Sedimentary cycles, ammonites and stratigraphic gaps: measuring the length of a Jurassic Stage using the Blue Lias Formation
  • Upper Lecture Theatre, Sherwell Centre

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The School of Geography, Earth and Environmental Sciences organises a regular series of research seminars throughout the academic year to which everyone is welcome to attend. Speakers - both external and internal to the University - will talk on topics related to all aspects of Earth Sciences.

Today's speaker is Dr Graham Weedon from the Met Office.

The Blue Lias Formation spans the uppermost Triassic and part of the Lower Jurassic. Deposition was in quiet water with alternations of marls with dark, laminated shales resulting from climatic cycles producing relatively dry and wet conditions. The climate associated with light marls included rare storms that caused increased bottom-water turbulence and pauses in deposition and/or hiatuses. The pauses promoted early diagenetic carbonate cementation and the formation of limestone nodules and beds.

High-resolution magnetic-susceptibility logs for four sections have been combined with the exceptionally detailed ammonite collections of Kevin Page (Plymouth). Standard spectral analysis of the logs, demonstrates several scales of regular cyclicity in depth. Bayesian probability spectra provide independent confirmation of at least one scale of regular cyclicity at each locality. The frequency ratios between the different scales of cyclicity are consistent with astronomical forcing of climate at the periods of the short eccentricity, obliquity and precession cycles. Orbital tuning has been used to construct local time scales. A composite time scale was constructed using the tuned time scales plus ammonite biohorizon limits allowing the integration of the local stratigraphic gaps. These time scales indicate that the Hettangian Stage lasted close to four million years.

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