Ślady organizmów w archaicznej strukturze Sejn

Wacław Ryka, Zygmund Waleńczak


Metodą ultramikroskopii przy zastosowaniu ciemnego pola zbadano skały archaicznej struktury Sejn - dwupiroksenowe gnejsy, anortozyty i leukogabra. Stwierdzono, że macierzysty materiał osadowy składał się głównie z drobnoziarnistych piaskowców, mułowców i iłowców. W strefie przydennej w warstwie fotycznej rozwijało się życie organiczne: biocenozy o charakterze kolonialnym, saprofity i formy przetrwalnikowe. Pospolite są nitkowate szczątki grzybów. Mimo przeobrażeń metamorficznych zachowały się ślady substancji organicznej w ilościach 0,06-0, 13 %.


The Sejny structure, situated east of the Wigry Lake, is built of Archaic rocks occurring beneath sedimentary cover about 550 m thick. The rocks are represented by two·pyroxene gneisses, hornfelses, amphibolites, anorthosites, leucogabbros, and pyroxene, pyroxene-magnetite, and hornblende restites. The maximum temperatures of differential anatexis responsible for origin of anorthosites and leucogabbros were estimated at 630-670°C. Rocks of the Sejny structure display drastic changes in texture, structure, mineral composition, and chemistry. Therefore, they were covered by detailed analyses, including the use of unconventional techniques of mineral memory analysis. One of these techniques - ultramicroscopy involving the use of dark field method - made possible studies of relics of parent sedimentary rocks and organic remains.

Metamorphic series of the Sejny structure were formed of sedimentary rocks, mainly fine-grained sandy sediments. The shares of muds, clays, and colloidal matter appear to be subordinate. The studies show fairly intense development of organic life in photic zone in times of sedimentation of these strata. The available record comprises various relics of these organisms from several types of sediments. Some relics occur in biocoenoses of the colony type, resembling stromatolites. Saprophytes, attached with acetabulum to the bottom, are fairly common here, similarly as spores and sclerotes, with chitinous coating resistant to various disadvantageous factors, and easily dispersed by water and wind There were also found fungi, especially thread-like forms representing siphonal saprophytic organisms. The thread-like forms were attached to the substratum with prehensile tail, and the opposite ends are often thickened (sporangia ?). When mud covered these thread-like organisms. those of the next generation began to develop at their upper ends, in direction of water table.

Metamorphic processes resulted in changes in mineral composition of the rocks and gradual destruction of coal and bituminous matter. Nevertheless, rocks of the Sejny structure still display traces of organic matter, with content of Corg equal 0.06-0.13%.

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