Orbitally Induced Small-Scale Cyclicity in a Siliciclastic Epicontinental Setting (Lower Lias, Yorkshire, UK)

The Yorkshire Lower Lias consists essentially of fine-grained sediments, in the clay to fine silt size range. The occurrence of several types of shellbed, siliciclastic layers and concretionary horizons makes it possible to distinguish five stratigraphically distinct facies, which can be interpreted in terms of specific depositional environments. One of these facies is the Banded Shales unit, which covers most of the jamesoni ammonite zone, and consists of 64 couplets of regularly alternating darker and lighter grey layers (layer thickness 10-70 cm), in general with plane gradational contacts. The layers consist mainly of quartz silt, clay, black and white micas, calcite and organic matter. Regular variation in the relative concentration of these components accounts for the rhythmically bedded nature of the unit. The clay mineral and total organic carbon (TOC) distributions also show evidence for a longer term variation covering four to five couplets. To investigate the presence of cyclicity, spectral analysis has been applied to three geochemical time series (isothermal remanent magnetization, TOC, Si/Al ratio) all covering the bottom 10 m (=20 couplets) of the Banded Shales unit, and to a gamma-ray log of the adjacent Felixkirk Borehole. Within the dating limitations of the ammonite zonation scheme it seems possible to distinguish the precession, obliquity and eccentricity cycles of the Earth's orbit. Based on this evidence it is suggested that orbitally forced climatic changes influenced the depositional system at two levels: (i) short-term variations expressed on a couplet scale, affecting the storm frequency and perhaps magnitude; and (ii) longer term variations affecting weathering and clay production in the source area. The temporal control established by the recognition of precession frequencies allows us to suggest that Jurassic ammonite subzones are markedly unequal in duration.