Early Badenian transgression on the outer flank of Western Carpathian Foredeep, Hluchov area, Czech Republic

Abstract

This multidisciplinary study, based on borehole cores from the Hluchov area in Czech Republic, documents an early Badenian marine transgression on the outer flank of the Western Carpathian Foredeep. The shallow-marine deposits represent coastal transgression over a terrestrial topography of weathered pre-Cenozoic bedrock. The lower facies association (FA1) consists of siliciclastic sediment derived from local substrate erosion. Facies indicate a wave-dominated environment with unstable bottom, variable rate of sediment supply and an incremental rise of relative sea level. The upper facies association (FA2) consists of carbonates indicating a major landward shift of shoreline, decline in siliciclastic input and further sea-level rise. The succession represents a transgressive to highstand systems tract.

The maximum flooding surface, ca. 1 m above the FA1/FA2 boundary, is signified by an anomalous decrease in K and Th, an increased Th/K ratio and highest U concentration. The heavy-mineral assemblages in FA1 confirm local sediment provenance, whereas those in FA2 indicate broader sediment derivation, including volcanic component from contemporaneous rhyolitic to rhyodacitic eruptions. The deposits contain a wide range of marine fauna, with the foraminifers and molluscs indicating an early Badenian age. Molluscs, bryozoans and echinoderms indicate a normal-salinity environment with a decreasing hydraulic energy. Foraminifers indicate salinity fluctuations in the lowest part of the succession. The isotopic composition of mollusc shells shows marked inter-species differences and a general negative shift in the δ¹³C and δ¹⁸O values, indicating diagenetic alteration. The impact of diagenetic processes appears to have been controlled by sedimentary facies. The highly negative δ¹³C and δ¹⁸O values correspond to sediment layers with the highest Th/K ratios and hence low clay content. Sediment permeability was thus probably crucial in controlling the differential circulation and impact of diagenetic fluids.