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Sinemurian to lowermost Toarcian ammonites of the Brescian Alps (Southern Alps, Italy): biostratigraphical framework and correlations

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Meister C. , Schirolli P. , Dommergues J.-L.

tom Vol. 4, no. 1

184-185

A set of 28 ammonite biohorizons or faunal assemblages can be proposed for the Sinemurian, the Pliensbachian and the lowermost Toarcian in the Brescian Alps, in part based on the data from Dommergues et al. (1997) and partly on new results, deriving both from recent field investigations and from the study of the historical collection of the Lower Jurassic ammonites preserved in the Museum of Natural Sciences of Brescia (Northern Italy), placed into their stratigraphical framework (Fig. 1). The biohorizons are referred to the Liassic carbonate succession of the Brescian Alps, outcropping between the eastern surroundings of Brescia (Botticino), to the East, and the Lake Iseo, to the West. Since the Hettangian the region was subjected to the paroxismal phase of the Jurassic rifting and the area of study was located in the eastern border of the wide Lombardian Basin, a part of the southern continental passive margin of the Tethys. At the beginning of Jurassic, an articulated fault-system, composed of N-S master faults (W-dipping) and W-E transfer faults, located from Brescia to the North, separated a western growing basinal area from the eastern Botticino structural high. Since Hettangian to the onset of Toarcian, almost 1000 m of well-stratified cherty marly limestones of the Medolo Group (Gardone Val Trompia Limestone and Domaro Limestone formations) represent the synrift deposition of the Sebino Basin, following the drowning of the Raethian-Hettangian Corna Platform. In contrast, after the Early Sinemurian a coeval reduced sequence (50 m thick), composed by the Rezzato Encrinite (Lower Sinemurian to Upper Carixian) and the overlying thinly-bedded and nodular ammonitic marly limestones of the Botticino Corso Rosso (Upper Carixian and Domerian), covered the Corna Platform in the Botticino High. The recognized ammonite biohorizons and assemblages are quite well integrated and correlable with either the NW European standard zonation or the different zonations given for the Tethyan realm (Appennines, Subbeticas and Austrian and Hungarian Upper Austroalpine). Most of these stratigraphical units are based on Tethyan taxa. Only two horizons (U. cf. jamesoni horizon and P. solare horizon) show NW European affinites. Moreover the E. quenstedti horizon is only known from the northern margin of the Tethys, mainly from the Upper Austroalpine units.