Mikrostruktury stromatolitów i onkoidów z wapienia cechsztyńskiego Polski północno-zachodniej

Abstract

Omówiono mikrostruktury utworów cjanobakteryjnych: stromatolitów i onkoidów występujących w wapieniu cechsztyńskim NW Polski oraz zinterpretowano środowiska ich powstania. Wyróżniono dwa zasadnicze typy mikrostruktur stromatolitów: zwartą i okienkową (fenestralną), związane z pierwotnym zróżnicowaniem środowiska powstania i składu mineralnego stromatolitów. Stwierdzono identyczność mikrostruktur utworów cjanobakteryjnych niezależnie od ich makrostruktur, co może wskazywać na biologiczne uwarunkowanie ich powstania.MICROSTRUCTURES OF STROMATOLITES AND ONCOIDS FROM THE ZECHSTEIN LIMESTONE OF NORTH-WESTERN POLAND Cyanobacterial deposits: stromatolites and oncolites, occurring mainly in the upper Zechstein Limestone of NW Poland (Figs 1 - 3) have been found both in the central and peripheral parts of the basin. Stromatolites originated in different environments. In a supratidal environment, stromatolites occurring at the Zechstein Limestone-Lower Anhydrite boundary (Tabl. IV, Figs 10, 11) in the whole area studied formed. In an intertidal environment and in the highest part of subtidal zone stromatolites from the well Grzybnica IG 1 (Tabl. V, Figs 12, 13: Tabl. VI, Figs 14, 15: Tabl. VII, Fig. 16) and some other stromatolites from the central part of the basin (Tabl. III, Fig. 8) formed. The majority of stromatolites from the central part of the basin formed in a subtidal environment (Tabl. I, Figs 4, 5; Tabl. Ill. Fig. 9). The occurrence of stromatolites usually shows a relation to occurrence of oncoids (Tabl. I. Fig. 4: Tabl. II, Figs 6, 7) that originated in a subtidal environment from action of periodic catastrophic events such as stroms and hurricanes. Very similar and identical microstructures, irrespective of macrostructures of cyanobacterial deposits have been stated (TabI. II, Fig. 7: Tabl. III, Fig. 9) what seems to indicate explicitly that the origin of microstructures has been biologically conditioned. In oncoids and stromatolites a dense microstructure has been stated that distinctly differs from fenestral microstructures characteristic of stromatolites that originated in the highest part of subtidal zone and in the intertidal and supratidal zones. The changes of fenestral microstructures (Tabl. V, Figs 12, 13; Tabl. VI, Figs 14, 15; Tabl. VII, Fig. 16), resulted from small changes of sea level, may be related to tides, and from changes of blue-green communities and sequences of diagenetic processes of differ.ent intensity that have been connected with those sea level changes. The poor preservation or even obliteration of microstructures of cyanobacterial deposits (Tabl. II, Fig. 7; Tabl. III, Fig. 9; Tabl. V, Figs 11, 13) resulted from the unstable primary mineralogical composition. Fenestral stromatolites and sabkha stromatolites were probably built of aragonite; far subtidal stromatolites the high Mg-calcite composition is assumed. It seems that the differences in microstructures of stromatolites of essentially different environments of their formation have been additionally emphasized during diagenesis by the different primary mineralogical composition.