Abstract

Detailed sedimentological examination of onshore sections through cyclic lacustrine deposits of the Middle Devonian succession of northern Scotland has been augmented with offshore data to allow the construction of a continuous 2.3 myr record of orbital forcing. These data provide an important record of climatic variance through the Devonian, which, by comparison with other records from this time, can significantly improve our understanding of the Devonian global climatic system. Precessional and eccentricity cycles are shown to be dominant by the direct measurement and extrapolation of depositional rates in well-exposed sections as well as the analysis of ratios between primary and modulating cycles. Spectral analysis has confirmed the presence of regular cycles throughout the offshore data, which provide a far longer record of orbitally forced cyclicity than previously available. The presence of a half precession cycle is also suggested. Similar up-section variations in cycle thicknesses are recorded from both the onshore and offshore succession. These variations and those in cycle symmetry are related to a trend from an underfilled to a balanced-fill basin state. Of further importance in controlling cycle thickness is the period of each cycle during which lacustrine conditions, and therefore lake level controlled accommodation, existed.

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