Epigenetic siliceous rocks from the southern part of the Kraków-Częstochowa Upland (Southern Poland) and their relation to Upper Jurassic early diagenetic chert concretions

The epigenetic siliceous rocks (ESR) are common in the southern part of the Kraków-Częstochowa Uplandwhere they are hosted in Upper Jurassic microbial-sponge megafacies as: (i) coatings on the surfaces of fractures and faults, (ii) irregular bodies enclosed within both the Upper Jurassic limestones and the overlying Cenomanian abrasion platform sediments, and (iii) concentric or irregular rims of silicified limestones on early diagenetic chert concretions. The distribution of ESR is limited to the topmost part of the preserved Upper Jurassic succession. The results of chemical analyses and fluid inclusion microthermometric data clearly demonstrate the hydrothermal origin of the ESR. The epigenetic silicification was triggered by low temperatures and low saline fluids. The age of the ESR is indirectly suggested by: (i) the early diagenetic cherts forming the crystallization nuclei, (ii) the silicification of pressure dissolution structures in limestones and (iii) the silicified carbonate infillings of fractures in chert concretions formed in the local extensional regime, enhancing themigration of thermal fluids during the main, Cenozoic faulting stage of Upper Jurassic sediments.