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.