Pierre Bultynck

Insight into the development of a carbonate platform through a multi-disciplinary approach: a case study from the Upper Devonian slope deposits of Mount Freikofel (Carnic Alps, Austria/Italy)

The development and behavior of million year-scaled depositional sequences recorded within Palaeozoic carbonate platform has remained poorly examined. Therefore, the understanding of palaeoenvironmental changes that occur in geological past is still limited. We herein undertake a multi-disciplinary approach (sedimentology, conodont biostratigraphy, magnetic susceptibility (MS), and geochemistry) of a long-term succession in the Carnic Alps, which offers new insights into the peculiar evolution of one of the best example of Palaeozoic carbonate platform in Europe. The Freikofel section, located in the central part of the Carnic Alps, represents an outstanding succession in a fore-reef setting, extending from the Latest Givetian (indet. falsiovalis conodont zones) to the Early Famennian (Lower crepida conodont zone). Sedimentological analysis allowed to propose a sedimentary model dominated by distal slope and fore-reef-slope deposits. The most distal setting is characterized by an autochthonous pelagic sedimentation showing local occurrence of thin-bedded turbiditic deposits. In the fore-reef slope, in a more proximal setting, there is an accumulation of various autochthonous and allochthonous fine- to coarse-grained sediments originated from the interplay of gravity-flow currents derived from the shallow-water and deepwater area. The temporal evolution of microfacies in the Freikofel section evolves in two main steps corresponding to the Freikofel (Unit 1) and the Pal (Unit 2) limestones. Distal slope to fore-reef lithologies and associate changes are from base to top of the section: (U1) thick bedded litho- and bioclastic breccia beds with local fining upward sequence and fine-grained mudstone intercalations corresponding, in the fore-reef setting, to the dismantlement of the Eifelian–Frasnian carbonate platform during the Early to Late Frasnian time (falsiovalis to rhenana superzones) with one of the causes being the Late Givetian major rift pulse; (U2) occurrence of thin-bedded red nodular and cephalopod-bearing limestones with local lithoclastic grainstone intercalations corresponding to a significant deepening of the area and the progressive withdrawal of sedimentary influxes toward the basin, in relation with Late Frasnian sea-level rise. MS and geochemical analyses were also performed along the Freikofel section and demonstrate the inherent parallel link existing between variation in MS values and proxy for terrestrial input. Interpretation of MS in terms of palaeoenvironmental processes reflects that even though distality remains the major parameter influencing MS values, carbonate production and water agitation also play an important role.

Conodont biofacies of the Taghanic transgressive interval (middle Givetian): Polish record and global comparisons

Abstract Conodont biofacies of the Lublin and Łysogóry–Radom basins in SE Poland have been analysed in five cored borehole sections in a narrow interval of the middle Givetian Polygnathus ansatus Zone, corresponding to the global Taghanic transgression. Assemblages exhibiting various proportions of dominant genera, Icriodus (I) and Polygnathus (P), as well as particular P species and a few accessory taxa, reflect both temporal transgression dynamics and lateral facies changes. The latter comprise transition from a brackish lagoon with intermittent open-marine influence, to a carbonate shoal and offshore marly shelf, generally characterized by P–I biofacies, but with a varying proportion of constituent genera and polygnathid species. Comparison of the Polish record with stratigraphically well-constrained, quantitative biofacies evidence worldwide allowed the construction of a 2D nearshore–offshore model for the Euramerican epicontinental faunas connected with Taghanic transgressive facies. The I/P ratio has a diagnostic value for specific sub-environments (very nearshore/shallow-water and drowned platform) but for other settings the Polygnathus ansatus to Polygnathus linguiformis ratio appears more useful. The Moroccan faunas display specific biofacies patterns tentatively explained by different climatic conditions. The conodont biofacies concept has a limited application for palaeogeographically isolated settings, including pelagic-oceanic areas of microcontinents or submarine rises. In other cases (Eastern Australia), palaeobiogeographical bias precludes direct comparisons with the Euramerican model.

A climate-driven model using time-series analysis of magnetic susceptibility (χ) datasets to represent a floating-point high-resolution geological timescale for the Middle Devonian Eifelian stage

Abstract Presented here are cyclostratigraphic time-series data, using magnetic susceptibility (χ) results from Devonian Moroccan rocks to establish a floating-point age chronology, and a method that can be applied to any geological stage using geochemical or geophysical datasets as a climate proxy. The χ data are fit to an independent uniform climate model for the entire Eifelian Stage. The procedure used comprised: (a) definition of a uniform c. 405 kyr eccentricity climate model for the Eifelian, with a published duration for the Eifelian; and (b) graphical testing of the model using χ data derived from outcrop samples, here including data from the Global Boundary Stratotype Section and Point for the Emsian–Eifelian and Eifelian–Givetian stage boundaries, and an overlapping succession from Bou Tchrafine, Morocco. The time-series methods used here identify χ cycles that conform to the c. 405 kyr by graphically comparing the χ zonation with the climate model. Well-established conodont zonations developed using graphic correlation are then compared with this model, allowing time estimates for Eifelian conodont zone ranges. The time-series data indicate that the Eifelian Stage in the Middle Devonian lasted for c. 6.28 myr, the Lower Eifelian Choteč bio-event lasted for c. 600 kyr, and the Kačák bio-event in the Upper Eifelian lasted for c. 370 kyr.

Taxonomy and biostratigraphic significance of Icriodus orri Klapper and Barrick and related Middle Devonian conodont species

Abstract. The present study of the Eifelian icriodid conodonts is based on collections from Belarus and the Michigan Basin (USA). It is here proposed that forms originally included in Icriodus orri Klapper and Barrick, 1983 can be attributed to I. retrodepressus Bultynck, 1970, Icriodus orri sensu stricto, and Icriodus michiganus new species, each displaying a distinct morphology, stratigraphical range, and geographic distribution. Icriodus retrodepressus, characterized by a triangular spindle, deep depression in its posterior part, and a well-pronounced spur and antispur, appeared in the lower partitus Zone of the lowermost Eifelian and disappeared in the upper Eifelian kockelianus Zone. Icriodus michiganus n. sp., distinguished by a lachrymiform spindle with a shallow posterior depression, ranges from the lower costatus Zone to the upper kockelianus Zone. Icriodus orri differs from the other two species by the occurrence of transverse denticle rows with lateral denticles displaying sharp edges. It ranges from the upper kockelianus Zone to the ensensis Zone of the uppermost Eifelian. Icriodus retrodepressus first occurred in the European part of the Euramerican continent and later migrated into the North American area. Icriodus michiganus n. sp. has been found in the interior part of the North American Craton and near the eastern Euramerican margin. Icriodus orri occurs in the North American interior, in British Columbia (Canada), and in the eastern part of Euramerica (Belarus). The introduction of I. retrodepressus can be related to the transgressive Choteč Event, whereas that of I. orri to a transgressive stage of the Ie eustatic cycle.