Tufity mioceńskie w Chełmie Wielkim nad Przemszą

Abstract

W rejonie Chełma Wielkiego koło Katowic w osadach mioceńskich (dolny baden) warstw skawińskich stwierdzono występowanie tufitów oraz tufu. Rozpoznano skład mineralny tych skał zwracając szczególną uwagę na najdrobniejsze frakcje ziarnowe oraz chemizm zawartego w nich szkliwa. Stwierdzono, że materiał piroklastyczny zawarty w badanych skałach ma charakter ryodacytowy. Otrzymane wyniki pozwoliły na dokonanie korelacji poszczególnych wkładek tufitowych.

MIOCENE TUFFITES FROM CHELM WIELKI ON PRZEMSZA RIVER

The Kłodnica and Skawina beds may be distinguished in the Miocene section from the vicinities of Chełm Wielki, about 20 km SE of Katowice (Fig. 1). The Kłodnica beds are represented by sandy claystones with Cepaea, grey and dark-grey clays with Congeria and Cerithium, mads and marly limestones with Planorbis and marly clays with diatomite intercalations. The Skawina beds are represented by grey marly clays with innumerous pelecypods and rich foraminifer assemblages. These assemblages are known from numerous Lower Badenian (Lower Tortonian) sections in the Upper Silesian Coal Basin and were usually denoted with symbols IIA, IIB and IIC (S. Alexandrowicz, 1963). Moreover, it is possible to distinguish glauconite sandstone layer with sponge spicules (Fig. 1–S_p) and a few tuffite intercalations in the Skawina beds.

Detailed correlation of tuffites was made on the basis of micropaleontological data. There were distinguished 5 tuffite intercalations the stratigraphic position of which is as follows (Fig. 1): tuffite Ta₁ – in marly clays with the foraminifer assemblage IIA in the sections I, V and VI; tuffite Ta₂ – in clays with the assemblage IIA in the section X; tuffite Tb₁ – in basal part of clay layer with the assemblage IIB; tuffite Tb₂ – in top part of the same layer, and tuffite Tc – two thin intercalations in clays with the assemblage IIC.

Petrographic studies showed that tuffite Ta₁ mainly consists of clay minerals, volcanic (obsidian, pumice and globular) glass, quartz, feldspars, biotite as well as glauconite and foraminifer tests. Samples A and B may be interpreted as typical tuffites formed of volcanic glass with a small admixture of detrital matter whilst sample C – mainly of volcanic glass. Biotite is well-preserved in tuffites and its scales are sometimes overgrown with plagioclases and glass (Fig. 2). The finest grain fractions of these rocks (below 2 mm) were studied with the use of X-ray method. The analyses of natural material saturated with glycole and 1 n KCI (Fig. 3) showed that it consists of pure montmorillonite only.

The rock Tal may be named as ryodacite tuffite. The chemical analyses showed uniform character of glass from all the tuffites, inherited after the parent magma (Table 1). The SiO₂ content is highly specific, equalling 67–69%. It should be emphasized that the other tuffite intercalation occurring in that horizon of marly clays (tuffite Ta₂), is characterized by glass yielding 73–75% of SiO₂ (W. Parachoniak, 1960, 1962). The latter intercalation is related to more acid magma and the presence of that tuffite in deposits was noted because of the finds of numerous pyroc1astic quartz grains (S. Alexandrowicz, 1957).