Onkoidy w słodkowodnych osadach wapiennych trzeciorzędu rowu Kleszczowa

Stanisław Lorenc

Abstrakt


ONCOIDS IN FRESH-WATER CALCAREOUS TERTIARY ROCKS IN THE KLESZCZÓW TROUGH

Summary
Oncoids occurring in fresh-water Tertiary calcareous rocks from the open-cast brown coal mine at Bełchatów are varying in size, shape and texture. They range from less than 1 mm to about 5 cm in size (most often from 1 to 2 cm in size). Their shape also appears highly diversified, ranging from spherical and subspherical to ellipsoid and finally cylindrical. Ellipsoidal forms with the ratio of the shorter axis to the longer up to 1:3 are most common here. Outer envelope of some oncoids is knobby. Nuclei are most often formed of fragments of phytogenic matter or more or less complete molluscan shells and, occasionally, lumps of carbonate-clay-coally matter and about a quarter of diameter of oncoid in length.
Cortex usually fairly clearly displays dark micritic laminae (0.1 -0.2 mm thick) and light microsparry ones (up to 2 mm thick). The latter are usually thicker and less regular in younger part of the cortex than in the older and display fine radial microstructure, due to arrangement of microsparry calcite along radially arranged calcified filaments of algae. The oncoids are often crushed and accompanied by fragments of algal mats which suggests their redeposition to that environment.

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