Stratigraphy and palaeoenvironmental analysis of the Triassic–Jurassic transition in the western Southern Alps (Northern Italy)

Abstract

High resolution stratigraphic studies were carried out on the Rhaetian–Hettangian succession of the Bergamasc Alps (western Southern Alps, Italy) in order to characterise the stratigraphic and palaeoenvironmental evolution of the Lombardian succession across the Triassic–Jurassic (T–J) boundary. The T–J boundary in the Bergamasc Alps is sandwiched between the Rhaetian carbonate ramp succession of the Zu Limestone Formation and the Malanotte Formation (a new lithostratigraphic unit introduced in this paper), deposited during relative sea-level rise in the earliest Hettangian. The Zu Limestone is organised into subtidal mixed shale-carbonate asymmetric cycles documenting the regional evolution from a distal to a proximal depositional environment of an articulated homoclinal carbonate ramp. The Malanotte Formation lacks cyclicity and shows thin bedding and a dominant micritic facies, documenting the subtidal evolution of a relatively homogenous outer carbonate ramp environment. The microfloral associations record a gradual evolutionary change at the T–J boundary, whereas the biocalcifying communities appear to be severely affected by the end-Triassic biotic crisis.