Organic-rich and manganese sedimentation during the Cenomanian–Turonian boundary event in the Outer Carpathian basins; a new record from the Skole Nappe, Poland

Abstract

Upper Cenomanian–Lower Turonian strata in the Spława section of the Skole Nappe, Outer Carpathians, Poland, provide a detailed palaeoenvironmental record of the oceanic anoxic event (OAE-2) from a deep-water basin below the CCD at the northern margin of the Western Tethys. The OAE-2 succession consists of black, parallel-laminated radiolarian shales, enriched in organic matter of mainly marine origin (TOC up to 8%). Organic carbon isotope variations, together with microfossil datum events allowed to correlate the studied organic-rich facies with other black shale successions along the Outer Carpathians and other areas of the Western Tethys. The mean values of sedimentation rate (3.5–5.5 mm/kyr) and organic matter accumulation rate (0.5 g/m 2/year), calculated for the OAE-2 interval in the Outer Carpathian basins, are comparable to the values from other deep-water marginal basins in the Western Tethys. The average redox-sensitive trace metal contents and their enrichment factors, indicate water column anoxia in the Outer Carpathian basins, intensified during the interval of the largest shift in δ 13C values. The periods of bottom anoxia were interrupted by intervals of variable Eh conditions with sedimentation of hemipelagic green shales, including poor agglutinated foraminiferal assemblages. An increase in productivity of organic and siliceous plankton during the OAE-2 is documented, based on increased values of silica and barium enrichments. This closely corresponds to an interval of the largest positive stable carbon isotope excursion, related to the beginning of the largest sea level rise, linked with intensive coastal upwelling. Extremely low sedimentation rate or even hiatuses and the increase in deep-water circulation causing basin oxygenation resulted in precipitation of two ferromanganese layers. Their formation was related to changes in hydrodynamic conditions at the basin floor, expressed by precipitation of Fe–Mn carbonates. Some chemical indices seem to suggest that the primary metal enrichments could be related to hydrothermal activity. However, the concentrations of some elements show also on hydrogenic to diagenetic influences in these metalliferous sediments. The precipitation of these Fe–Mn sediments was probably synchronous with the manganese enrichment across the C–T transition recorded in other Western Tethyan basins and in the adjacent epicontinental seas.