Cretaceous anoxic–oxic changes in the Moldavids (Carpathians, Romania)

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Abstract

This study focused on the Cretaceous black shale successions, followed by red shales that crop out at the outer regions of the Romanian Carpathians, in the Moldavids. The oldest parts of the black shale units deposited in an abyssal plain during Late Valanginian–Late Barremian time; they are mainly characterized by hemipelagic and pelagic muddy siliciclastic rocks and carbonates, commonly intercalated with fine-grained turbidites.

During the sedimentation of the middle part of the black shale units in the Late Barremian–Early Albian interval, the depth of the basin increased, as the carbonate hemipelagic sedimentation was replaced by a mainly siliceous one. Only a few thin turbidite intercalations are present.

The youngest part (Albian pro parte) of the black shale units is characterized by a turbiditic sedimentation, with mainly sandy sequences of middle and lower deep-water fans. We may assume that the depth of the basin continuously decreased. The presence of authigenic glauconite in the Albian sandstones suggests a palaeoenvironmental change, linked to the occurrence of oxygenated turbidity current circulation.

A significant shift in the sedimentation regime in the Eastern Carpathian Moldavids took place in the Late Albian, when Cretaceous Oceanic Red Beds (CORB) occurred. This type of sedimentation lasted up to the Coniacian. The lower part of the CORBs that contains radiolarites intercalated with variegated shales, pyroclastic tuffs and thin sandstones is interpreted as a hemipelagic and pelagic sedimentation in the abyssal plain environment, where rarely turbidites occurred. Upwards, there are mainly burrowed variegated red and green shales. The youngest parts of CORBs are characterized by increased thickness and frequency of the turbidites. While the main part of the CORB is carbonate free or has very low carbonate content, the upper part of these strata becomes rich in marl and mudstone strata, indicating a decrease of the basin-depth.

The accumulation of black shales in the Eastern Carpathians during the Late Valanginian–Late Albian interval is linked to the existence of a small, silled basin of the Moldavian Trough, in which restricted circulation led to the density stratification of the water column, resulting in the deposition of anoxic Lower Cretaceous sediments (i.e., the black shales). Because of the tectonic deformation that took place within the Lower–Upper Cretaceous boundary interval, the restricted circulation had changed to an open circulation regime in the Moldavian Trough. Hence, the anoxic regime was progressively replaced by an oxic one, across the Albian–Cenomanian boundary interval. The beginning and the end of the CORBs in the Moldavid units depend thus on various palaeogeographic and palaeoenvironmental settings, and it was controlled by the regional tectonic activity.

Introduction

The transition from Cretaceous anoxic to oxic environments was observed in many Tethyan areas (Jansa et al., 1979, Arthur & Premoli Silva, 1982, Hu et al., 2005, Jansa & Hu, 2009), including the Carpathians (Švábenická et al., 1997, Bąk, 1998, Michalík et al., 2002, Wang et al., 2005, Melinte-Dobrinescu et al., 2009, Skupien et al., 2009). In the Carpathian mountain belt, the sedimentation of the Upper Cretaceous Oceanic Red Beds (referred as CORB by Hu et al., 2005) follows the Lower Cretaceous black shale deposition. It appears that the development of Cretaceous black shales and CORBs is somehow causally related (Hu et al., 2006). The anoxic–oxic changes may reflect various Earth processes, such as palaeoceanographic shift, tectonic movements, and/or climatic fluctuations that changed the balance between carbon sources and sinks in the world ocean (Wang et al., 2005, Hu et al., 2009).

Colour of sedimentary rocks is an important indicator of environmental conditions in which they have formed. Gray and black shale occurrence is commonly linked to increases in organic matter content, and low oxygen content in bottom waters. In turn, CORB indicate low organic matter content, but well-oxygenated bottom conditions.

Numerous dark-coloured, commonly laminated, organic-rich sediments occur within Mesozoic sequences; they are frequently associated with Upper Jurassic to Lower Cretaceous organic carbon enriched claystones, clayey sandstones and limestones (Wignall, 1994, Pancost et al., 2004). Such beds that are more than 1-cm-thick and contain more than 1% of total organic carbon (TOC) are described as black shales (Pettijohn, 1957, Stow et al., 2001). In general, the term of black shales is used for any dark-colored fine-grained organic carbon rich sediment, in which TOC contents typically range from 1 to 15%. Many black shales are hemipelagites; some, such as black cherts and organic rich limestones, are pelagites; others are fine grained turbidites (Stow et al., 1996). Black shales are also characterized geochemically by high Fe2+ and S2 contents and enrichments in trace elements, such as Ba, Bi, Cr, Ni, Mo, V, Zn (Nijenhuis et al., 1999, Lipinski et al., 2003). The sedimentation of black shales is mostly ascribed to oxygen-poor bottom waters, typically accumulated in deep-ocean basin setting, but they may also occur in shallower sites (Wignall and Newton, 2001).

