Provenance and early diagenetic processes of the Ordovician Sarka Formation at Praha - Cerveny vrch Hill (Barrandian, Czech Republic)

Shales and siliceous nodules from Praha - Cerveny vrch Hill (Middle Ordovician, Sarka Formation, Barrandian area) were studied by chemical and X-ray phase analyses as well as by K-Ar age dating to specify their formation and provenance. White micas occurring as detrital components within the shales of the Sarka Formation reveal a Neoproterozoic (Cadomian) K-Ar age (599 plus or minus 12 Ma) of the source rocks. The enrichment of heavy rare earth elements (HREE) over light rare earth elements (LREE), high Th/U ratios, and a distinct kaolinite content in the shales of the Sarka Formation point to strong chemical weathering of the source area in pre-Middle Ordovician times. Major element data indicate a passive margin setting for these shales, whereas trace element data and discriminant function analysis display the inherited island arc signature of the Neoproterozoic basement. Very low sulphide sulphur (S sub(pyrite)) and organic carbon (C sub(org)) contents as well as the depletion in Fe and Mn are interpreted as a tracer for oxic bottom-water conditions during deposition and transport of these elements by bottom currents. The growth of the siliceous nodules is supposed to be connected with the replacement of former sediment by SiO sub(2) and with leaching of most elements simultaneously with the enrichment in Mn, Ca, Co, Pb, Y and HREE.