Petrografia osadów kambru środkowego w syneklizie perybałtyckiej

Abstract

Na podstawie szczegółowych badań petrograficznych opisano skład mineralny badanych skał i wyróżniono cztery litofacje w obrębie syneklizy perybałtyckiej. Określono stopień dojrzałości strukturalnej i mineralogicznej osadów oraz źródła i kierunki transportu materiału okruchowego. Przedstawiono również zagadnienie procesów postsedymentacyjnych w powiązaniu z porowatością skał.PETROGRAPHY OF MIDDLE CAMBRIAN DEPOSITS IN THE PERIBALTIC SYNECLISE (NE POLAND)The paper deals with deposits of the Middle Cambrian (Paradoxides oelandicus and Paradoxides paradoxisimus zones) of the Peribaltic syneclise. The petrographic characteristics of these deposits is given on the basis of core material from 12 boreholes (Fig. 2). The studies included microscopic analysis ,of 500 thin sections and 170 samples of heavy minerals.The Middle Cambrian is represented by clastic deposits, mainly fine-grained quartz arenites (Fig. 3), accompanied by mudstones and claystones developed as interbeddings varying in thickness.Detrital material mainly consists of quartz, which is accompanied by micas, feldspars, heavy minerals and glauconite occurring in small amounts. Within intergranular spaces of the deposits studied there occur: quartz overgrowths most frequently), carbonates, clay minerals and matrix. The group of allogenic heavy minerals includes primarily zircon, tourmaline and rutile, as well as some amounts of minerals less resistant to weathering and transport - hypersthene and amphiboles (Fig. 4). The latter were most probably derived from the Kaszuby Sfecofenno-Karelian complex which stretches northwards beneath the Baltic Sea floor. The studied sandstones are structurally and mineralogically mature or, sometimes, super-mature.Four lithofacies were distinguished in the studied part of the Peribaltic synclise: sandy on the east, passing westwards successively into sandy-day, clay-sandy and clay lithofacies (Fig. 1).The paper presents an attempt to reconstruct directions of transport of detrital material. The material was presumably coming from NE and NW in the Paradoxides oelandicus zone, and from E, NE and NNE in the Paradoxides paradoxissimus zone.The studied deposits are strongly diagenesed in result of the following processes: compaction, silification and carbonatization, which resulted in a marked decrease in their primary porosity. That is why these rocks are characterized by low reservoir properties.