Abstract
The Middle Devonian lacustrine sediments of Orkney, off the NE Scottish mainland, are composed largely of the Lower and Upper Stromness formations and overlying Rousay Formation. These three formations have been subdivided and defined by vertebrate biostratigraphic biozones with recent division of the Rousay Formation into three further units based on characteristic fish fossils. The division of the Rousay Formation has enabled a map to be constructed of the solid geology of the island of Westray, Orkney, based on fish identification, detailed logging of sedimentary cycles throughout the Rousay succession, parameters of divisional boundaries, and a survey of faults marking sinistral transtensional movement parallel to the Great Glen Fault. Post-Carboniferous shortening and basin inversion led to uplift, folding and reactivation of normal faults as reverse faults, to form a positive strike-slip flower structure in Westray. A suite of Permian igneous dykes intruded across Orkney include three minor offshoots in Westray. The resulting map is the first to make use of biostratigraphic units within the Rousay Flagstone, which are now regarded as Members.
This paper provides a new map and summary of the solid geology of the islands of Westray and Papa Westray Orkney, Scotland (Fig. 1). Westray measures 17 km from Noup Head to Weather Ness and about 8 km west to east across the widest part. Steep cliffs, flat rocks and sandy bays make up the highly indented 80 km of coastline. The bedrock largely consists of a thick series of lacustrine flagstones of Middle Devonian age, c. 393–383 Ma (Gradstein et al. 2012), a succession of cyclic fossiliferous, mudstones, siltstones and fine sandstones. The oldest rocks on Westray are of the Upper Stromness Flagstone Formation conformably overlain by the Rousay Flagstone Formation, which was divided by Michie et al. (2015) into three biozones on the basis of their fish fauna. A north–south anticline bisects the island with minor folds on each flank (British Geological Survey 1999). Mapping demonstrated that the northern extension of the East Scapa Fault forms a flower splay across Westray. Superficial deposits of glacial till and wind-blown sand cover more than half of the island.
Solid geology of Westray and Papa Westray, Orkney, Scotland, showing line of section of Figure 15 and localities of biozone fish. Dt; Dickosteus threiplandi; Mm, Millerosteus minor; Op, Osteolepis panderi.
Abbreviations used: WHT, Westray Heritage Trust; NMS, National Museum of Scotland.
Methods
Research on Westray geology led to the publication of Westray Flagstone for the Westray Heritage Trust (Leather 2006), followed in 2009 by a geology exhibition staged at the Heritage Centre in Pierowall, Westray, repeated in 2010, and awarded the ‘ENI Geological Challenge 2009’. A long shoreline, broad exposures at lowest tide and gentle dips of 10° or 12° ensure a vast area of bedding surfaces. With no recourse to thin sections or laboratory techniques, research was concentrated along the coastline, initially logging a succession of 17 cycles (Table 1) in the Osteolepis panderi Pander, 1860 (formerly Osteolepis microlepidotus) biozone scattered across the island. A survey of overlying Millerosteus minor Miller, 1858 (which Miller originally placed in the genus Coccosteus) beds on the NW coast of Westray established a 215 m succession of 17 cycles. The Upper Rousay Flagstone unit was found to be present in Westray along the Weather Ness shore east of Rapness harbour where 8 cycles were mapped, with detailed logging of transition zones below and above four consecutive lake beds. A survey of strike-slip faults along the 100 km of combined coastlines of Westray and Papa Westray revealed an extensional fault complex across the two islands. A brief account is given of rhythmite cyclicity, a description and interpretation of facies changes within typical cycles, with descriptions of the three biozone units of the Rousay Flagstone Formation in Westray and a summary of the structural geology of Westray and Papa Westray.
