THE TRIASSIC-JURASSIC ECOSTRATIGRAPHIC TRANSITION IN THE LOMBARDIAN ALPS, ITALY

Three paleoenvironmental phases and two declines in diversity characte rize the Late Triassic to Early Jurassic history of the Lombardian Pla tforms. The first phase, of Late Triassic time (?Choristoceras Zone), consists of 1-5 m thick shallowing-upward subtidal cycles of molluscan , coralline, and echinoderm wackestone and packstone of the Zu Limesto ne. Biotic and ecostratigraphic characteristics such as typical Rhaeta vicula contorta fauna and facies allow correlation to the Kossen Forma tion of the Northern Calcareous Alps. The second phase, of latest Tria ssic time (?upper Choristoceras Zone), consists of shallow restricted marine or peritidal carbonates of the Conchodon Formation dominated by barren lime mudstone and dolostone, algal laminites, and oolitic grai nstone. The Zu-Conchodon transition predates the Triassic-Jurassic bou ndary and represents the first, and most severe, diversity decline for the Lombardian fauna corresponding to a fall in sea-level. Where obse rved, the upper and lower contacts of the Conchodon Formation are conf ormable and do not constitute sequence boundaries as suggested by some workers. The Lower Jurassic (?Psiloceras Zone) Sedrina Limestone mark s the beginning of the third phase with the onset of transgression and return of normal marine conditions. Typical microfacies include mollu scan, echinoderm, and sponge wackestone and packstone with abundant an omuran microcoprolites. The second diversity decline occurred at, or j ust above, the Triassic-Jurassic boundary at the Conchodon-Sedrina tra nsition, where the remaining restricted marine forms disappeared with the transgression. Anoxia was not a factor in this decline in diversit y, although other mechanisms in addition to sea-level change cannot be discounted.