Global correlations of mid Early Triassic events: The Induan/Olenekian boundary in the Dolomites (Italy)

The Dolomites (Southern Alps, Italy) are a reference-area for research on the end-Permian mass extinction and its Early Triassic aftermath. The effects on shallow marine benthic biota are recorded in the Werfen Formation, a thick mixed carbonate-siliciclastic sedimentary succession. Only in its lower (Griesbachian) and upper (Spathian) parts, this formation is bio-chronologically constrained by means of conodonts and ammonoids, whilst no significant bioevent occurs in its middle part. This represents an impediment to the biochronologic recognition of the Induan/Olenekian boundary (IOB). The Bulla/Pufels (Val Gardena) succession is a key-section for the P/T boundary and Early Triassic for global correlation due to the abundance of studies on biostratigraphy (mostly on conodonts), magnetostratigraphy and chemostratigraphy carried out there by stratigraphers of various nationalities. Recent chemostratigraphic studies have permitted the recognition of some carbon isotope positive peaks, the strongest of which is considered to approximate the IOB. However, various authors have reached different conclusions on the position of the maximum peak and thus on the IOB location. This leads to important stratigraphic consequences for the calibration of conodont biostratigraphy. The critical revision of the traditional stratigraphic units (litho- and biostratigraphy), under-evaluated in most of the recent literature, and magneto-, chemo- and sequence stratigraphic units allowed herein an integrated stratigraphic scale for the Bulla/Pufels section to be proposed. This contribution highlights the mid Early Triassic Dolomites record for regional and global correlations. The most significant results attained herein regard the different lithostratigraphic subdivisions of the middle Werfen Formation and its consequences on the position of the IOB with respect to the conodont and bivalve biostratigraphy and sequence stratigraphic units. The upper part of the section is attributed herein to the Gastropod Oolite Member, which is represented by the lithozone A, a predominant supratidal episode, and the lower part of the subtidal lithozone B. Between the lithozones A and B, a sequence boundary of 3th order (Sc2/Sc3) is located. The maximum carbon isotope excursion is near this boundary, which therefore approximates the IOB in the Dolomites. This proposal suggests a Dienerian age for the FO of the conodont P. obliqua, which occurs about 60 m below the stage boundary. No significant biotic event, either for molluscs or conodonts, occurred across this stage boundary.