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The Structure of the West Orkney Basin, Northern Scotland

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The Structure of the West Orkney Basin, Northern Scotland Journal of rhe Geological Society, London, Vol. 144, 1987, pp. 871-884, 17figs. Printed in Northern Ireland The Structure of the West Orkney Basin, northern Scotland M. A. ENFIELD & M. P. COWARD Department of Geology, Imperial College, London SW72BP, UK Abstract: The West Orkney Basin developed in Devonian times, as the western part of the Orcadian intermontane basin. It has been studied using commercial speculative seismic reflection dataand the MOIST deep seismic data. The NW edge of the West Orkney Basin is formed by listric faults which are also strongly arcuate in plan, while the SE part is composed of straight domino-type faults which formedparallel to earlier (Caledonian) layering in thebasement. Fault restoration and balancing suggest that initial extension in the basin occurred on low-angle reactivated Caledonian thrust faults. Steeper breaching faults cut the low-angleset, forming planar (domino-type) faults in the centreof the basin but listric faults at the NW margin. The maximum extension is about 45% in the basin centre, most of this being taken up on the later breaching fault system. This extension decreases to the SW, where fault tips occur on-shore, but some may transfer to fault systems in the Minches. The faults apparently decouple at a depth of18-20 km and the extension suggests an initial post-Caledonian crustal thickness of up to 40 km. However, the sedimentary thickness is an averageof only 3 km in the basin centre, much less than would be expected had the lithosphere thinned homogeneously, and there is no evidenceof a thermal subsidence phase to the basin. This suggests that the extension shown by the West Orkney Basin was transferred to lower lithospheric levels to the east along the deep decoupling zone. The Devonian sediments on-land show facies changes and periods of uplift and erosion which may be relatedto extension during basin development. They also show a phase of pre-Late-Permian tectonic inversion where the beds are locally folded and thrusted, probably related to the Hercynian events further south. The West Orkney Basin is capped by Mesozoic sediments and was probably reactivated during Mesozoic basin development in the Minches and Moray Firth. The shape of the faults, their orientation and decoupling levels are strongly controlled by the earlier Caledonian structure, in particular by the layering and crustal anisotropy developed along and above the Moine thrust. The West Orkney Basin with its 20 km deep decoupling level formed by extension of Caledonian thickened crust. It is notablethat the major basin-bounding faults to the NW, the Outer Islesand Flannan faults, whichdeveloped where the crust was thinner and henceless ductile at depth, decoupled at much deeper structural levels, at the Moho or below. The West Orkney Basin lies off the north coast of Sutherland British Institutions Reflection Profiling Syndicate (BIRPS), and Caithness, northern Scotland, and west of the Orkneys to 15 S TWT (seeSmythe et al. 1982; Brewer & Smythe (Fig. 1). Shallow bore-hole data suggest thatthe basin 1984; Cheadle et al. 1987). The West Orkney Basin overiies carries at least a capping of Permo-Triassic red sandstones the northern continuation of the early Palaeozoic Caledo- (Kirton & Hitchen 1987). However, Devonian age Old Red nian orogen and this paper will discuss aspects of the basin Sandstone sediments are seen on-land on the N Sutherland geometry and its evolution in relation to the structure of the coast (Figs 1 & 14) and more extensively in the Orkneys and Caledonides. Caithness, which representthe emergent portions of the Orcadian basin. These sediments are structurally continuous with those of the West Orkney Basin and indicatea Regional Geology Devonian age of initiation for the West Orkney Basin (see The main features of the Caledonides are shown in Fig. 1. discussion later).These sediments showevidence of both The Moines are Proterozoic metasediments with minor basic normal faulting, producing the basins, and also contractional and acid intrusions which were generally intensely foliated deformation (sensu Norris1958), producing folding and and metamorphosed to upper greenschist and amphibolite thrusting within the sediments, some of which is strike-slip facies during the Caledonian orogeny. Along the N coast of related. Scotland the Moines show a complex interlayering of This paper aims to describe the structure and structural intensely foliatedsediments, metabasite intrusions and history of the West Orkney Basin and adjacent parts of the earlier basement due to isoclinal folding and closely spaced, Orcadianbasin, using datafrom field work on-landand WNW-directed thrust imbrication (Butler & Coward 1984; fromthe interpretation of offshorecommercial anddeep Barr et al. 1986). The foliation dips E to SE about 15" in the seismic reflection data.The commercial dataare froma western part of the section, but at steeper dips, sometimes speculative survey by Western Geophysical, and consist of nearly vertically, in the eastern part. 48-fold multichannel reflection profiles (Fig. 1) shot using a The Moines were thrust to the WNW over a foreland, 930cm airgun source and 2400 m streamers. Data were consisting of Lewisian (Archaeanto lower Proterozoic) recorded to 6s two-way traveltime (TWT) and all lines basement,overlain by a thick sequence of Torridonian were time migrated. The deep data are from theMoine and (upper Proterozoic)arkosic sandstones, grits andcon- Outer Isles Seismic Traverse (MOIST), undertaken by the glomerates, again overlain by Cambro-Ordovician shelf 871 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/6/871/4888586/gsjgs.144.6.0871.pdf by guest on 30 September 2021 872 ENFIELDM. A. & M. P. COWARD I I 5 4 3 Fig. 1. Location map for the Western Geophysical linesof the West Orkney Basin, north of the Scottish coast. MT, Moine Thrust;Ns, Naver slide; SBS, Sgurr Beag slide; OIT, Outer Isles thrust; OIF, Outer Isles fault (possible offshore continuation of the OIT); GGF, Great Glen fault;72/25, BGS shallow borehole number 72/25. Limit of the restored Cambrianof the Moine Thrust Zone after Butler& Coward (1984). sediments (Peach et al. 1907; Butler & Coward 1984). A the Middle ORS on Orkney mainland (Wilson et al. 1935), minimum overthrust displacement of 54 km on the Moine where the Lower ORS is considered to have been uplifted thrust is indicated from the restoration of imbricated Middle and eroded prior to Middle ORS deposition. and Upper Cambrian sedimentsin the Eriboll and Foinaven The Middle ORS is strongly diachronous, onlapping to areas(Butler & Coward 1984). Thusthe Moine thrust the west in Caithness and Orkney. The early Middle ORS cannot have cut up through basement or Lower Cambrian succession is a relatively quiescent lacustrine facies, sediments within 54 km of its present outcrop trace in these producing a flagstone sequence 4 km thick in Caithness and areas (Fig. 1). 2 km thick in Orkney. The Upper ORS is separated from Thestrata of the West Orkney Basin have only been the Middle ORS by an unconformity, which apparently testedfrom shallow boreholes off-shore (for example, occurs throughoutthe basin. OnHoy this unconformity borehole 72/25, Dunham 1973; see Fig. 1 for location), but post-dates a set of earlier normal faults (Wilson et al. 1935). thesouthern continuation of the West Orkney Basin Upper ORS sedimentation on Orkney is characterized by sedimentsform aseries of conglomerates and sandstones high energy fluvial deposits, with aeoliandunes locally occupying half-grabens at Tongue, Kirtomy and Poulouris- developed(Astin 1985). On Hoy the development of caig and are considered to be of Old Red Sandstone (ORS) calc-alkaline lavas, dated at about 370Ma (Halliday et al. age (Peach & Horne 1914; Blackbourn 1981). 1977), marks the onset of Upper ORS sedimentation. The ORS deposits of Caithness andthe Orkney Isles unconformablyoverlie an irregularlandscape in the Caledonian rocks and are considered to have been deposited Deep structure as seen on MOIST in a majorinter-montane basin, theOrcadian basin TheMOIST profile (see Smythe et al. 1982; Brewer & (Anderton et al. 1979). Smythe 1984) shows the West Orkney Basin to be comprised The Lower ORS coarselenticular breccias and con- of half-grabens,formed onthe hanging-walls of easterly glomerates, interdigitated with finer bedded sandstones, dippingfaults (see also Brewer & Smythe 1984, 1986; havebeen deposited within small isolated intermontane Blundell et al. 1985; Cheadle et al. 1987; Kirton & Hitchen basins (Mykura1976). Local thick fanglomerates suggest 1987). Beneath the West Orkney Basin, within the lower deposition within basins which are controlled by active fault crust, the MOIST section shows azone of sub-horizontal scarps. An angular unconformity is present at the base of reflectors down to 9 S TWT,where a strongly reflecting zone Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/6/871/4888586/gsjgs.144.6.0871.pdf by guest on 30 September 2021 STRUCTUREOF WEST ORKNEY BASIN 873 is interpreted as representing the Moho (Smythe et al. 1982; structures onthe map (Fig. l), but may be linked by a Brewer & Smythe 1984). The upper and middle crust in the NW-trending fault N of Lewis. eastern part of the MOIST profile is dominated by strong, The basins appear to be in isostatic equilibrium (Cheadle east-dippingreflectors, which have beeninterpreted as et al. 1987) andthe Moho is at approximately the same lower Palaeozoic sediments overlain by Caledonian foliated TWT throughout (Blundell
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