Depositional versus ecological control on the conodont distribution in the Lower Frasnian fore-reef facies, Holy Cross Mountains, Poland

Abstract

Lithofacies and conodont biofacies of reef-slope deposits of the Wietrznia Beds (Lower Frasnian Palmatolepis transitans Zone) were studied in three measured sections at the Wietrznia quarry (Holy Cross Mts. Poland). The diverse reef-derived grained deposits interrupted the background sedimentation that included lime mudstones, deposited largely at 50 to 90m depths in oxygen-depleted middle slope to toe of slope settings. The mainly storm-controlled proximal gradient is laterally marked by gradual changes from coarse-grained tempestites, represented by flat-pebble conglomerate fabric, to diluted muddy tempestites. The mostly sparse polygnathid and mesotaxid–polygnathid biofacies characterize the hemipelagic facies, whilst polygnathid–ancyrodellid biofacies dominates in the event layers. Three-step taphofacies scheme is proposed, distinguishing settled (parautochthonous), residual (winnowed) and displaced (settled from waning currents and/or transported in sediment-gravity flows) conodont element accumulations. The primary ecological signals are still partly preserved in most of the tempestite-derived assemblages, indicating scarcity of truly residual, lag-like taphofacies. The hydraulic sorting was most effective in the middle-slope setting affected by unidirectional and/or oscillatory flows. Syndepositional mixing was strengthened upslope by an importation of off-reef mesotaxid faunas due to storm surge onto the reef. This reworking pattern as a key factor in biofacies interpretation may be applicable only on a local scale. However, similarly biased conodont signature is probably common in other hurricane-affected carbonate complexes, even if without a disrupting impact on major biofacies belts.