| Summary: | Current time scales disagree on age estimates for Jurassic stage boundaries and carry large uncertainties. The
U-Pb or ⁴⁰Ar-³⁹Ar dating of volcaniclastic rocks of precisely known biochronologic age is the preferred method to
improve the calibration.
Eighteen new U-Pb zircon dates were obtained on volcaniclastic rocks from Jurassic volcanic arc
assemblages of the North American Cordillera. The volcanic rocks are interbedded in fossiliferous marine
sedimentary rocks, which in turn were dated by ammonite biochronology at the zonal level. In the Queen Charlotte
Islands, a tuff layer directly beneath the Triassic-Jurassic boundary was dated at 200±1 Ma. Lowermost Jurassic
(middle and upper Hettangian) volcaniclastic rocks from Alaska yielded ages of 200.8+2.7/-2.8 Ma, 197.8±1.0 Ma,
and 197.8+1.2/-0.4 Ma. An ash layer near the base of the Middle Toarcian Crassicosta Zone in its type section in
the Queen Charlotte Islands was dated at 181.4+1.2 Ma. The other new dates furnish additional time scale
calibration points.
The biostratigraphy of measured sections contribute to the understanding of North American Jurassic
ammonite successions, especially for the Hettangian of Alaska. In an attempt to quantify biochronologic correlation
uncertainty, the computer-assisted Unitary Association method was used to correlate a Toarcian North American
regional ammonite zone with the northwest European standard. The correlation uncertainty between the two
regions is not more than ±1 standard subzone.
A radiometric age database consisting of 50 U-Pb and ⁴⁰Ar-³⁹Ar ages was compiled to construct a revised
Jurassic time scale. Apart from the newly obtained U-Pb ages, several recently reported Cordilleran dates are
included with revised biochronologic ages. Additional dates were compiled from previous time scales and recent
literature.
Only a few boundaries are dated directly. The chronogram method was applied for the first time to estimate
all Early and early Middle Jurassic chron boundaries, as well as late Middle Jurassic substage boundaries and Late
Jurassic stage boundaries. The most significant improvement concerns the Pliensbachian and Toarcian, where six
consecutive chron boundaries were determined. The derived chron durations vary between 0.4 and 1.6 Ma. Their
disparities argue against the assumption of equal duration of chrons or subchrons, which was used as a basis for interpolation in several previous time scales. Interpolation based on magnetochronology improved chronogram
estimates for the latest Jurassic, where the isotopic database remains too sparse.
The initial boundaries of Jurassic stages are proposed as follows: Berriasian (Jurassic-Cretaceous):
144.8 +2.61-2.1 Ma; Tithonian: 151.5 +1.0/-1.4 Ma; Kimmeridgian: 154.7 +1.0/-0.9 Ma; Oxfordian:
156.6 +2.0/-2.7 Ma; Callovian: 159.7 ±1.1 Ma; Bathonian: 166.0 +0.6/-5.4 Ma; Bajocian: 174.0 +1.0/-7.3 Ma;
Aalenian: 178.0 +1.0/-1.5 Ma; Toarcian: 183.6+1.6/-1.1 Ma; Pliensbachian: 192.0 +3.8/-5.2 Ma; Sinemurian:
197.0 +1.2/-4.2 Ma; Hettangian (Triassic-Jurassic): 201±1 Ma.
The revised time scale was used to assess the timing of mass extinction events and subsequent biotic
recoveries. Marine and terrestrial extinctions at the end of the Triassic both occured at about 201 Ma, later than
suggested in most previous time scales. During the Pliensbachian-Toarcian mass extinction, elevated extinction
rates were sustained between 185.7 and 181.4 Ma and apparently followed by an immediate recovery. The
hypothetical 26 Ma extinction periodicity is plausible between the end-Permian and end-Triassic, between the
Pliensbachian-Toarcian and Callovian, and among the Tithonian, Cenomanian-Turonian, and end-Cretaceous
events. However, the spacing of the end-Triassic and Pliensbachian-Toarcian events is less than 20 Ma,
significantly less than predicted by the periodicity model. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === Graduate
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