New biostratigraphic data significantly improve the age assignment of
the Ladinian succession of Monte San Giorgio (UNESCO World Heritage List
site, Southern Alps, Switzerland), whose world-famous fossil marine
vertebrate faunas are now dated to the substage and zone levels.
High-resolution single-zircon U-Pb dating was performed using ID-TIMS
and chemical abrasion (CA) pre-treatment technique on volcanic ash
layers intercalated in the biostratigraphically-defined intervals of the
Meride Limestone. It yielded ages of 241.07 +/- 0.13 Ma (Cava superiore
beds, P. gredleri Zone), 240.63 +/- 0.13 Ma (Cassina beds, P gredleri/P.
archelaus transition Zone) and 239.51 +/- 0.15 Ma (Lower
Kalkschieferzone, P. archelaus Zone). Our results suggest that the time
interval including the vertebrate-bearing Middle Triassic section spans
around 4 Myr and is thus significantly shorter than so far assumed. The
San Giorgio Dolomite and the Meride Limestone correlate with intervals
of the Buchenstein Formation and the Wengen Formation in the reference
section at Bagolino, where the Global boundary Stratotype Section and
Point (GSSP) for the base of the Ladinian was defined. The new
radio-isotopic ages of the Meride Limestone are up to 2 Myr older than
those published for the biostratigraphically-equivalent intervals at
Bagolino but they are consistent with the recent re-dating of the
underlying Besano Formation, also performed using the CA technique.
Average sedimentation rates at Monte San Giorgio are by more than an
order of magnitude higher compared to those assumed for the Buchenstein
Formation, which formed under sediment-starved pelagic conditions, and
reflect prevailing high subsidence and high carbonate mud supply from
the adjoining Salvatore/Esino platforms. Finally, the high-resolution
U-Pb ages allow a correlation of the vertebrate faunas of the Cava
superiore/Cava inferiore beds with the marine vertebrate record of the
Prosanto Formation (Upper Austroalpine), so far precluded by the poor
biostratigraphic control of the latter.