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Cretaceous oxic-anoxic changes in the Romanian Carpathians.

Domenii publicaţii > Ştiinţele pământului şi planetare + Tipuri publicaţii > Articol în revistã ştiinţificã

Autori: Melinte-Dobrinescu, M.C. and Roban, R.-D.

Editorial: Elsevier, Sedimentary Geology , 235, p.79-90, 2011.

Rezumat:

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.

Cuvinte cheie: Creatcic; sisturi negre; strate rosii marine; sedimentologie; bisotratigrafie. // Cretaceous; black shales; marine red beds; sedimentology; biostratigraphy.