Ammonoids of the middle/late Anisian boundary (Middle Triassic) and the transgression of the Prezzo Limestone in eastern Lombardy-Giudicarie (Italy)

Abstract

This study documents ammonoids with a precise stratigraphic control at the middle/late Anisian (Pelsonian/Illyrian) boundary from a new locality in eastern Lombardy-Giudicarie (Monte Guglielmo) and from classical sections in Giudicarie. These ammonoid faunas allow revising the taxonomic interpretation of Ceratites cimeganus Mojsisovics 1882 and of the genus Paraceratites Hyatt 1900. Ceratites cimeganus is here assigned to the North American genus Rieppelites Monnet & Bucher 2005. In eastern Lombardy-Giudicarie, R. cimeganus is diagnostic of a distinct biochronological unit (cimeganus Zone) bracketed between the older Bulogites zoldianus Zone and the younger Judicarites euryomphalus-Paraceratites trinodosus zones. The recognition of this cimeganus Zone significantly improves worldwide correlation since it is recognized in several other Tethyan basins (Dolomites, Northern Calcareous Alps) as well as in North America (Nevada). These new data allow a redefinition of the middle/late Anisian boundary in the western Tethys, which is here intercalated between the zoldianus and cimeganus zones. This limit is marked by a clear ammonoid turnover (e.g. disappearance of Acrochordiceras and Balatonites, appearance of Rieppelites). Finally, the presence of sections including the cimeganus Zone in eastern Lombardy-Giudicarie allow the establishment of local gaps in sedimentation, which may reflect the regional and important transgression of the pelagic Prezzo Limestone over the shallow water platform carbonates of a "Camorelli-Dosso dei Morti barrier", as also underlined by the spatial distribution of brachiopod lumachellas. © 2008 Birkhaueser.