Glaucony from the condensed Lower-Middle Jurassic deposits of the Križna Unit, Western Tatra Mountains, Poland

Abstract

Glaukonit ze skondensowanych osadów dolnej-środkowej jury jednostki krizniańskiej Tatr Zachodnich, Polska Lower-Middle Jurassic glaucony-bearing deposits crop out in the Polish part of the Krizna Unit in the Western Tatra Mts. These deposits, up to 20 cm thick, consist of glaucony-rich marls and limestones. The glaucony grains constitute up to 30% volume of the deposits. They represent an evolved stage of glauconitization since they contain more than 7% K2O. The content of AI2O3 is high (up to 19.97%, average 16.98%) while the content of Fe2C>3 is low' (not more than 23.48%, average 12.84%). These features are interpreted as a product of diagenetic processes. The glaucony-bearing deposits were formed at an upper bathyal depth and their rate of deposition was very low, what allowed long-lasting evolution of the glaucony grains. The K-Ar age of the glaucony grains is much younger than the biostratigraphic age of the studied section. The lowering of the K-Ar dates is interpreted as a result of loss of radiogenic Ar from the lattice of the glaucony.