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

Jach R.

Annales Societatis Geologorum Poloniae

2002

Vol. 72, No 2

131-144

Lower Jurassic spiculite series was studied in the Polish part of the Križna Unit in the Western Tatra Mts. This series consists of interbedded spiculites and crinoidal limestones. The spiculites are built almost entirely of 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 of 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.