The Middle Old Red Sandstone (mid Devonian) of the Orcadian Basin in northern Scotland is composed predominantly of cyclic lacustrine deposits. The cyclicity recorded reflects regular fluctuations in lake levels that have been attributed to climatic variability resulting from regular variations in the Earth's orbital parameters (Milankovitch cycles). However, previous estimates and calculations of the periodicities of these events have produced highly variable results, and no consistent approach has emerged. Variability in the estimated thicknesses of the sequences and regular revisions of the Devonian timescale, combined with poorly defined absolute dating of stratigraphic boundaries, have hindered accurate calculations of cycle periodicities.
This study reviews the factors involved in the calculation of published periodicities for the lacustrine cycles of the Orcadian Basin and considers the effect of recent revisions to the Devonian timescale and stratigraphic thicknesses. When considered alongside detailed examination of well exposed sections from Easter Ross, Caithness and Orkney, the calculation of cycle periodicities provides consistent results reflecting the dominance of Precessional (19 886 years) and modulating Eccentricity (100 000 years) cycles calculated for the Devonian.