Carbonate microfacies and depositional environments of the Silurian-Devonian boundary strata in the Barrandian area (Czech Republic)

The sedimentary record of the Silurian-Devonian (Pridoli-Lochkovian) boundary interval was assessed using eight sections, which essentially represent two different environments and facies types. The local predominance of bioclastic packstones to grainstones indicates the presence of relatively shallow marine environments, between the upper carbonate slope and lower subtidal. The thickest limestone beds show locally wave ripple-bedding structures, erosional surfaces, and hardgrounds with accumulated angular lithoclasts; these features may have been markers of deposition above the storm wave base level. Slightly beneath these environments, the irregular rhythmites of the upper slope contain erosional channels, which are also frequently associated with hardgrounds. Here, the debris-flow material fills braided-channel systems and contains various clasts from the slope. Rhythmites deposited in relatively deeper water are dominated by platy limestones: bioclastic and peloidal wackestones, mudstones. They represent typical lower fan calciturbidites interbedded with basin-floor mudstones and shales. Very thin-bedded distal calciturbidites occur here frequently. Normal-graded calciturbidite beds with horizontal lamination and current-ripple bedding have large proportions of Tc and Td units of the Bouma sequence, the indication of lower flow-regime deposition. Calcareous shale interbeds represent background hemipelagic sediments (Tep-Tet). Occurrences of several meters thick layers composed of coarse-grained proximal calciturbidites with submarine mass-flow conglomerates are typical of the lowermost part of the Lochkov Formation (Lochkovian). These deposits were accumulated near the carbonate shelfbreak and on adjacent parts of the slope.