Stratigrafie sedimentů doprovázejících vulkanity těšínitové asociace u Bruzovic

Abstract

The rocks of the teschenite association form smaller scattered occurrences in the upper part of the Hradiště Formation of the Silesian Unit. The age of the igneous rocks corresponds to Berriasian to Barremian (Menčík et al. 1983). The teschenite association includes a relatively heterogeneous group of rocks with variable abundances of felsic and mafic minerals. Three outcrops of the teschenite association rocks (both typical teschenites and strongly altered ultramafic types) embedded in non- volcanic sediments were studied in the bed of the Pazderůvka River between Bruzovice and Žermanice (Bz1, Bz2, and Bz3; Fig. 1). At Bz1 and Bz2 outcrops, bodies of teschenite are hosted by sedimentary rocks affected by caustic metamorphism. The volcanic rocks at Bz3 are represented by strongly altered, markedly clayey dark green rocks with a macroscopically apparent high proportion of dark micas and pyroxenes. The bedrock of the outcrop is composed of calcareous claystone, limestone, and sandy limestone, likely representing Těšín Limestones. The phase composition of the volcanic rocks from the Bz1 and Bz2 localities is to a certain extent similar, which indicates that the same body was observed at both localities. Macroscopically, the rocks are medium- to coarse-grained with a variable proportion of dark minerals, indicating internal differentiation of the body. The very high proportions of natrolite (up to about 35 %, up to 54 % in the leucocratic type) are particularly notable. The volcanic rocks from the Bz3 locality are quite different. Their composition corresponds to strongly argilitised mafic rocks (and their volcaniclastic derivates), containing more than 50 % of clay component with a dominance of smectite, as documented by X-ray diffraction data. As for the primary components, pyroxene, biotite, plagioclase, and accessory amphibole were detected. Calcareous dinoflagellate cysts Cadosina semiradiata fusca, Colomisphaera vogleri, and Cadosina semiradiata representing the lower Valanginian to upper Barremian were identified in polished sections of the calcareous rock. Non-calcareous dinoflagellate cysts in Bz1 and Bz2 samples are almost black, reflecting the strong thermal effects of the teschenite intrusion at the contact with with the sediments. The dinoflagellate cysts in the Bz3 outcrop are very well preserved with no evidence of thermal alteration. The overlying body of the volcanic rock can be therefore considered as effusive. Circulodinium vermiculatum, Cymososphaeridium validum, Muderongia tabulata, Oligosphaeridium albertense, Oligosphaeridium asterigerum, Pseudoceratium pelliferum, Systematophora palmula from the Bz3 outcrop can be considered as stratigraphically significant. Based on the first occurrence of C. validum and the last occurrences of Circulodinium vermiculatum and Systematophora palmula, Leereveld (1995) delineates a dinoflagellate zone of Cymososphaeridium validum which represents the upper Valanginian to the lowermost Hauterivian. The composition of palynofacies, in which palynomorphs predominate, is characteristic of a distal well-oxygenated shelf.