KARBONÁTOVÉ KONKRECE „KOBLÍŽKY“ Z LOKALITY NOVÉ DVORY U SUCHDOLA V MORAVSKÉM KRASU (ČESKÁ REPUBLIKA)

Nove Dvory “donuts” are globular to elongated concretions, up to 10 cm in diameter, with remarkable radial structure. They were described from sandstone and limestone cavities in the Moravian karst. Microscopic analyses indicate that these nodules consist of about 45 mod. % of elongated calcite crystals (Ca0,999–0,998 Mg0,002–0,001 CO3). Angular to subrounded quartz grains form the dominant clast type. K-feldspar, strongly weathered plagioclase grains and muscovite are present in small amount. Mineralogical composition of the translucent heavy fraction of “donuts” and surrounding sandstones is interpreted as result of mixing of the detritus coming from the typical Cretaceous (Cenomanian) sediments and of detritus coming from local sources (such as Devonian clastic sediments). Translucent heavy mineral assemblage typical for the Cretaceous clastic sediments includes staurolite, kyanite, tourmaline, rutile and sillimanite, while the local material (Devonian) is characterized by a dominance of epidote and/or zircone. The Nove Dvory “donuts” can be interpreted as a product of precipitation of calcium carbonate in sand that filled the karst cavities (paleo-sinkholes or paleo-cave).