Termination of Organic-Rich Accumulation of the Oceanic Anoxic Event 2 in the Deep-Water Carpathian Basins Based on Carbon Stable Isotope Data

Marta Bąk,Krzysztof Bąk

doi:10.3390/min11040420

Marta Bąk, Krzysztof Bąk

Open Access

https://doi.org/10.3390/min11040420

Copy DOI

Abstract

Organic-rich black shales intercalated with green radiolarian shales and bentonites, 2.2 m thick, represent an equivalent of the Bonarelli Level in the Outer Carpathian deep-water succession. Carbon stable isotope data from four sections in the Outer Carpathians show that termination of organic-rich accumulation, related to the oceanic anoxic event 2 (OAE2), occurred at the same time in this part of the Western Tethys. The excellent marker of this event is a first horizon of Fe–Mn layer (nodules), a few centimeters thick, directly covering the youngest black shale layer of the Bonarelli-equivalent Level, which is regarded as the regional chronohorizon. The youngest succession of the organic-rich shales, ca. 30 cm thick, corresponds to the latest Cenomanian interval of δ13Corg values displaying a negative trend, which represents a terminal part within the OAE2 carbon isotope excursion. A deep negative shift which ends this falling trend, close to the Cenomanian–Turonian boundary in δ13C curves from many sections around the world, is not visible in the Outer Carpathian successions. The reason for this was the long period of stratigraphic condensation, which is reflected in the ferromanganese sediments of this area.

Highlights

In deep-sea sediments of the Western Tethys, thick black shale intervals of the oceanic anoxic event (OAE) 2, like Bonarelli Level in the Umbria–Marche Basin [1] or Thomel Level in Vocontian Basin [2] and its equivalent (Bonarelli-equivalent Level) in the Carpathian basins [3] are characterized by repeated occurrences of organic-rich and biogenic silica-rich shales
Our point “C” corresponds to the point “III” from the Bahlul section (Figure 4; see figure 11 in [44]). Both marker points are characteristic of other oceanic anoxic event 2 (OAE2) sections, e.g., from the Umbria–Marche Platform [45], Vocontian Basin [46], European epicontinental sea (e.g., [47]), Demerara Rise in the Central Atlantic [48], continental margin of the South
The chemostratigraphic (δ13 Corg ) data related to non-calcareous, hemipelagic shales enriched in marine organic matter in the Cenomanian–Turonian boundary (CTB) interval have been analyzed in the Silesian, Subsilesian, and Skole nappes of the Outer Carpathians

Summary

Introduction

In deep-sea sediments of the Western Tethys, thick black shale intervals of the oceanic anoxic event (OAE) 2, like Bonarelli Level in the Umbria–Marche Basin [1] or Thomel Level in Vocontian Basin [2] and its equivalent (Bonarelli-equivalent Level) in the Carpathian basins [3] are characterized by repeated occurrences of organic-rich and biogenic silica-rich shales. Tarfaya coastal basin vs Western Interior Seaway [9,10]) It seems that the time interval for such accumulation in the Western Tethys was similar, the stratigraphic correlation of this phenomenon is not unequivocal and easy, especially for deep environments below CCD, due to lack of calcareous microfossils. An example of such environment were marginal basins in the Carpathian domain, where the CTBI record containing the Bonarelliequivalent Level was studied in detail based on deep-water agglutinated foraminifers (DWAF), radiolarians [11], and using chemical indices (summary in [3,12,13])

Methods

Discussion

Conclusion