Piroklastyczne skały ogniwa obrzyckiego w zachodniej części Niżu Polskiego

Abstract

Najwyższą część eruptywnej formacji autunu, wyodrębnioną jako ogniwo obrzyckie, stanowią skały piroklastyczne miąższości do 229,5 m, średnio – kilkadziesiąt metrów.Są to skały litokrystalo-witroklastyczne, wykształcone najczęściej w litotach aglomeratowo-lapillowej i piaskowcowej, rzadziej skały witroklastyczne litofacji popiołowej.Wyróżniono tufy złożone wyłącznie z materiału pirogenicznego i tufity zawierające domieszkę składników terygenicznych. Stwierdzono, że są to zlityfikowane osady potokówaglomeratowych i laharów. Uległy one intensywnym przemianom hydrotermalnym, związanym z posteruptywną działalnością wulkanizmu autuńskiego. PYROCLASTIC ROCKS OF THE OBRZYCKO MEMBER (AUTUNIAN) IN WESTERN PART OF THE POLISH LOWLANDSThe paper presents lithological-petrographic characteristics of Autunian pyroclastic series differentiated as the Obrzycko member by J. Pokorski (1976). These rocks, up to 229.5 m thick, represent the uppermost part of Autunian volcanogenic series comprising effusive rocks with some tuff intercalations. They usually overlay rhyolites and trachyandesites or, sometimes, they rest directly on the Carboniferous. Maps (Figs. 1–2), stratigraphic columns (Fig. 3) and correlative profiles (Figs. 4–7) show distribution of the Obrzycko member. Classification triangles (Figs. 8–9) show petrographic character of these deposits, and photographs (Tables I–XII) – typical structures and components.Pyroclastic rocks of the Obrzycko member are variable in granulation. They usually represent agglomeratic-lapilli and sandstone or, sometimes, ash lithofacies. Intercalations of fine-grained terrigenic rocks are sometimes found. Coarse-grained rocks mainly consist of rhyolite lapilli and, sometimes, debris of trachyandesites or welded tuffs. Sandy fraction of tufts comprises pyrogenic quartz grains, albite, sanidine, biotite and hematite pseudomorphoses after mafic minerals and glassfragments. Tuffites also contain rounded grains of terrigenic quartz, microcline, heavy minerals, muscovite, and debris of quartzites and crystalline schists. Cement is made of clay minerals (mainly illite and chlorite) coming from devitrification of glass. Mixed montmorillonite/illite packets are also sometimes found. Other components of cement include quartz dust, hematite, chalcedony and authigenic quartz, carbonates, and anhydrite. Sandstone tuffs and tuffites differ from agglomeratic- lapilli ones only in granulation whereas petrographic composition is the same and all the rocks are lithocrystallo-vitroclastic. Rocks of the ash lithofacies belong to vitro clastic or vitrocrystalloclastic, mainly consisting of clay pelite and chalcedony coming from devitrification of glass. Both mono- and polimictic tuffs are present here, presumably depending on lithology of volcanic cones and petrographic variability of underlying covers of intrusive rocks. The Obrzycko member represents a series of lithified deposits of agglomeratic streams and lahars. Some tuff layers are made only of pyroclastic material consolidated in situ in water reservoir. An increased content of terrigenic material and the presence of intercalations of wackes, siltstones and claystones without pyroclastic material, found in some places, indicate breaks in volcanic activity.Rocks of the Obrzycko member underwent intense hydrothermal alterations connected with post-eruptive phase of Autunian volcanism, i. e. devitrification and neomorphism of volcanic glass and, in some places, bentonitization, carbonatization and silification. Alteration of rock material varied in space, becoming' weak in some places but some ash tuff layers turned into limestones, dolomites of flints in result of infiltration of mineral solutions.