TOWARDS A BETTER DEFINITION OF THE ANISIAN LADINIAN BOUNDARY - NEW BIOSTRATIGRAPHIC DATA AND CORRELATIONS OF BOUNDARY SECTIONS FROM THE SOUTHERN ALPS

Stratigraphic sections of basinal ''Buchenstein Beds'' in the northwes tern Dolomites and eastern Lombardy are correlated on the basis of the distribution of macrofossils (ammonoids, Daonellas) and volcaniclasti c layers. In Lombardy, these strata are bracketed by other Middle Tria ssic basinal sediments (Prezzo Lst., ''Wengen Beds''), and the entire succession hosts a clear macrofossil (ammonoids, Daonellas) record ran ging from the Late Anisian to the Late Ladinian. In the Anisian/Ladini an boundary interval this record appears to be relatively coherent whe n compared to equivalent standard sections in the western Tethys area. It allows the full integration of rich faunas from isolated localitie s in coeval platform carbonates (Latemar and Cernera) and intra-platfo rm deposits (Monte San Giorgio). The combined series of fossils includ es successive levels with key species of Judicarites, Paraceratites, K ellnerites, Hungarites, Reitziites, Parakellnerites, Aplococeras, Tici nites, Halilucites, Stoppaniceras, Nevadites, Chieseiceras, Eoprotrach yceras, Arpadites and Protrachyceras among other ammonoids and Daonell as. The non-condensed ammonoid succession is suitable for a partial re vision of the Tethyan zonal subdivision. It also indicates a slightly, but distinctly diachronous base of the ''Buchenstein Beds'' or its si ngle members. Thus the original location of the Anisian/Ladinian bound ary at the base of the ''Buchenstein Beds'' (Bittner 1892) is ambiguou s. The boundary between the Nevadites Zone and the Curionii Zone is at present the best constrained alternative. Not only can this marker be pinpointed in stratigraphic columns in the Southern Alps (i.e. the or iginal ''type-area'' of the stage boundary) but it can also be traced to sections further afield in western Tethys and in North America. Rad iometric age data are available on volcaniclastic rocks within the stu died sections but do not yet allow a conclusive correlation of the Lad inian Stage with the numerical time-scale. The best estimate at presen t is from 232 to 225 Ma. These values are in conflict, however, with o ther current estimates. The recognition of corresponding levels in sta rved basinal sediments and carbonate platforms with clear large-scale architectures promises a detailed calibration and comparison of the in dividual platform to basin evolution histories. For the Upper Anisian to Ladinian platforms in the western Dolomites our data suggest a peri od of initially rapid but then decreasing up- and outbuilding at Latem ar (Reitzi/Kellnerites Zone to Gredleri Zone) followed by a short phas e of distinct lateral progradation (late stage at Rosengarten). Even h igher rates of creation of accommodation space at Cernera in the centr al Dolomites prevented any significant progradation of this platform b efore its ultimate drowning close to the Anisian/Ladinian boundary. Ba sed on new fossil collections the generic and specific assignments of several ammonoids and Daonellas are revised in the paleontological par t. This includes a description of the new genera Reitziites and Latema rites and of the new species Kellnerites bagolinensis n. sp., Latemari tes latemari n. sp., Parakellnerites zoniaensis n. sp., Ticinites bres cianus n. sp., Ticinites dolomiticus n. sp., Stoppaniceras evolutum n. sp., Nevadites avenonensis n. sp., Nevadites bittneri n. sp., Nevadit es secedensis n. sp., Nevadites crassiornatus n. sp. and Daonella cern eraensis n. sp., Daonella sotschiadensis n. sp.