Paper No. 2
Presentation Time: 8:15 AM
LATE JURASSIC TIME SCALE: INTEGRATION OF AMMONITE ZONES, MAGNETOSTRATIGRAPHY, ASTRONOMICAL TUNING AND SEQUENCE INTERPRETATION FOR TETHYAN, SUBBOREAL AND BOREAL REALMS
Several inter-disciplinary teams have enabled the assembly of a composite astronomically-scaled Late Jurassic time scale with implications for oceanic spreading rates and origin of depositional sequences:
- Intercalibration of the Boreal, Sub-Boreal and Tethyan faunal realms. Standardization of zonal definitions for each faunal realm with recognition of inter-regional incursions of marker taxa. The detailed magnetic polarity patterns enhance the correlation of subzones among faunal realms.
- Durations for each ammonite zone and most subzones. Expanded sections in basal settings display a signature of superimposed cyclic oscillations. Durations for ammonite zones/subzones in Oxfordian, Kimmeridgian, Volgian, and portions of the Berriasian are derived from the distinctive 405 kyr eccentricity signal.
- Determination of spreading rates for the M-sequence of marine magnetic anomalies. The cycle-scaled magnetic polarity pattern of the Oxfordian through Berriasian can be matched to the “M-Sequence” derived from Pacific marine magnetic anomalies. Spreading rates for the Hawaiian spreading center gradually slowed from an early Oxfordian rate of 29 km/myr to the middle Berriasian with 26 km/myr.
- Quantifying offsets of regional usage of “base-Kimmeridgian” and other stage/substage boundaries. The combined ammonite-magnetic time scale indicates that the proposed base-Kimmeridgian GSSP (base of the Pictonia baylei Zone of Subboreal ammonite province) is equivalent to a level within the middle of the E. bimammatum Zone) in the Sub-Mediterranean standard zonation, or 1.3 myr older than its traditional placement in that Sub-Mediterranean zonation.
- Many of the sedimentary sequences correspond to 405-kyr eccentricity cycles. The Oxfordian shelfal successions in England have pronounced facies changes and/or recognized hiatuses within their ammonite biostratigraphy. These interpreted depositional sequences display a consistent relationship to the shifts in clay/carbonate content in SE France that correspond to the 405-kyr eccentricity orbital-climate oscillation.
Acknowledgements: Many colleagues contributed to this synthesis, including Angela Coe (Open Univ., UK), Ewa Glowniak (Univ. Warsaw, Poland), Juan Herrara (Purdue Univ., USA), and John Wright (Royal Holloway, UK).