A U-Pb age from the Toarcian (Lower Jurassic) and its use for time scale calibration through error analysis of biochronologic dating

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• 作者：Pá ; lfy ; Jó ; zsef ; Parrish ; Randall R. ; Smith ; Paul L.
• 关键词：absolute age ; U/Pb ; Toarcian ; biochronology ; calibration ; Ammonites
• 刊名：Earth and Planetary Science Letters
• 出版年：1997
• 出版时间：February, 1997
• 年：1997
• 卷：146
• 期：3-4
• 页码：659-675
• 全文大小：1.26 M

文摘

The Early Jurassic is the most poorly constrained part of the geochronologic 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 (2σ) 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 between local observed and true ranges, uncertainties of correlation, and taxonomic noise) are assessed to determine the maximum biochronologic dating error. The local ammonite distribution data are robust in that 95 % confidence level extensions of observed ranges of common ammonite taxa would not significantly modify the zonal assignment. The computer-assisted unitary association (UA) method is used for world-wide biochronologic correlation. Representative North and South American, western Tethyan, and northwest European sections were selected to establish the global maximum ranges for 103 Middle Toarcian ammonite taxa and a sequence of 40 UA. Maximum permissible correlatives of the Crassicosta Zone lie between the northwest European standard Semipolitum Subzone (late Bifrons Zone) and Bingmanni Subzone (early Thouarsense Zone). In particular, the dated tuff layer cannot be older than the Semipolitum Subzone or younger than the Illustris Subzone (Variabilis Zone). Taxonomic inconsistencies are present in the source data but their effect in corrupting UA-based correlation is shown to be negligible. The precise U-Pb age (2σ error < ± 1 % ) that is constrained at the (sub)zonal level furnishes a useful new time scale calibration point. It warrants adjustments to previous Toarcian stage boundary age estimates and allows a significant reduction of their associated uncertainties.