea digitalina is the most frequent fossil in those sands. At Lgota the Tortonian gravels and sands also overlie the Miocene clays with no fossils. These clays are dipping under the Cretaceous of the Silesian nappe. Thus in both areas, near Benczyn and at Lgota, similar conditions are developed at the border of the Carpathian overthrust. A lower division of the Miocene, developed as clays, dips under the Flysch; an upper division composed of gravels and sands covers trangressively both the Flysch and the lower division of the Miocene. Up to the present it has not been possible to determine the age of the lower argillaceous Miocene in a more precise way as no fossils have been known from the clays. The discovery of a fauna in the clays at Benczyn permits the determination of their age. The fauna occurs nest-like in clays in three points but only one is very rich in fossils. The fauna is composed of numerous Foraminifera and mollusks, but also corals, Bryozoa, Crustacea, fish remains and Echinoidea abound. Among Foraminifera Heterostegina is the most frequent beside Robulus, Nodosaria, Pyrgo and Miliolidae. Corals nearly all are simple (CaryophyIlia, Discotrochus, Flabellum) but also colonial corals occur. Lithothamnia are fairly frequent. Mollusks are the most numerous. 144 species of them have been determined (see the list p. 283 of the Polish text). Among them 37 belong to Lamellibranchia and the rest to Gasteropoda which are the most numerous group in the Benczyn fauna. The most frequent are the following species: Area diluvii, Corbula gibba, C. carinata, Chlamys scabrella var. Lornnicki, Venus multilamella var. marginalis, Meretrix italica, Cardita scalaris, Turritella badensis, Vermetus intortus, Ancilia glandiformis, Conus Dujardini, Natica helicina. The presence of Gasteropods provided with siphon (Fu sus, Murex, Nassa, Mitra, Daphnella, Pleurotoma etc) indicates a muddy bottom. The depth of the Miocene sea may be estimated on the ground of the fauna as ca 300 m. The fauna is markedly composed of two groups. One group seems to be attached to a deeper and muddy bottom and another possesses characteristics of a shallow water fauna. It may be concluded that the Benczyn fauna consists of autochtone elements which lived on the clay bottom and of allochtone elements brought by currents from shallow waters. A number of fossils exhibit some rounding of their shells. The age of the Ben czyn clays may be inferred from the age limits of 104 species. In the Benczyn fauna 9 species are such which have been living since the Aquitanian, 31 since the Burdigalian, 72 since the Helvetian, 100 since the Tortonian and 42 since the Pliocene; there are 24 species living at present. The fauna of Benczyn contains no species living exclusively in the Aquitanian and Burdigalian. Only one species is exclusively Helvetian, 14 are exclusively Helvetian and Tortonian, 21 species are exclusively Tortonian and 6 Pliocene. Other species living in large stratigraphical limits are indifferent. The above indicates the Tortonian age of the fauna. A more precise determination of the age may be obtained by the comparison of the fauna with other Tortoniah faunas of Southern Poland. Benczyn possesses 23% of common species with the Wieliczka salt formation, 14% with the clays of Zgłobice and Grabowiec, only 7% with the Żegocina clays and 26% with the Pleurotoma clays of Korytnica. Compared with the Bogucice sands (covering the salt formation) the Benczyn fauna has 24% of common species, with Lithothamnium and Heterostegina marls of Książ Wielki (NE of Cracow) — 39% and with the Heterostegina marls of Pińczów and Korytnica — 40%. According to the percentage of common species the Heterostegina and Lithothamnium beds of the southern slope of the Święty Krzyż (Holy Cross) Mountains are the most related to the Benczyn fauna. The second place is taken by the Pleurotoma clays of Korytnica (lying below the Heterostegina - Lithothamnium beds) and the last by the Wieliczka salt formation (younger than Lithothamnium beds) and Bogucice sands There are very few index fossils in the Tortonian, but Pectinidae may give a closer determination of the age. All Pectinidae occuring at Benczyn point to the Lithothamnium division of the Lower Tortonian. The same conclusion may be based on the abundance of Heterostegina in the Benczyn clays. The Heterostegina clays of Benczyn belong thus to the Lower Tortonian. Heterostegina sands2) of the Cracow area may be considered as their stratigraphical equivalent. The Heterostegina clays of Benczyn have been deposited in the same but somewhat deeper sea nearer the axis of the Subcarpathian foredeep. The Heterostegina sandstones lying on the Flysch near Tarnow, described by F. Bieda 3) may mark the southern littoral zone of the Heterostegina sea. The deposition of the clays was followed by an orogenic stage of the Carpathian movements. During that stage the Flysch had been overthrust on the Lower Tortonian clays. In the next stage a transgression followed which deposited gravels and sands with Ostrea digitalina both on the Lower Tortonian clays and the border of the Flysch. The gravels and sands correspond to the Bogucice sands transgressing at Wieliczka on the folded salt formation and may be regarded as the Middle Tortonian. 1) M. Książkiewicz, Annales de la Soc. Geol. de Pologne, 8, Rocznik Pol. Tow. Geol. XVIII, 19, 1932. 2) W. Friedberg. Annales de la Soc. Geol. de Pologne, 9, 1933. 3) Annales de la Soc. Géol. de Pologne, 12, 1936.