Compostional variations and patterns of condont reworking in Late Devonian and early carboniferous calciturbidites (Moravia, Czech Republic)

Summary Compositional variations and grain-size properties of both carbonate constituents and conodonts as an alternative component group were used for interpreting the processes governing the deposition of upper Famennian and middle Tournaisian calciturbidites in Moravia, Czech Republic. Both the composition and grain-size properties of conodont element associations showed to be markedly dependant on facies type of their host sediment. Upper Devonian calciturbidite successions deposited on flanks of wide, Moravian-Silesian carbonate platform are composed mainly of echinoderm-and peloid-rich wacke/packstones and intraclastic float/rudstones (fine-grained calciturbidites, “normal” calciturbidites with Tab Bouma sequences, debris-flow breccias) with abundance of shelf-and shelf margin conodont taxa and epipelagic and “mesopelagic” conodonts. Upper Devonian calciturbidites deposited on slopes of volcanic sea-mounts are composed of echinoderm-and peloid-rich wacke/packstones and float/rudstones with increased proportion of intraclasts and volcanigenic lithoclasts (fine-grained calciturbidites, normal calciturbidites), yeilding abundant conodont associations with higher proportion of “mesopelagic” taxa compared to the platform-flank examples. Middle Tournaisian calciturbidite succession composed of crinoid-, peloid-, intraclast-and lithoclast-rich lime mudstones, wacke/packstones and float/rudstones (normal calciturbidites and debris-flow breccias) yielded conodont element associations rich in shelt-and shelf-margin taxa, “mesopelagic” conodonts and reworked Middle-and Upper Devonian conodonts. In general, the ratio of shelf-and shelf margin conodont taxa to “mesopelagic” taxa is distinctly lower in finegrained calciturbidites than it is in normal calciturbidites and debris-flow breccias. Grain-size properties (mean grain size and sorting) and percentage of fragmented conodont elements, too, are markedly dependant on the facies type: in fine-grained calciturbidites the values of mean grain-size and fragmentation are low and the sorting is good to very good whereas in normal calciturbidites and debris-flow breccias the values of mean grain-size and fragmentation are distinctly higher and the sorting is poorer. The interdependence of facies type and composition and grain-size properties of conodont element associations in gravity-flow deposits is explained as resultant from hydrodynamic sorting during turbidity current flow and final deposition of the bed. Compositional variations observed in our sections may thus be attributed to facies variability (coarsening-and thickening-upward trends) rather than to sea-level fluctuations (highstand shedding of carbonate platforms). On the other hand, significant enrichment in reworked conodont taxa in middle Tournaisian normal calciturbidites compared to scarcity and/or absence of such conodonts in essentially identical facies of upper Famennian age indicate sea-level to be the major control governing such compositional variations, with low relative sea-level stand in middle Tournaisian and high relative sea-level stand in upper Famennian. Thorough analysis of conodont evolution, palaeoecology and taphonomy, with emphasis on understanding the processes of deposition of their host rock, are recommended for any biostratigraphic and biofacies study to be done in carbonate sediments deposited under strong hydrodynamic regimes, such as calciturbidites, temperstites, debris-flow deposits, shelf-edge oolitic sands, tidal-channel facies etc.