Geochemistry of intercalated red and gray pelagic shales from the Mazak Formation of Cenomanian age in Czech Republic

Abstract

Pelagic red and gray shales are intercalated within the lower part of the Mazak Formation of Middle Cenomanian age in Czech Republic. A detailed geochemical study of major, trace and rare earth elements and carbon isotopic compositions of organic carbon has been conducted on sixteen red and gray shales. The data suggest that the shales were most likely accumulated in well-oxygenated bottom waters with very limited organic matter supply and consisted of marine organic matter mixed with minor amounts of terrestrial organic matter. The shales were deposited below CCD in one of the tectonic troughs developed along northern margin of the western Tethys. Similar geochemical covariances of major, trace and rare earth elements for the shales suggest palaeoceanographic conditions and provenance during their deposition. The most probable cause for the variation of redox bottom conditions in the mid-Cretaceous deep ocean was periodic changes in the concentration of dissolved oxygen in bottom waters, due to changes in deep water circulation and processes driven climate changes.