Lower Jurassic spiculite series from the Križna Unit in the Western Tatra Mts, WesternCarpathians, Poland

Renata Jach


Dolnojurajskie spikulity jednostki kriżniańskiej w Tatrach Zachodnich, Karpaty Zachodnie, Polska

Lower Jurassic spiculite series was studied in the Polish part o f the Kriżna Unit in the Western Tatra Mts. This series consists o f interbedded spiculites and crinoidal limestones. The spiculites are built almost entirely o f siliceous sponge spicules belonging to Hexactinellida and Demospongiae classes. The prolific growth of siliceous sponge community was caused by favourable topographic and bathymetric conditions and by increased content of dissolved silica in the seawater. The spicules were not transported. Crinoidal limestones intercalating with spiculites are composed predominantly of crinoidal ossicles redeposited from shallower parts of the basin by gravity currents generated by storm events. The deposits o f the studied series reveal a shallowing upward trend, marked by upward gradual replacement of hexactinellids by demosponges and by thickening and coarsening of the crinoidal limestone layers.

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