Genetic significance of the Cretaceous black and red shales from the Eastern Carpathians (Romania)

In the outer nappe systems of the Eastern Carpathians, namely the Moldavids, marine Upper Cretaceous red sediments overlay the Lower Cretaceous organic-rich black shales. The oldest parts of the black shale units are composed of Upper Valanginian-Upper Barremian hemipelagic and pelagic muddy siliciclastic rocks and carbonate muds, commonly intercalated with fine-grained turbidites. These features indicate an abyssal plain setting. 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. This assumption is based on the occurrence of mainly hemipelagic sediments, with a few thin turbiditic intercalations.
   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. A continuous decreasing of the basin depth is to be assumed. The presence of the 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 of 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, composed of radiolarites intercalated with variegated shales, pyroclastic tuffs and thin sandstones, is interpreted as a hemipelagic and pelagic sedimentation in an 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 marls and mudstones, indicating a decreasing of the basin depth.
   The accumulation of the 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 during the Late Albian time, the restricted circulation 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.