A revision of the classical Aalenian succession in the Middle Jurassic of San Vigilio, Lake Garda, Northern Italy

Abstract

The outcrops around Capo San Vigilio include the type horizons of many Aalenian ammonite species created by Benecke (1865) and Vacek (1886), important in interregional and interbioprovincial correlations. Their biostratigraphy at the type locality remains however highly uncertain. A reinvestigation has begun, based on three sources of information: (a) detailed lithostratigraphy at Capitello quarry and on Monte Pomo; (b) new collections made in situ; and (c) a careful re-examination of the types in the Vacek collection in Vienna. All the ammonites came from an interval 0.9 m thick, divisible into four units (the Scissum Bed, 2a-d) at the top of the San Vigilio Oolite, and a fifth (3) at the base of the overlying “Calcari a Skirroceras” in the Rosso Ammonitico Veronese. Three faunal horizons can sofar be distinguished: 2b, topmost Lower Aalenian (with Leioceras comptum and Erycites fallifax)), upper Scissum Zone sensu anglico; 2d, lowest Middle Aalenian (with Ancolioceras opalinoides)), basal Murchisonae Zone; and 3, condensed, the source of the majority of Vacek's species, ranging from middle Murchisonae Zone to Concavum Zone, near the top of the Aalenian, but no higher. A closer time-resolution within this interval should be attainable but will need detailed taphonomic analysis and further collecting. The faunal horizons of many of Vacek's species are listed, as deduced from the lithologies of his types and from newly collected topotypes.