Abstract
AbstractThe lithostratigraphic characteristics of the iconic Blue Lias Formation of southern Britain are influenced by sedimentation rates and stratigraphic gaps. Evidence for regular sedimentary cycles is reassessed using logs of magnetic susceptibility from four sites as an inverse proxy for carbonate content. Standard spectral analysis, including allowing for false discovery rates, demonstrates several scales of regular cyclicity in depth. Bayesian probability spectra provide independent confirmation of at least one scale of regular cyclicity at all sites. 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. Using local tuned time scales, 62 ammonite biohorizons have minimum durations of 0.7 to 276 ka, with 94% of them <41 ka. The duration of the Hettangian Stage is ≥2.9 Ma according to data from the West Somerset and Devon/Dorset coasts individually, increasing to ≥3.7 Ma when combined with data from Glamorgan and Warwickshire. A composite time scale, constructed using the tuned time scales plus correlated biohorizon limits treated as time lines, allows for the integration of local stratigraphic gaps. This approach yields an improved duration for the Hettangian Stage of ≥4.1 Ma, a figure that is about twice that suggested in recent time scales.
Highlights
The Blue Lias Formation comprises alternating centimetre- to metre-scale beds of homogeneous light grey limestone; light grey marl associated with homogeneous light grey limestone nodules; dark grey marl; and black, organic-rich laminated shales associated with nodules of very dark grey laminated limestone (Hallam, 1960; Weedon, 1986)
It was demonstrated that hiatuses are prevalent throughout the formation in southern Britain, as inferred from field observations and from graphic correlation using the locations of numerous ammonite biohorizons (Weedon et al 2018)
If the biohorizon boundaries as located in the Blue Lias Formation can be treated as reliable approximations to time lines, the anomalously reduced relative thicknesses combined with the reduced numbers of inferred 100 ka cycles between common biohorizon levels can be explained in terms of local stratigraphic gaps or sedimentologically condensed intervals
Summary
The Blue Lias Formation (uppermost Triassic and Lower Jurassic) comprises alternating centimetre- to metre-scale beds of homogeneous light grey limestone; light grey marl associated with homogeneous light grey limestone nodules; dark grey marl; and black, organic-rich laminated shales associated with nodules of very dark grey laminated limestone (Hallam, 1960; Weedon, 1986). Weedon et al (1999) obtained a high-resolution log of volume magnetic susceptibility at Lyme Regis with a fixed stratigraphic spacing of measurements at 2 cm at Lyme Regis (as utilized here and by Weedon et al 2018) These data were used as an inverse proxy for calcium carbonate content and were subjected to spectral analysis in a series of subsections, rather than as one long record, in order to produce stationary time series by allowing for long-term (ammonite-zone scale) changes in accumulation rate. Paul et al (2008) noted ‘bundles’ or groups of limestone beds and groups of laminated limestone beds in the lowest part of the Blue Lias Formation at Lyme Regis (Tilmanni and Planorbis zones) They inferred, without time-series analysis, that the bundles represent 100 ka eccentricity cycles. The vol MS data from the lowest 9 m of the Blue Lias in St Audries Bay, Somerset, show a close, inverse correspondence to the high-resolution calcium carbonate content record of Clémence et al (2010)
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