Summary of 42 logged cycles in the Rousay Flagstone Formation, Westray, including identified fossil fishes, coprolites, plant remains, stromatolites, pseudomorphs, chert, desiccation cracks, synaeresis cracks and décollement
History of research
Peach and Horne (1880) described the general structure of Westray as a broad anticline with its core in the Bay of Tuquoy. The fish fauna of Orkney has been widely studied (e.g. Miller 1841, 1858; Sedgwick and M'Coy 1855) and revised by Traquair (1888); Geikie (1878) and Flett (1898) recognized that certain fish species were confined to the upper sequence in Orkney and Caithness that contained the characteristic species Osteolepis panderi, Thursius pholidotus Traquair, 1888 and Millerosteus minor. Flett introduced the term Rousay Beds for the lacustrine sequence above the Stromness Flags that contains these species. The Geological Survey (Wilson 1935) separated the full sequence into Lower and Upper Stromness Flags, divided by the Achanarras–Sandwick horizon, followed by the overlying Rousay Flags and Eday Group. Miles and Westoll (1963) correlated the Spital beds of Caithness with the Upper Stromness Flagstone in Orkney, based on the presence of the placoderm Dickosteus threiplandi Miles and Westoll, 1963, which also occurs in Westray. Mykura (1976) estimated a thickness of 1500 m for the Rousay Flags, and noted fish beds with abundant M. minor in Rousay, similar to those on Westray and Papa Westray, and rare specimens of Asterolepis orcadensis Watson, 1932 ex Traquair MS near the top of the Rousay group on Eday, now also found to be present low in the Rousay beds on Westray. Astin (1990), using the Sandwick Fish Bed as the key correlative horizon, made a broad survey across the NE coast of Orkney Mainland, Rousay and south Eday. He numbered cycles from a base at the Sandwick Fish Bed to the Eday Flagstone and established the Rousay Flagstone Formation. Astin (1990) noted the presence of fossil fish but ignored species and thus, on Rousay, omitted the sequence of Rousay beds below his cycle 25 that is now known to extend down to his cycle 16 at Mid Howe (Michie et al. 2015) with the occurrence of O. panderi. Speed (1999) logged 19 sections across northern Orkney based on Astin's work, including two in Westray. The Rousay Flagstone Formation was re-established by Michie et al. (2015) and subdivided into three biostratigraphic units, Lower, Middle and Upper Rousay Flagstones (Fig. 2).
Biostratigraphic table of the principal units of the Stromness and Rousay Flagstones and biozone fossil fish.
Flagstone cyclicity
The periodicity of rhythmic sedimentation within the Orcadian Basin has long been under discussion. For example, Andrews et al. (2016) favoured 19.9 kyr, driven by precession of the Earth's spin axis, while Brown et al. (2018) interpreted it as reflecting orbital eccentricity of 100 kyr. These Milankovitch cycles resulted in alternating long-term periods of wet and dry climate with fluctuating lake levels, varying from permanent lake to playa. In the Middle Devonian, c. 390 Ma ago, the Orcadian Basin was about 15–20° south of the Equator (Scotese 2003; Cocks and Torsvik 2011) and, although land-locked, was never far from adjacent seas, the Palaeo-Tethys Sea to the east and the Rheic Ocean to the south. According to Berner (2006), levels of CO2 were up to 10 times higher than the present day, keeping worldwide surface temperatures c. 7.5° above the recent global average of 14°C for the 30-year period 1951–80 (Hansen et al. 2010), maintaining temperatures in the Orcadian Basin of between 21 and 29°C (De Vleeschouwer et al. 2015). Middle Devonian palaeo-wind directions were reconstructed by measuring the orientation of over 500 wave ripple marks in Westray (De Vleeschouwer et al. 2015). The presence of two sets of wave oscillation ripples, separated by a single seasonal lamina (Fig. 3), provided a clue to the palaeoclimate. It was found that the predominant ripple orientation was produced by winds from the SE (trade winds). However, the small percentage (7%) of ripples created by northeasterly winds implies that the normal movement of the low pressure Inter Tropical Convergence Zone, as it crossed the Equator, moved far enough south to bring rainfall from the North East Trades for a short period in the southern summer, before moving north again with the Sun, bringing further precipitation in the autumn. This suggests a typical double monsoon climate with heavy seasonal precipitation (Fig. 4). The proximity of nearby oceans thus resulted in a hothouse monsoonal palaeoclimate with strong seasonal variability and rainfall in the southern summer. In the permanent lake, seasonality produced annual couplets and triplets (Rayner 1963; Andrews et al. 2010) of clastic, carbonate and organic components with varying thicknesses from 0.16 to 1.0 mm (Trewin 1986; Andrews 2008; Andrews et al. 2010; Brown et al. 2018) accumulating as lake laminites.
Paired wave ripples in situ separated by a single seasonal lamina indicate two wind directions: North East Trades and South East Trades. For scale, compass is 5 cm wide, and indicates present-day north. Rapness, Westray.
Simulated climate for the Orcadian Basin, using the HadSM3 general circulation model after De Vleeschouwer et al. (2015). The climate-graph shows a double monsoon with rainfall peaks in southern spring and summer with dry winters.
