Claudia Spalletta

Upper Devonian intraclast parabreccias interpreted as seismites

Abstract Parabreccias found across a Devonian carbonate platform-basin margin, in the Carnic region (NE Italy), coeval to extensional tectonic fragmentation are interpreted as seismites s.l. They are characterized by plate-shaped, edge-smoothed, early lithified intraformational clasts of basinal micritic limestone embedded and usually floating in a calcarenite matrix made of shallow-water debris. Such features are explained as a result of a first seismic autoclastic brecciation, due to shallow-seated earthquakes, of a thin pelagic mud layer lithified at the sediment—water interface; the brecciation was followed by disruption, small-scale transport and embedding of the clasts in a gravity-driven turbidity flow from the shallow-water platform, triggered by the same seismic event. A seismic origin of the clasts is preferred to an erosional one because of composition and morphology of the clasts, as well as of geometry of the layers (length in the order of 10–15 km, model thickness of 0.5–1 m, estimated minimum volume of 0.5–1 km 3 , relative abundance of parabreccias versus all remaining lithofacies about 70%). The suggested interpretation of the parabreccias fits very well the tecto-sedimentary evolution of the Carnic Devonian since the parabreccias appear only associated with independent evidences of synsedimentary palaeofaulting. Parabreccias of this kind, if distinct and mapped separately from polymictic “common” carbonate megabreccias of seismotectonic origin could become an additional tool for detecting timing, spacing, zonation and magnitude of palaeoearthquakes. An uncommon lithofacies of “intraclast parabreccia” interbedded with calcareous turbidites (and/or allodapic beds) within a pelagic carbonate sequence of Upper Devonian age, from the Carnic Alps (NE Italy), has been interpreted as a special type of seismite (Spalletta et al., 1983). Aim of this short note is to summarize the main features of these deposits and to discuss the evidences for such an interpretation.

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.

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 .