Carlo Corradini

The early siphonodellids (Conodonta, Late Devonian-Early Carboniferous): overview and taxonomic state

Previous conodont studies on the Devonian/Carboniferous (D/C) boundary revealed that the current definition of the D/C boundary cannot be maintained due to several problems relating to the index taxon. The base of the Carboniferous System is defined by the first occurrence of the cono- dont species Siphonodella sulcata, within the S. praesulcata - S. sulcata lineage. However, problems in discriminating S. praesulcata from S. sulcata have been evidenced by the conodont studies of a variety of authors. To provide an overview about the early siphonodellids and the taxonomic state, conodonts from the current GSSP La Serre (Montagne Noire, France), as well as all data from the literature were taken into account. Based on these data, several morphotypes could be distinguished, although the great majority of specimens show intermediate features between the two taxa. There- fore, precise identification of such specimens is very much dependent on personal interpretation, thus strongly influencing the precise position of the D/C boundary which, as a result, is not synchronous. Alternative scenarios for the definition of the D/C boundary need to be considered in the future.

PROTOGNATHODUS (CONODONTA) AND ITS POTENTIAL AS A TOOL FOR DEFINING THE DEVONIAN/CARBONIFEROUS BOUNDARY

The current definition of the Devonian/Carboniferous boundary is the first occurrence of the conodont Siphonodella sulcata . Due to difficulties in identification of the early siphonodellids, such as S. praesulcata and S. sulcata , investigation of Protognathodus which enters in the latest Devonian and extends into the Mississippian, was undertaken to determine use as a better indicator of the base of the Carboniferous. During the D/C boundary interval, Protognathus is represented by four species: Pr. meischneri , Pr. collinsoni , Pr. kockeli and Pr. kuehni. Although Pr. kockeli can be abundant in boundary interval sections, none of the four Protognathodus species has a high potential as a tool for redefining the D/C boundary, based on regional variation in first occurrence data, restricted stratigraphic ranges and global distribution, poorly understood facies occurrences, as well as general rarity of the taxa.

Palaeoenvironmental changes at Col des Tribes (Montagne Noire, France), a reference section for the Famennian of north Gondwana-related areas

We present detailed biostratigraphy based on conodonts and palaeoenvironmental trends deduced from microfacies and conodont abundance through the Famennian (Late Devonian) at Col des Tribes (Montagne Noire, France). The succession is characterized by micritic limestones deposited in settings oscillating between mid to outer ramp. Facies contain poor fauna, widely dominated by nektonic organisms. This section is complete and one of the most conodont-rich for the Famennian of the north Gondwana-related area. The Upper Kellwasser event (Frasnian–Famennian boundary) and the Hangenberg (Devonian–Carboniferous boundary) have been lithologically identified. They are characterized by decimetre-thick black dysoxic to anoxic argillaceous sediments. The Condroz and annulata events, although not materialized by lithological changes, have been positioned due to the precise stratigraphy. The first event occurred during the deposition of condensed ferruginous facies (griotte limestones) and the second event during the deposition of micrites barren of benthic fauna. The combination of information from both facies and conodont biofacies changes allows a general sea-level curve through the entire Famennian for north Gondwana to be proposed for the first time. At Col des Tribes, the general trend is a slight deepening upwards from triangularis to trachytera zones, then a pronounced shallowing-upwards trend from upper trachytera to praesulcata zones. This curve correlates with the well-known reference curve from Euramerica concerning the late Famennian ( trachytera to praesulcata Zones). There are some discrepancies in minor cycles which can be explained by tectonical phenomena at the onset of the edification of the Variscan belt in Europe.

Late Ludfordian Correlations and the Lau Event

Changes in whole conodont faunas and δ13C values are combined to achieve high-resolution correlations of Upper Silurian successions in many areas (primarily Gotland, Skane, Lithuania, Bohemia, Austria, Sardinia and Queensland); other areas are correlated with lower precision. Four of the widely recognised subzones average considerably less than 0.1 Ma and a fifth interval less than 0.01 Ma. The main constraints on resolution and precision now achievable are the amount of, and the precision in, new or previously published data from each local section; centimetric scale collecting would be worthwhile in many sections. Some stratal characters are more widespread during certain intervals than might have been expected, for example, the presence of oncolites and algal coatings in the Icriodontid Zone and the lower part of the O. snajdri Zone. Similarly, the Dayia navicula bloom in the Upper P. siluricus Subzone was widespread. So also were muddy-sandy sediments followed by oolite low in the O. snajdri Zone; crinoids flourished widely when the lower part of the Lower Icriodontid Subzone was formed. Closely spaced samples show that, in some intervals, the now well-known δ13C spike was modified considerably by fluctuations; that is, it is not a smooth plateau. The best-documented fluctuations (19 analyses) are 2.5 smooth cycles with up to 2.7‰ in amplitude during ca 12,000 years or less in the base of the O. snajdri Zone. An enigma is the depletion of the spike in some sections, especially in the best Bohemian section.

