Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)

Corrosion and colour alterations of lydite clasts (Cenomanian-lower Turonian, Bohemian Cretaceous Basin)


Jiří Žítt, Čestmír Nekovařík

Geoscience Research Reports 33, 2000 (GRR for 1999), pages 88–89

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A study of lydite clasts from the late Cenomanian-early Turonian conglomerates (Korycany Member) of the Bohemian Cretaceous Basin shows that they sometimes experienced very complicated set of processes. In adittion to the physical and biological processes, the chemical effects, recorded as corrosion and colour alteration are the most important. Soft corrosion of clasts often affects their surfaces but the deep one may change even the clast shape. The colour alterations of clasts may be only superficial in the form of patinas or may deeply penetrate the clast matter (alteration into greenish and mainly yellow colours). The age of both the corrosion and colour alterations was proved by Cretaceous encrustation (epibionts), boring and mineralization (phosphatic crusts) of affected surfaces and by the occurrence the "in situ" Cretaceous deposits. Corrosion and colour alterations undoubtedly reflect special (possibly subaerial) palaeoenvironmental conditions affecting the coarse marine elastics, most probably during their subaerial exposure. The problems are now being studied in more detail.