Sedimentary conditions in the Siary zone of the Magura basin (Carpathians) in the late Eocene-early Oligocene

Stanisław Leszczyński, Ewa Malata


Warunki sedymentacji w strefie Siar basenu magurskiego (Karpaty) w późnym eocenie-wczesnym oligocenie

The sediments of the upper Eocene (Priabonian)-lower Oligocene (Rupelian) o f the Siaiy zone in the Magura nappe (basically Magura Beds in glauconitic facies) display features indicating their origination in sedimentary conditions quite different from those in which coeval sediments from the other parts o f the Carpathians were deposited. This paper aims at interpreting the sedimentary conditions on the basis o f lithofacies, their vertical and lateral distribution, foraminifera assemblages, CaC03 and organic matter content determined in six transects through the entire Siary zone, the contents of main and trace elements determined in 11 samples from one section and different published data.
Lithofacies and the present day shape of the Siary zone suggest sedimentation of the entire succession in a strongly elongated confined basin. Facies distribution indicates deposition on a submarine ramp that gradually expanded to NE and E along the slopes of the outer basin margin. Assemblages o f foraminifera suggest location of the basin bottom below the foraminiferal lysocline. Moreover, foraminifera together with bioturbation structures and sediment geochemistry indicate sedimentation mainly in weakly oxic, close to dysoxic bottom waters.
The differences in facies in relation to the coeval deposits of the other parts of the Carpathians are interpreted as resulting mainly from intense resedimentation in the Siary zone during the Priabonian-Rupelian, and in part from the relatively low calcium carbonate supply. Gradual expansion o f resedimentation towards NE, followed by a decrease in the supply of coarse-grained material is regarded as due to lithosphere rollback beneath the evolving Carpathians. This process is interpreted as o f superior significance in controlling sedimentation during the Priabonian-Rupelian not only in the Siary zone but also in the entire Magura basin. This was the factor responsible for forcing the subsidence of the Magura basin, accretionary wedge development in its inner part, and fore-bulging of the area at some distance in front of the zone o f the rolling back lithosphere, i.e. in the source area of the sedimentary succession in question. Finally, this process also caused drowning and burying of the source area.

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