Abstract:

(Pre)-Cretaceous sediments that fill extensive hydrothermal dissolution cavities in Devonian limestones in the Certovy schody Quarry contain locally abundant halloysite clays. In most samples halloysite coexists with metahalloysite and kaolinite that form snowy-white and blue-green masses intimately intergrown with manganese oxides. The determination of members of kaolinite group minerals was the main purpose of this study. X-ray diffraction method and thermal analyses enabled to differentiate halloysite from metahalloysite and kaolinite. Tubular morphology of halloysite was observable in electron micrographs. A study of the stored material confirmed continuous transition from the halloysite form over interstratified states (mixed-layer halloysite-metahalloysite crystal structure) to metahalloysite. Field observations indicate that halloysite formed due to selective in situ hydrothermal alteration of clayey matrix of quartz sandstone and/or breccias of unclear original composition that occur within the sedimentary fill. The proposed model for halloysite formation involves alteration by warm (< 80°C) acid solutions that migrated through the system of tectonic fractures, dissolved limestone host and altered overlying sedimentary members that collapsed into the resulting hydrothermal depressions. Since halloysite partially replaced also Cenomanian sediments, the hydrothermal alteration must have occurred only during post-Cenomanian time.