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Stacked Gilbert-type deltas filling an incised palaeovalley along the cratonward margin of a foreland basin (Miocene, Western Carpathian Foredeep)

Nehyba Slavomír

Geological Quarterly

2022

Vol. 66, No. 3

art. no. 28

The coarse-grained Brno sands are an important groundwater aquifer and represent a volumetrically significant component of the Lower Badenian/Middle Miocene sedimentary infill of the Western Carpathian Foredeep, a foreland basin formed on the eastern margin of the Bohemian Massif in front of the Western Carpathians. The integration of core and outcrop facies analysis, summary of facies features, depositional processes and architectures, and integration of provenance analysis have led to a new depositional model for the Brno sands as the product of Gilbert-type delta systems prograding into an incised palaeovalley developed on the outer foreland basin margin. The position, shape and morphology of the palaeovalley were influenced by pre-Neogene basement faults trending NW–SE. The Gilbert-type deltas recognized can be classified as foreset-dominated with other elements subordinate. The foreset beds mainly reflect low-density turbidites, with varied but important evidence of deposits of high-density turbidity currents, cohesionless debris flows and debris falls. The thick foreset deposits show, in detail, repetitive successive prograding-retrograding episodes. These are interpreted as rapid high-frequency pulses of relative sea level change i.e. as high-frequency depositional sequences. The deltaic deposition of the Brno sands was terminated by deposits of a shoal-water delta and the sands were finally drowned by a thick pile of offshore pelagic clays. The provenance of Brno sands is located into the nearby geological units. Redeposition from the older basin infill also played an important role.