DEVONIAN MARINE INCURSIONS INTO THE ORCADIAN BASIN, SCOTLAND

BP/Chevron well 14/6-1 drilled in the East Orkney Basin has penetrated Devonian sedimentary rocks which can be dated palynologically as with in the latest Givetian to early Frasnian interval. This microflora con tains the first substantiated record in Western Europe of Archaeoperis accus, a spore hitherto regarded as restricted to northern Laurasia. T hree grey-green intervals within the well contain marine microfossils which provide the first direct evidence for the movement of marine wat ers into the northern part of the Old Red Sandstone continent. Onshore an Eday Marls section in the Bay of Berstane, Orkney contains a marin e microfauna which demonstrates the presence of a marine incursion of mid-late Givetian age. This is the first record of a Devonian marine i ncursion in Scotland. Combination of this palaeontological data with s edimentological studies of the Eday Marls and Upper Old Red Sandstone suggests that a wide flat sabkha plain extended eastwards from the ons hore Orcadian Basin during the later stages of its infill, and was sub jected 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 in terval of 14/6-1, and are interpreted as resulting from orbital cyclic ity (at periodicities of 39.5 and 413.9 ky). This indicates that the c ontrol on the Devonian sea level maxima may have been orbital cyclicit y. 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 Rhines treet, Middlesex and Genundewa transgressions. Reconsideration of the palaeogeography of the northern margin to the marine Devonian indicate s that the sea entered the Orcadian Basin from the east along the Torn quist Zone at the margin of the Fenno-Scandian High.