Temporal constraints to major events of the Triassic of the Dolomites (Southern Alps, Italy)

Five new U/Pb ages obtained from Triassic ash-fall beds are presented, and preliminary data on other five layers are discussed. Adopting an improved pre-treatment of the samples (Chemical abrasion; Mattinson, 2005) and a standard, highly-precise method to determine isotope ratios from single grains (Isotope Dilution Thermal Ionization Mass Spectrometry or ID-TIMS), the calculated ages approach the nowadays maximum possible accuracy. The volcanic horizons are interbedded with marine fossiliferous sedimentary rocks, which in turn were dated by ammonoid or conodont biostratigraphies at the zonal or subzonal level. A database of 18 recent CA-ID-TIMS ages from Triassic rocks have been compiled then to apply the Maximum Likelihood fitting of a Functional Relationship method of Agterberg (2004). The method allows estimating stages and substages boundaries’ ages. The new numerical time scale, integrated by the most updated biostratigraphies for the Western Tethys has been adopted to quantify subsidence rates and carbonate production rates in the Dolomites Area. The Dolomites are an exceptional area to study Triassic carbonate buildings for many reasons; one more reason is now the exceptional control over the time variable, given by extremely evolved biostratigraphies and by availability of direct, highly precise numerical ages. These new ages independently demonstrated the enormous duration of the Late Triassic and, on the other hand, the short time required building carbonatic edifices higher than 800 m at that time. Sedimentation rates recorded in the Dolomites are then some of the higher ever recorded, but still comparable with rates recorded during the last Holocene transgression. Some hypothesis about factors controlling sedimentation rates rise from previous points. They’ve been discussed to assess a hierarchy on their importance across the Triassic.