Famennian Chondrichthyan Microremains from Morocco and Sardinia

Abstract New material from the Famennian of Morocco in the southern Maider comprises chondrichthyan teeth (Thrinacodus, Cobelodus, Denaea, Stethacanthus), actinopterygian remains (scales, teeth and hemilepidotrichium) and one acanthodian scale. The absence of crushing teeth suggests deeper water environments for the Maider Basin than the Tafilalt Basin. Vertebrate microremains from the Famennian of Sardinia, including Siamodus and Jalodus teeth, are illustrated and described. Ichthyofaunal relationships of the North Gondwanan platform during the Famennian are examined.

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.

UPPER DEVONIAN CONODONT BIOSTRATIGRAPHY OF SHAMS ABAD SECTION, KERMAN PROVINCE, IRAN

The study of the conodont fauna from the Upper Devonian Shams Abad section provides new data on the stratigraphy of the Bahram Formation in the Kerman province, in southeastern Iran. The fauna includes twenty-four species, mainly belonging to genera Polygnathus and Icriodus , and gives evidence of a late Frasnian to middle Famennian age, in contrast to previous interpretation of the same unit. The shallow water conodont biofacies confirms an inner shelf to foreshore shallow marine depositional environment.

Conodonts across the Devonian/Carboniferous boundary: a review and implication for the redefinition of the boundary and a proposal for an updated conodont zonation

This paper is a contribution to the redefinition of the base of Carboniferous system. At present the criterion for the definition of the Devonian–Carboniferous boundary is the first occurrence of a conodont species. In order to evaluate the stratigraphic potential for new criteria for the definition of the Devonian–Carboniferous boundary, the distribution of conodont species of Bispathodus, Branmehla, Palmatolepis, Polygnathus, Protognathodus, Pseudopolygnathus and Siphonodella across the boundary is presented and discussed. An updated biozonation scheme across the boundary based on the First Appearance of Bispathodus ac. aculeatus, Bispathodus costatus, Bispathodus ultimus, Protognathodus kockeli, Siphonodella bransoni and Siphonodella duplicata is proposed, and it is suggested that the new criterion for the definition of the base of the Carboniferous system be the First Appearance Datum of Pr. kockeli or Si. bransoni .

Conodont and crinoid stratigraphy of the upper Silurian and Lower Devonian scyphocrinoid beds of Tafilalt, southeastern Morocco

Three sections (Atrous 3, Atrous 7 and Bou Tchrafine N2) in the “Scyphocrinites Limestones” of Tafilalt (southeastern Morocco) have been investigated for conodont and crinoid stratigraphy. Conodonts allow to discriminate four biozones (Ozarkodina eosteinhornensis s.l., Lower Oulodus elegans detortus, Upper O. elegansdetortus, and Icriodus hesperius) of Pridoli and the lowermost Lochkovian. Cirrus loboliths occur only in the Pridoli, whereas plate loboliths are present in the uppermost Silurian and the basal Devonian; the two types overlap in the uppermost Silurian. As for calices, Scyphocrinites and Carolicrinus occur in the whole Pridoli; Camarocrinus and Marhoumacrinus are present across the Silurian–Devonian boundary and in basal Devonian beds.

Graphic correlation of the Sardinian Ockerkalk (upper Silurian): Implications on the conodont biostratigraphy

Abstract Eight sections representing the upper Silurian of SE Sardinia have been correlated using Graphic Correlation. New data combined with already published data allow high-resolution correlations between the sections and a subdivision of the upper Silurian of SE Sardinia into 24.7 Composite Standard Units (CSU). The well-known Silius section was chosen as reference section for the studied area. Correlations of the SE Sardinian composite standard with sections outside the studied area (Cellon in the Carnic Alps, Klonk in Bohemia and Mason Porcus in SW Sardinia) show an earlier first occurrence of Oulodus elegans detortus in Sardinia; therefore, the use of this taxon as marker for interregional correlations is to be discouraged.