A U-PB AGE FROM THE TOARCIAN (LOWER JURASSIC) AND ITS USE FOR TIME-SCALE CALIBRATION THROUGH ERROR ANALYSIS OF BIOCHRONOLOGIC DATING

The Early Jurassic is the most poorly constrained part of the geochron ologic scale within the Mesozoic, owing to the scarcity of calibration points. We report a new U-Pb zircon age of 181.4 +/- 1.2 Ma(2 sigma) from an ash layer within the type section of Crassicosta Zone, a North American regional standard ammonite zone in the Middle Toarcian. For time scale calibration, the isotopically dated level needs to be tied to the Jurassic chronostratigraphic framework established in northwest European successions. Three major contributing factors (difference be tween local observed and true ranges, uncertainties of correlation, an d taxonomic noise) are assessed to determine the maximum biochronologi c dating error. The local ammonite distribution data are robust in tha t 95% confidence level extensions of observed ranges of common ammonit e taxa would not significantly modify the zonal assignment. The comput er-assisted unitary association (UA) method is used for world-wide bio chronologic correlation, Representative North and South American, west ern Tethyan, and northwest European sections were selected to establis h the global maximum ranges for 103 Middle Toarcian ammonite taxa and a sequence of 40 UA. Maximum permissible correlatives of the Crassicos ta Zone lie between the northwest European standard Semipolitum Subzon e (late Bifrons Zone) and Bingmanni Subzone (early Thouarsense Zone), In particular, the dated tuff layer cannot be older than the Semipolit um Subzone or younger than the Illustris Subzone (Variabilis Zone). Ta xonomic inconsistencies are present in the source data but their effec t in corrupting UA-based correlation is shown to be negligible. The pr ecise U-Pb age (2 sigma error < +/-1%) that is constrained at the (sub )zonal level furnishes a useful new time scale calibration point. It w arrants adjustments to previous Toarcian stage boundary age estimates and allows a significant reduction of their associated uncertainties.