Integrated Astronomical Calibration of the Jurassic Time Scale
Astronomical forced cyclostratigraphy has become an important tool in defining Jurassic geologic time and establishing ‘floating’ astronomical time scales. The astronomical time scale provides much higher resolution (0.02-0.4 Myr) than the Geologic Time Scale 2004 (GTS2004), which has errors on the order of ±0.6 to ±4.0 Myr for the Jurassic. Recent work on Jurassic cyclostratigraphy reveals that 405 kyr long-eccentricity cycles dominated climate and sea-level changes through many of the Jurassic stages; obliquity-scale (~40 kyr) forcing had a somewhat lesser, although still important role. There is now also clear evidence for ~2.0 Myr long eccentricity cycles in Jurassic cyclostratigraphy. The ~2 Myr and 405-kyr eccentricity cycles are clearly visible in the Upper Jurassic Kimmeridge Clay Formation (KCF) in southern UK. The astronomical calibrated duration of the Upper KCF (lower part of the Tithonian Stage, Fittoni to Elegans zones) is 3.25 Myr, and the Lower KCF (Kimmeridgian Stage, Autissiodorensis to Cymodoce zones) is 3.63 Myr. These estimates differ significantly from GTS2004 (2.8±4 and 3.75±4 Myr), and from an earlier astronomical calibration (3.91 and 3.61 Myr). Cyclic magnetic susceptibility (MS) data in the Lower and Middle Oxfordian Terres Noires Formation of SE France indicates a duration of 4.1 Myr, comparable to 3.8 ±0.4 Myr in GTS2004; however, the data yield a duration of 2.2 to 2.4 Myr in the Mariae zone, where GTS2004 assigns only 0.6 Myr. The Pliensbachian to Aalenian stages are represented by >300 m of clay-rich marine deposits in the Paris Basin (France). MS data reveal ~2 Myr and 405-kyr eccentricity cycles in these stages, as well as short eccentricity, obliquity and precession scale cyclicity. Astronomically calibrated durations are 3.85, 7.3 and 3.0 Myr for the Aalenian, Toarcian and Upper Pliensbachian, respectively. In comparison, GTS2004 assigns 4.0±3.0, 7.4±2.0 and 4.0±1.5 Myr. The duration of the entire Pliensbachian is 4.82 Ma, based on the Belemnite Marls and Ironstone Shales (UK), and the Morbio Formation (Switzerland). The Sinemurian is only partially calibrated, 0.342 Myr for the Bucklandi zone in the Blue Lias Formation (UK). Finally, the duration of the Hettangian Stage is a minimum of 1.29 Myr interpreted from obliquity cycles in the Blue Lias Formation; lacustrine sequences in North America suggest a 2 Myr duration. To date, no astronomically forced stratigraphy has been reported for the Bajocian-Callovian stages.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009