Stratigraphical and palaeoenvironmental analysis of the Permian-Triassic transition in the Badia Valley (Souther Alps, Italy)

A sedimentological, palynological and microfaunal integrated analysis was carried out across the Permian-Triassic transition in a 35 m thick section near Seres (Badia Valley, northern Italy). This section includes the uppermost part of the Bellerophon Formation and the base of the Werfen Formation (Tesero Horizon and the lowermost Mazzin Member). The Bellerophon Formation (”badiota facies”) displays a general shallowing-upward trend. It has been subdivided into three intervals on the basis of facies, microfacies, biofacies and palynofacies associations. The Bellerophon-Werfen Formation boundary was located at the boundary between a thin black organic-rich layer and a 50 cm thick limestone bed, containing in the upper part the first oolitic beds. The palynological assemblages show throughout the Bellerophon Formation a gradual decrease of exclusive Permian palynomorphs and a concomitant increase of Triassic ranging forms. Triassic palynomorphs were found in the lower part of the Tesero Oolitic Horizon (TOH), where the Permian-Triassic transition can be recognized. Foraminifers with Triassic affinity appear just above the proposed Permian Triassic transition. This multidisciplinary approach led to evidence the palaeoenvironmental evolution which was characterized by variations in the water column oxygenation from dysoxic to oxic conditions related to climatic and/or physiographic changes influencing the water layering. Palynofacies studies show a shift from relatively distal to more proximal conditions which reached the maximum during the deposition of the black organic-rich level at the upper boundary of the Bellerophon Formation. The overlying shallowing upward cycle at the base of the TOH starts with an anoxic transgressive layer and ends with the oolitic beds.