The Triassic-Jurassic ecostratigraphic transition in the Lombardian Alps, Italy

Abstract

Three paleoenvironmental phases and two declines in diversity characterize the Late Triassic to Early Jurassic history of the Lombardian Platforms. 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 Limestone. Biotic and ecostratigraphic characteristics such as typical Rhaetavicula contorta fauna and facies allow correlation to the Kössen Formation of the Northern Calcareous Alps. The second phase, of latest Triassic time (?upper Choristoceras Zone), consists of shallow restricted marine or peritidal carbonates of the Chonchodon Formation dominated by barren lime mudstone and dolostone, algal laminites, and oolitic grainstone. The Zu-Conchodon transition predates the Triassic-Jurassic boundary and represents the first, and most severe, diversity decline for the Lombardian fauna corresponding to a fall in sea-level. Where observed, the upper and lower contacts of the Conchodon Formation are conformable and do not constitute sequence boundaries as suggested by some workers. The Lower Jurassic (? Psiloceras Zone) Sedrina Limestone marks the beginning of the third phase with the onset of transgression and return of normal marine conditions. Typical microfacies include molluscan, echinoderm, and sponge wackestone and packstone with abundant anomuran microcoprolites. The second diversity decline occurred at, or just above, the Triassic-Jurassic boundary at the Conchodon-Sedrina transition, where the remaining restricted marine forms disappeared with the transgression. Anoxia was not a factor in this decline in diversity, although other mechanisms in addition to sea-level change cannot be discounted.