Stefano Manfrin

THE PRATI DI STUORES/STUORES WIESEN SECTION (DOLOMITES, ITALY): A CANDIDATE GLOBAL STRATOTYPE SECTION AND POINT FOR THE BASE OF THE CARNIAN STAGE

The Prati di Stuores/Stuores Wiesen section (Dolomites, Italy) is proposed as a candidate Global Stratotype Section and Point for the base of the Carnian Stage. In addition to being a famous, richly fossiliferous locality, it includes the type-section of the Cordevolian substage. The section is located near Pralongia, along the southern slope of the crest separating the Badia/Abtei and Cordevole valleys. Below the levels with Trachyceras aon , the section contains a rich ammonoid fauna that chacterizes the lower part of the Regoledanus Subzone and subsequently records the first appearances of the mid-high latitude genus Daxatina ( Daxatina sp., D. cf. canadensis ) and of traditional Trachyceras with species different from T. aon . Moreover, the Daxatina cf. canadensis Subzone is recognised above the Regoledanus Subzone. Very rare conodonts of the Budurovignatus group and species of Gladigondolella from the diebeli Assemblage Zone occur. Gondolella polygnatyformis, already known from the Aon Subzone, is absent. Palynomorphs, foraminifers, gastropods, bivalves, brachiopods, microcrinoids and holothurian sclerites were studied. Variations in frequency and taxonomic diversity of these faunas suggest anaerobic-disaerobic bottom conditions for the lower-middle part of the section (0-105 m), followed by a more stable oxygen content in the upper portion. Magnetostratigraphy showed four intervals with normal polarity and three intervals with reversed polarity. The Daxatina cf. canadensis Subzone falls close to the normal polarity interval S2n. The present study proposes the FAD of the cosmopolitan genus Daxatina as a marker of the base of the Carnian Stage, placing it at a lower stratigraphic level than previously indicated in the Stuores area. The Prati di Stuores section is proposed as GSSP of the Ladinian-Carnian boundary.

SELECTED AMMONOID FAUNA FROM PRATI DI STUORES/STUORES WIESEN AND RELATED SECTIONS ACROSS THE LADINIAN-CARNIAN BOUNDARY (SOUTHERN ALPS, ITALY)

The ammonoid fauna of the Prati di Stuores/Stuores Wiesen section (Dolomites, north-eastern Italy) was studied in detail. This section was proposed as stratotype for the Carnian GSSP, with the first appearance of Daxatina cf. canadensis as primary marker, based on preliminary data. The validity of this proposal is confirmed by the present study, which integrates collections from the Prati di Stuores/Stuores Wiesen section and from others, particularly Bec de Roces (Passo Campolongo) and Antersass (Badia Valley). Ammonoids collected in these localities belong to three biostratigraphic units: regoledanus , canadensis and aon subzones. These subzones constitute, in the Southern Alps, the Ladinian/Carnian boundary interval. Regoledanus Subzone: the north American species Zestoceras enode is most probably documented in this biostratigraphic interval. Canadensis Subzone: the base is marked by the first appearance of Daxatina canadensis , and at least, another north American species, Daxatina laubei , seems to be present. Two species of Zestoceras were found: Z. barwicki e Z. lorigae sp.n. Two species o f Trachyceras , distinct from T. aon , are represented in the canadensis Subzone: T. muensteri and T. bipunctatum . These species predate the first appearance o f Trachyceras , justifying the canadensis Subzone to be included in the Carnian also on the basis of nomenclatural stability. Other significant taxa of this subzone are Rossiceras ? armatum and Sirenotrachyceras thusneldae . Aon Subzone: the ammonoid fauna of this subzone is extensively treated in existing literature and thus not further discussed. Taxonomical notes and stratigraphic consequences: the new subfamily Anolcitinae (family Trachyceratidae) is erected. At present, Frankites regoledanus , F. apertus and F. sp. A have been recognized in the Southern Alps. The lectotype of Ammonites ( Trachyceras ?) Regoledanus was defined. The latter species is exclusive of the regoledanus Subzone, whereas the distribution of the two other species encompasses the defined GSSP. The shell morphology of F. apertus and the north American species F. sutherlandi is identical. The suture line is also comparable, hence the two species are considered synonyms. This synonymy is relevant for long-distance correlations. The lectotypes o f Trachyceras ruturanum and T. laricum were also defined.

REFINED AMMONOID BIOCHRONOSTRATIGRAPHY OF THE BAGOLINO SECTION (LOMBARDIAN ALPS, ITALY), GSSP CANDIDATE FOR THE BASE OF THE LADINIAN STAGE

A refined ammonoid biostratigraphy is reported for a critical interrval of the Bagolino section (Giudicarie Area, Northern Iraly), a candidate for the Anisian/Ladinian stage boundary. The Avisianum Subzone has been documented, and its boundary with the Crassus Subzone has been redefined, on the basis of new findings of Reitziites reitzi, Aplococeras aff. smithi, Aplococeras avisianum, Halilucites rusticus, Parasturia sp. and other significant ammonoids. Six potential criteria for the definition of the Anisian/Ladinian boundary can now be considered in this single stratigraphic section, so that the FO of Eoprotrachyceras curionii is no longer the unescapable choice at Bagolino. Despite several drawbacks, including the absence of palynomorphs and daonellid bivalves, the thermal history preventing the recovery of magnetic and isotopic signals, and a lower sedimentation rate with respect to many other sections of the Southern Alps, the Bagolino section is here accepted as stratotype for the Anisian/Ladinian boundary, but further paleontological and sedimentological studies should be considered.