Astronomical calibration of the Jurassic time-scale from cyclostratigraphy in British mudrock formations

Three British Jurassic mudrock formations have been investigated, via time–series analysis, for evidence of sedimentary cyclicity related to orbital–climatic (Milankovitch) cyclicity: the Blue Lias, the Belemnite Marls and the Kimmeridge Clay Formation.Magnetic–susceptibility measurements through the Blue Lias (uppermost Triassic to Sinemurian) were used to generate high–resolution time–series. The data indicate the presence of a regular sedimentary cycle that gradually varies in wavelength according to sedimentation rate. Tuning of this cycle to the 38ka Jurassic obliquity cycle produces spectral evidence for two additional regular cycles of small amplitude. These correspond to the 95 ka component of orbital eccentricity and the 20 ka orbital–precession cycles. Cycle counting allowed the minimum duration of four ammonite zones to be estimated and the duration of the Hettangian stage is estimated to be at least 1.29 Ma. Calcium carbonate measurements through the Belemnite Marls (lower Pliensbachian) are characterized by two scales of cyclicity that can be firmly linked to orbital–precession (20 ka) and the 123 ka component of eccentricity. A time–scale has been developed from the precession–related sedimentary cycles, with cycle counts used to constrain the duration of two ammonite zones. In the Kimmeridge Clay Formation (Kimmeridgian–Tithonian), magnetic–susceptibility measurements made on exposures, core material and down boreholes can be correlated at the decimetre scale. Only measurements of magnetic susceptibility made below the Yellow Ledge Stone Band (midway through the formation) are suitable for analysis of the bedding–scale cyclicity. A large–amplitude sedimentary cycle detected in the lower part of the formation is probably related to the orbital–obliquity cycle (38 ka). In certain stratigraphic intervals, there is evidence for small–amplitude cycles related to orbital precession (20 ka).This study of the British Jurassic shows that, in the Rhaetian–Sinemurian, the dominant cyclicity was related to obliquity. In the Pliensbachian this had shifted to dominantly precession, and in the Kimmeridgian obliquity again dominated. These shifts in cycle dominance presumably reflect changing local or global palaeoclimatic and/or palaeoceanographic conditions.