Throughout the deposition of lacustrine sediments in the Middle Devonian, Milankovitch cycles gave rise to a series of at least 125 lake cycles. Each cycle is lithologically diverse and averages 12 m thick. Sediments are remarkably fine-grained with clasts no bigger than 0.5 mm. A typical cycle (Figs 5 and 6) resembles that of Donovan (1980) who named four lithological associations, A to D, based on logging on the north coast of Caithness between Brims and Dounreay, and interpreted these as recording progressive shallowing of the lake, culminating in the formation of ephemeral playa lakes ‘D’. Retaining Donovan's lithological associations, and subdividing his lithological association C into C1 and C2 and lithological association D into D1 and D2, a typical cycle may be divided into six distinct lithologies (Fig. 5). Taking the permanent lake laminites as the base of each cycle, the lithologies are ordered as follows: B, A, B, C2, D1, D2, D1, C1, where B–A–B comprise permanent lake, C1 the transition zone from playa to permanent lake and C2 from permanent lake to playa. D1, and D2, represent ephemeral lakes and fluvial beds, respectively (Fig. 6).
Summary of the characteristics of a typical flagstone cycle in the Middle Devonian of Westray, Orkney.
Graphic log of a typical cycle from the Lower Rousay Flagstone Member, based on cycle 8, Stancro, Rapness, Westray, illustrating variable palaeo-environmental conditions and including Donovan's adapted lithological associations.
Description of the geology covered by the map of Westray (Fig. 1)
The Upper Stromness Flagstone Formation
The upper part of this unit (c. 175 m) occupies a north–south anticlinal axis in the centre of the island. Plates of the biozone fish Dickosteus threiplandi (Fig. 7a) were found in a 2.4 m fish bed at the Ness of Tuquoy [HY 4605 4326], including several median dorsal and parietal plates. The fish bed also contained Gyroptychius milleri Jarvik and Dipterus valenciennesi Sedgwick and Murchison. This fish bed is the highest in the Upper Stromness Flagstone in which the biozone fossil occurs and is overlain by four cycles prior to the first Rousay Flagstone cycle. The four cycles, once described as barren, lack D. threiplandi but contain the lung fish D. valenciennesi, the large placoderm Homostius milleri Traquair, and lobe-fins Glyptolepis paucidens Agassiz and G. milleri (Table 1). A further outcrop of the Upper Stromness Flagstone, lower in the succession, also containing the biozone fossil D. threiplandi, occurs on the east side of Tuquoy Bay at Cubbigeo.
Biozone fossils: (a) median dorsal plate of Dickosteus threiplandi Upper Stromness Flagstone Formation, Ness of Tuquoy [HY 4605 4347], WHT 2016-4.12; (b) disjointed 3D specimen of Thursius pholidotus, Lower Rousay Flagstone Member, West Sous, Rapness [HY 5005 3983], WHT 2016-4.4; (c) parietal plate of Osteolepis panderi, Lower Rousay Flagstone Member, Stancro foreshore [HY 5004 4046], author's collection; (d) small entire specimen of Osteolepis panderi, cycle 1, Lower Rousay Flagstone Member, Tuquoy Bay [HY 4533 4439], NMS G.2017.9.4; (e) median dorsal plate of Millerosteus minor, Middle Rousay Flagstone Member, The Sneck, Papa Westray [HY 4907 5386], author's collection.
The Rousay Flagstone Formation
In Orkney the Rousay Flagstone Formation extends from the Upper Stromness Flagstone Formation to the Eday Group. It forms the bedrock across about a third of Orkney (Fig. 8), that is, a large part of east Mainland, Burray, South Ronaldsay, Flotta and Hoy and most of the northern isles (except Eday). The rhythmically bedded, predominantly fine-grained flagstones are lithologically similar to the underlying Upper Stromness Flagstone and are differentiated largely by their fossil content. Michie et al. (2015) divided the Rousay Flagstone Formation into three biozone units, the Lower Rousay, characterized by Osteolepis panderi, the Middle Rousay by the small placoderm Millerosteus minor and the Upper Rousay with no biozone fossil then assigned to it. Michie gives a total thickness for the Formation of over 800 m.
Geological map of Orkney, after Mykura (1976) [Permit Number CP21/023 BRITISH GEOLOGICAL SURVEY © UKRI. All Rights Reserved.], shows widespread distribution of Rousay Flagstone from Pentland Firth to Orkney's northern isles.
As the three Rousay Flagstone biozones of Michie et al. (2015) have been shown to be useful mappable units, it is proposed to designate them as Members, Lower, Middle and Upper Members of the Rousay Flagstone Formation.
