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Elemental and organic carbon proxies for redox conditions of the Oligocene formations in the Ropa Tectonic Window (Outer Carpathians, Poland) : palaeoenvironmental implications

Wójcik-Tabol P.

Annales Societatis Geologorum Poloniae

2017

Vol. 87, No. 1

41--53

The Oligocene Grybów Succession is recognized as a counterpart of the anoxic Menilite Formation. Its comprehensive geochemical investigations are made in the key sections of the Ropa Tectonic Window (the Grybów Unit, Polish Outer Carpathians). The maceral assemblages, dominated by land-plant liptinite, vitrinite and intertinite, correspond to kerogen types II and III. A Tmax vs. HI diagram shows terrestrial kerogen type II with various additions of type III and algal kerogen type I. A variation in δ13Corg. (from –25.21 to –27.38‰) may have resulted from variations in the composition of organic matter (the content of terrestrial vs. marine organic matter), controlled by depositional setting (turbidite vs. hemipelagic). The highest TOC contents are associated with an enhanced influx of land-derived organic matter. The redox-sensitive trace elements positively correlate with TOC and TS contents. Redox conditions varied between oxic and anoxic, as was concluded from TOC-TS, V/(V+Ni) and U/Th. The turbidity currents might have ventilated the bottom waters, especially more efficiently in the proximal zone of turbidite sedimentation. Moreover, oxygenated bottom waters may have also affected the concentration of trace metals, owing to migration of the redox interface downward within the sediments.

Depositional redox conditions of the Grybów Succession (Oligocene, Polish Carpathians) in the light of petrological and geochemical indices

Wójcik-Tabol P.

Geological Quarterly

2015

Vol. 59, No. 4

603--614

This study details the petrology and chemistry of the Oligocene succession of the Grybów Nappe in its stratotype-locality in the Grybów tectonic window (Polish Carpathians). The section studied is composed of the Sub-Grybów Beds, the Grybów Marl Formation (GMF), and the Cergowa Beds, representing the middle to upper part of the Oligocene succession. The rocks studied consist of quartz, calcite, Na-plagioclase, muscovite and clay minerals (illite-smectite with 25–30% of smectite and kaolinite). Additionally, hematite occurs in the GMF and chlorite in the Cergowa Beds, respectively. The macerals assemblage of the GMF is dominated by landplant-derived compounds of liptinite, associated with minor amounts of vitrinite representing type II kerogen. The total organic carbon (TOC) content is between 0.45 and 6.16 wt.%. The δ13Corg values of the GMF vary between –27.1 and –27.9‰. The values of both carbon and oxygen isotopic ratios of carbonates range for δ13C from –1.1 to –4.3‰ VPDB, and for O from –1.5 to –4.8‰ VPDB. The concentrations of Co, U, Ni, As, and Mo are higher in the GMF than in the adjacent strata and positively correlate with TOC and S. Values of the TOC/S and V/V+Ni ratios are 0.7 to 3.5 and 0.67 to 0.78, respectively, and indicate anoxic conditions. The ratios of U/Th and V/Cr (0.3–2.2, 1.18–3.18, respectively) suggest the change of oxic conditions to reducing conditions occurred during the GMF deposition. This change could have been preceded by a plankton bloom, initiated by a nutrient-rich freshwater inflow that is inferred from the decrease of the δ13Ccarb values and the terrestrial detritus supply. Thermal alteration of the Grybów Succession is concluded on the basis of smectite illitisation and low δ18O values.