Pacific Spreading Rates during Middle Jurassic through Early Cretaceous: Astronomical Cycle-Derived Durations of M-Sequence Polarity Chrons

High-resolution cyclostratigraphic correlation from magnetic susceptibility was performed on multiple Lower-Middle Oxfordian and Lower Kimmeridgian hemipelagic marl to pelagic marl-limestone successions in southeastern France.  The oscillations of magnetic susceptibility are consistent among the overlapping sections, which have well-delimited ammonite zones and subzones.  Spectral analysis reveals the complete suite of orbital frequencies (precession, obliquity, and short- and long-eccentricity).  The most distinctive features are a dominant envelope of 405 kyr long-eccentricity in all intervals and precession-driven marl-limestone couplets within the Kimmeridgian. An independent array of magnetostratigraphic studies of the full succession of the Oxfordian through Tithonian ammonite zones in France, Poland and Britain enabled a correlation to the M-sequence of marine magnetic anomalies.  The assignment of marine-derived magnetic chrons to the magnetostratigraphic pattern in the Callovian-Oxfordian boundary interval was greatly improved by the cycle-scaling, which implied a much longer time span than is normally indicated by Jurassic time scales.  The cycle-derived durations for the associated sets of magnetic chrons (M28D-M37n of Early-Middle Oxfordian, and M24r-M25r of Early Kimmeridgian) imply a near constant Pacific spreading rate through this Late Jurassic interval.  The same spreading rate is implied by previously published cyclostratigraphic studies of lowermost Cretaceous (equivalent of M16n-M16r), and by a trio of basement radiometric ages from ODP sites on Middle Jurassic through earliest Cretaceous marine magnetic anomalies.  Cyclostratigraphic studies of mid-Early Cretaceous (Valanginian-Barremian) ammonite-zoned sections and corresponding scaling of magnetic chrons imply that Pacific spreading rates slowed after the Berriasian.  The apparent constancy of Pacific spreading rates from Bathonian through Berriasian (ca. 20 myr) enables magnetostratigraphic correlation of geologic events, including stage-boundary definitions, to be projected onto a well-constrained M-sequence time scale, similar to how the C-sequence provides a reliable calibration of timing of Cenozoic events.