LITHOLOGY OF THE BULLA MEMBER (BELLEROPHON FORMATION) IN THE PERMIAN-TRIASSIC BOUNDARY INTERVAL (SOUTHERN ALPS, ITALY)

The Bulla Member was recently proposed by Perri and Farabegoli as a lithostratigraphic unit of the uppermost Bellerophon Formation in the western Dolomites (Southern Alps). According to the proposers, it is separated from the overlying Werfen Formation and underlying divisions of the Bellerophon Formation by unconformities. The Bulla Member was measured and sampled for this study at two well-known localities: Bulla (Val Gardena), and Tesero (Val Fiemme). The Permian-Triassic boundary, identified by Perri via conodonts, falls within the Tesero Member of the lower Werfen Formation, ~1.3 m above the top of the Bulla Member at both localities. The Bulla Member at the Bulla section can be separated into two lithologic units. The lower unit is an ostracod- and bivalve-bearing carbonate mudstone with quartz silt and authigenic iron, distinguished from the underlying dolomitic facies of the Bellerophon Formation. The upper unit is a slightly thicker (~1 m) packstone interval with abundant algae, foraminifera, and subordinate ostracods and bivalves. The overlying basal Tesero Member of the Werfen Formation is foraminiferal packstone with abundant echinoderms, bivalves and algae, a lithology that is not markedly different from that of the Bulla Member. The Tesero Member is lithologically more variable higher in the section. The Bulla Member at the Tesero section is uniformly algal and foraminiferal packstone. It is immediately underlain by dolomudstone with quartz silt and fine sand, rare muscovite, and birdseyes. At this locality the Bulla Member is readily distinguished from oolitic grainstones of the basal Tesero Member.