During the mid-Cretaceous, red oxic sediments, with low content of preserved organic carbon, replaced the deposition of organic-rich black shales in many of the Tethyan areas (Wang et al., 2005). This type of sedimentation is found from low to middle palaeolatitudes up to the end of the Cretaceous (Arthur & Premoli Silva, 1982, Hu et al., 2005, Melinte & Jipa, 2005, Hu et al., 2006). Probably, CORB sedimentation is a response to a palaeoclimatic and/or palaeoceanographic fluctuations, but also is effected by tectonic activity and synorogenic depositional processes (Hu et al., 2005, Wagreich & Krenmayr, 2005, Neuhuber et al., 2007, Melinte-Dobrinescu et al., 2009). The occurrence of CORB may also be linked to changes in bioproductivity, triggered by major palaeoceanographic modifications of the world oceans.

In this paper, we will discuss stratigraphy and examine genetic relationship between Lower Cretaceous organic-rich black shales deposition and overlaying Upper Cretaceous oceanic red beds from the eastern part of the Romanian Carpathian region. We also comment on causes of the Cretaceous anoxic–oxic changes as seen in the Moldavid nappes of the Eastern Carpathians.

Section snippets

Geological setting

The Eastern Carpathians represent a segment (over 600 km long) of the Carpathian tectonic chain. Inwards (westwards), this mountainous chain is bordered by the Transylvanian Basin and the easternmost part of the Pannonian Basin, while outwards (eastwards), it is bordered by the Moldavian and Scythian Platforms (at the E) and by the Moesian Platform (towards SE and S). The Eastern Carpathians are mainly composed of Jurassic–Miocene sedimentary rocks, deposited in several basins, folded and

Materials and methods

We have studied several sections located in the Inner and Outer Moldavids, displaying a continuous transition from organic-rich black shales to CORB. However, we have selected to present herein only three of the most representative sections, located at the southern and central parts of the Eastern Carpathians, in the Audia and the Tarcău nappes. The most complete investigated section is placed in the Audia Nappe, being located in the Carpathian Bend region (Bota River, tributary of the Buzău

Organic-rich black shales

In the southernmost investigated section, located in the Audia Nappe (A in Fig. 1), the Audia Formation, 190 m in thickness, displays all three members of the black shale strata (Fig. 2, Fig. 3), which are from the base to the top:

  • (i)

    The Lower Member, 80 m in thickness, that contains at its lower part mainly laminated black shales. The upper part is composed of decimetre-thick rhythmically alternating lithic and sublithic sandstones, siltstones and dark grey shales. The organic rich black shales

Black shales

The laminated organic-rich black shales are the main lithological component of the Upper Valanginian–Upper Albian Audia Formation. They also occur, within the Albian–Cenomanian boundary interval, as cm levels interbedded in the CORB units of the Moldavids. These shales are composed of phyllosilicates, quartz and subordinately siliceous bioclasts (radiolarian and sponge spicules); they also include carbonate bioclasts with foraminifera, bivalve and echinids (Grigorescu, 1971; Papiu &

Conclusions

In the Eastern Carpathians, black shales intercalated with carbonates (marls and micrites) firstly appeared in the Valanginian, with the latter rocks progressively diminishing up to the Aptian. During the Late Valanginian–Late Barremian interval, carbonate hemipelagites and pelagites, with rare siliceous rocks, accumulated in the Moldavids. These sediments of an abyssal plain are interbedded with thin distal siliciclastic and bioclastic turbidites. Taking into account the presence of the green

Acknowledgments

We thank to Dan Jipa (National Institute of Marine Geology and Geo-ecology, GEOECOMAR Bucharest) for the fruitful discussion and comments on an earlier version of this paper. The authors are also indebted to Titus Brustur and Stefan-Andrei Szobotka (National Institute of Marine Geology and Geo-ecology, Bucharest), for the assistance during field trips in the Eastern Carpathians. This paper is a contribution to the IGCP Project 555 ‘Rapid Environmental/Climate Change in the Cretaceous Greenhouse

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      The paleosetting is mainly anoxic in the Moldavide nappes and subordinately dysoxic, as grey shales with cm-scale turbidites are rarely encountered, typical for basin plain and mud-rich slope apron systems. Anoxic sedimentation in the Outer Moldavide nappes, such as at Tarcău and Vrancea (Figs. 10A, B, 11, 12), started in the Hauterivian (Melinte-Dobrinescu and Roban, 2011), as in some parts of the Inner Moldavides, e.g., Audia Nappe, where the black shales had accumulated since the Hauterivian (Alexandrescu, 1966, 1971; Ion, 1975; Melinte-Dobrinescu et al., 2009). We assume that the deep basin of the Moldavides was mostly fed by fluvial systems from the Eastern Carpathian Foreland, reworked on the shelf (Fig. 10A, B).

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