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

A set of 28 ammonite biohorizons or faunal assemblages is proposed for the Sinemurian, the Pliensbachian and the lowermost Toarcian of the Brescian Prealps, in part based on the published data of the authors of this contribution and partly on new results, derived both from recent field investigations and from the study of the historical collection of Lower Jurassic ammonites preserved in the Museum of Natural Sciences of Brescia (Northern Italy). The biohorizons are present in the Liassic carbonate succession of the Brescian Prealps, cropping out between the eastern surroundings of Brescia (Botticino), to the east, and Lake Iseo, to the west. Since the Hettangian this region was subjected to Jurassic rifting. The area of study was located on the eastern border of the wide Lombardian Basin, a part of the southern continental passive margin of Tethys. An articulated fault-system, trending from Brescia to the North, separated the western subsiding area of the Val Trompia-Sebino Basin from the eastern Botticino structural high. After the drowning of the Rhetian-Hettangian Corna Platform, the very thick synrift succession of the Medolo Group accumulated in the Val Trompia-Sebino Basin, whereas the coeval reduced sequence of the Rezzato Encrinite and the overlying Botticino Corso Rosso covered the Botticino High, subsequent to the Early Sinemurian. The ammonite biohorizons and assemblages recognised are quite well integrated and correlable with either the NW European standard zonation or the different zonations proposed for the Tethyan Realm.