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1 Department of Geology, Southampton Oceanography Centre, Empress Dock, European Way, Southampton SO14 3ZH, UK
2 Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK Present address: 27 Athole Gardens, Glasgow G12 9BB, UK
3 Murphy Eastern Oil Company, Winston House, Dollis Park, Finchley, London N3 1HZ, UK
BP/Chevron well 14/6–1 drilled in the East Orkney Basin has penetrated Devonian sedimentary rocks which can be dated palynologically as within the latest Givetian to early Frasnian interval. This microflora contains the first substantiated record in Western Europe of Archaeoperisaccus, a spore hitherto regarded as restricted to northern Laurasia. Three grey-green intervals within the well contain marine microfossils which provide the first direct evidence for the movement of marine waters into the northern part of the Old Red Sandstone continent.
Onshore an Eday Marls section in the Bay of Berstane, Orkney contains a marine microfauna which demonstrates the presence of a marine incursion of mid––late Givetian age. This is the first record of a Devonian marine incursion in Scotland.
Combination of this palaeontological data with sedimentological studies of the Eday Marls and Upper Old Red Sandstone suggests that a wide flat sabkha plain extended eastwards from the onshore Orcadian Basin during the later stages of its infill, and was subjected to episodic marine incursions. These incursions are tentatively linked with marine transgressions into the Argyll area of the Central North Sea.
Two orders of cyclicity occur within the dated Devonian interval of 14/6–1, and are interpreted as resulting from orbital cyclicity (at periodicities of 39.5 and 413.9 ky). This indicates that the control on the Devonian sea level maxima may have been orbital cyclicity. These marine incursions are speculatively correlated to high-stands on the Devonian sea-level curve. The Bay of Berstane marine incursion representing the Taghanic Onlap whilst those in 14/6–1 are the Rhinestreet, Middlesex and Genundewa transgressions. Reconsideration of the palaeogeography of the northern margin to the marine Devonian indicates that the sea entered the Orcadian Basin from the east along the Tornquist Zone at the margin of the Fenno-Scandian High.
Keywords: Old Red Sandstone, transgression, cycles, palaeogeography.
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