Organic geochemistry and palynofacies of the Early–Middle Frasnian transition (Late Devonian) of the Holy Cross Mountains, Southern Poland

Geochemical and palynological investigation of the Lower to Middle Frasnian reef-to-basin succession at the well-known Kowala Quarry (Holy Cross Mountains) demonstrates the variable character of organic matter (OM) deposited in different oxidative–reductive conditions. During sedimentation of the basinal deposits, anoxic conditions in the water column prevailed. In the organic-rich sediments deposited during the Palmatolepis punctata Zone (set D), anoxia occurred in the photic zone, as inferred from the presence of isorenieratane, aryl isoprenoids and gammacerane. In the same kind of deposits, but occurring during the Palmatolepis hassi Zone (set F), anoxia apparently occurred mainly in the deeper parts of the basin and only sporadically reached the photic zone. This interpretation is based on the lack of isorenieratane, traces of aryl isoprenoids and the presence of gammacerane in the investigated samples. The basinal clay-enriched sediments of sets D and F are divided by an organic-poor set E, typical for the carbonate platform (fore-reef) environment characterized by oxic conditions. A palynostratigraphical study of the Frasnian section from the Kowala Quarry allowed recognition of two miospore zones: Geminospora semilucensa–Perotrilites donensis (SD) and Archaeoperisaccus ovalis–Verrucosisporites grumosus (OG). Relatively small quantitative and qualitative changes of the organic particles (miospores, tracheids, prasinophytes and acritarchs) are interpreted primarily in terms of sea-level fluctuations and enhanced productivity. In particular, the appearance of abundant, abnormally large mature leiospheres in set D can be the result of progressive eutrophication, related to environmental changes recorded in the recently detected major positive carbon isotope anomaly. As during the Frasnian–Famennian mass extinction interval, this eutrophication could have been stimulated by a rapid increase in primary production and subsequently expanding anoxic conditions, but it is evident only in the early phase of the Middle Frasnian δ13C excursion. The processes of the secondary hydrothermal fluid migration, taking place after the lithification of the sediments, caused partial OM oxidation. The products of these processes are such phenyl derivatives of aromatic hydrocarbons as phenylphenanthrenes, terphenyls, phenyldibenzothiophenes and phenyldibenzofurans. Moreover, an oxygen compound rarely described from sedimentary OM, dibenzo-p-dioxin and its methyl- and dimethyl-derivatives, seems to be the additional product of hydrothermal OM oxidation.