Compositional variations and patterns of conodont reworking in late Devonian and early carboniferous calciturbidites (Moravia, Czech republic)

Compositional variations and grain-size properties of both carbonate consti tuents and conodonts as an alternative component group were used for interp reting the processes governing the deposition of upper Famennian and middle Tournaisian calciturbidites in Moravia, Czech Republic. Both the compositi on and grain-size properties of conodont element associations showed to be markedly dependant on facies type of their host sediment. Upper Devonian ca lciturbidite successions deposited on flanks of wide, Moravian - Silesian c arbonate 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-now breccias) wit h abundance of shelf- and shelf margin conodont taxa and epipelagic and "me sopelagic" conodonts. Upper Devonian calciturbidites deposited on slopes of volcanic sea-mounts are composed of echinoderm- and peloid-rich wacke/pack stones and float/rudstones with increased proportion of intraclasts and vol canigenic lithoclasts (fine-grained calciturbidites, normal calciturbidites ), yielding abundant conodont associations with higher proportion of "mesop elagic" taxa compared to the platform-flank examples. Middle Tournaisian ca lciturbidite succession composed of crinoid-, peloid-, intraclast- and lith oclast-rich lime mudstones, wacke/packstones and float/rudstones (normal ca lciturbidites and debris-flow breccias) yielded conodont element associatio ns rich in shelf- and shelf-margin taxa, "mesopelagic" conodonts and rework ed Middle- and Upper Devonian conodonts. In general, the ratio of shelf- and shelf margin conodont taxa to "mesopela gic" taxa is distinctly lower in fine-grained calciturbidites than it is in normal calciturbidites and debris-flow breccias. Grain-size properties (me an grain size and sorting) and percentage of fragmented conodont elements, too, are markedly dependant on the facies type: in fine-grained calciturbid ites the values of mean grain-size and fragmentation are low and the sortin g is good to very good whereas in normal calciturbidites and debris-flow br eccias the values of mean grain-size and fragmentation are distinctly highe r and the sorting is poorer. The interdependence of facies type and composi tion and grain-size properties of conodont element associations in gravity- flow deposits is explained as resultant from hydrodynamic sorting during tu rbidity current now and final deposition of the bed. Compositional variatio ns observed in our sections may thus be attributed to facies variability (c oarsening- and thickening-upward trends) rather than to sea-level fluctuati ons (highstand shedding of carbonate platforms). On the other hand, signifi cant enrichment in reworked conodont taxa in middle Tournaisian normal calc iturbidites compared to scarcity and/or absence of such conodonts in essent ially identical facies of upper Famennian age indicate sea-level to be the major control governing such compositional variations, with low relative se a-level stand in middle Tournaisian and high relative sea-level stand in up per Famennian. Thorough analysis of conodont evolution, palaeoecology and t aphonomy, with emphasis on understanding the processes of deposition of the ir host rock, are recommended for any biostratigraphic and biofacies study to be done in carbonate sediments deposited under strong hydrodynamic regim es, such as calciturbidites, tempestites, debris-flow deposits, shelf-edge oolitic sands, tidal-channel facies etc.