Organic matter variations in the deep marginal basin of the Western Tethys and links to various environments in isotopic Albian–Cenomanian Boundary Interval

Abstract

In the latest Albian–lowermost Cenomanian, the palaeoenvironments along the northern margin in the Carpathian domain of the Western Tethys were characterized by deep basins with floors below the calcium compensation depth, which were filled with flysch and hemipelagic sediments. On the other hand, the terrestrial areas surrounding the Western Tethys to the north were covered by a diverse flora of gymnosperms and experienced an increase in the diversity of angiosperms; the latter was characterized by a rapid spread in most terrestrial environments. The flysch/hemipelagic succession of the Silesian Basin (the Polish part of the Outer Carpathians) contains a sedimentological record of organic-rich accumulation that is related to the carbon isotope excursion, known as the Albian–Cenomanian Boundary Interval (ACBI); its older part corresponds to Oceanic Anoxic Event 1d (OAE1d). Organic matter (OM) records from these noncalcareous sediments are characterized by the occurrence of both terrigenous and marine organic compounds, and an increase in their accumulation intensity occurred during OAE1d, similar to other Western Tethyan areas and their northern surroundings. Quantitative analyses of the OM compounds using petrographic and chemical methods indicate that the majority were redeposited from land and estuarine environments; however, the OAE1d strata (Alternans Level) and the lower part of the post-OAE1d black shale succession are also enriched in OM of marine origin. This enrichment is interpreted as the result of enhanced surface water productivity, most likely related to redistribution of nutrients from coastal areas during the sea level rise of the KAl8 eustatic cycle. OAE1d was also the period in which the degradation of OM took place under reducing conditions in the bottom waters (although they were not anoxic), which was possibly caused by poor ventilation. A high proportion of terrigenous/estuarine OM compounds redeposited in the Silesian Basin is characteristic, especially for the post-ACBI black and dark grey shales. A large amount of this OM type has undergone relatively rapid degradation as it has settled from the shallows through a well‑oxygenated water column in the epi-, meso- and bathypelagic zones. Some terrigenous OM came from forest fires that occurred in the catchment area and supplied material to the Silesian Basin.