Lithology A and B: permanent lake
In the Rousay Flagstone Formation of Westray (Fig. 5), deep lake ‘lithology A’ is present in most lake beds and consists of a central band of black, silicate and carbonate laminae 0.16 to 0.2 mm. Fossil fish tend to be well preserved in this facies, occasionally as articulated specimens. Above and below this core, lithology B is of wider extent and laminae are thicker and siltier, representing a shallower regressing lake (Janaway and Parnell 1989). Evidence for a shallow lake and occasional exposure to the atmosphere includes the presence of stromatolites that require sunlight, plant remains and synaeresis cracks (formed by changes in salinity). Gypsum pseudomorphs, believed to have been formed subaerially, are present in 8 lake beds. The strongest evidence is that many lake beds (13 out of 42 surveyed) contain subaerial desiccation cracks with polygons up to 25–35 cm (Fig. 9) that are commonly lined with stromatolites. Intervals of a dried-up lake suggest Westray was on the margin of the Orcadian Lake.
Desiccation cracks (within lake bed), Lower Rousay Flagstone, West Kirbest, Westray, Orkney. Polygons average 25–35 cm.
Lithology C1 and C2: transition from playa to permanent lake and vice versa
Donovan's lithological association C is here referred to as a transition zone; C1 is the transgressive phase from playa to re-established permanent lake and C2 the regressive phase from permanent lake to playa. Both occur below and above each lake bed sequence, so that the grouping C1–B–A–B–C2 creates a unit comprising c. 40% of each cycle. The C1 succession begins abruptly and grades upwards from thick laminae comprising 80% pale coarse siltstone/fine sandstone and 20% grey to greenish-grey mudstone, to about 20% sandstone and 80% mudstone at the top, ending abruptly against overlying lake laminites. In this phase there is a grading of laminae thickness from 10 mm at the base to 4 mm at the top, from mainly fine sandstone to predominantly mudstone, reflecting the change to permanent lake conditions. The vertical sequence is similar to horizontal changes from playa to shallow lake. A survey of the Stancro foreshore, Rapness peninsula, of four pairs of transition zones, below and above four consecutive lake beds 6, 7, 8 and 9 of the Lower Rousay Flagstone showed that the pre-lake transgressive phase is thinner, 0.7 m mean thickness and contains fewer laminae, (average 110). The post-lake regressive phase is thicker, averaging 1.1 m, with a mean of 270 laminae, representing a greater length of time for the lake to dry out than it did to become established. Laminae are thinner, averaging 3–5 mm, and begin with a high proportion of mudstone, grading upwards to predominantly coarse siltstone and fine sandstone as lake influence diminishes. If laminae are annual, then, in this part of the cycle, 110 years elapsed for the lake to become permanently established and 270 years for it to completely desiccate. See ‘Discussion’ below for interpretation.
An outstanding feature of the transition zones is the presence of gypsum pseudomorphs (Fig. 10). There has been debate as to whether the small star-shaped clusters are synaeresis cracks or pseudomorphs. Donovan and Foster (1972, p. 315) described the structures as ‘subaqueous shrinkage cracks’. Parnell (1985, p. 377) states that ‘pseudomorphs after gypsum are widespread’ [in the Orcadian Basin], while Astin and Rogers (1991) reinterpreted subaqueous shrinkage cracks of Donovan and Foster as pseudomorphs after gypsum and as subaerial in origin and Andrews and Hartley (2015) prefer syneresis cracks. In transition zones, synaeresis cracks are rare and are only seen in thicker couplets where mud is dominant. Pseudomorphs after gypsum commonly form single rhomboid or lensoid crystals and, of 100 measured, the average size is 18 × 6 mm. Their distribution is densest in laminae where there is a higher proportion of sandstone and typically have a contrasting pale colour, weathering out of the surface where resistant to erosion. Where well developed, they form desert rose clusters that may completely cover exposed bedding planes with a star-like pattern. In laminae with a small proportion of siltstone, pseudomorphs appear as widely spaced single crystals or, closest to lake laminites, as micro-pseudomorphs in a scatter of small spots. Gypsum pseudomorphs are present throughout transition zones but very rarely elsewhere in the cycle. The transition zones are tight units, commonly forming a hard rock platform on the shore, easily mappable and always adjacent to a lake laminite bed.
Single gypsum pseudomorphs in transition zone siltstone, Westray.
Lithology D1 and D2: ephemeral lake and fluvial deposition
D1 above the transition zone consists of a discontinuous series of pale grey or greenish-grey siltstones and fine-grained sandstones intercalated with darker greenish mudstones deposited in a broad playa environment of ephemeral lakes, salt flats, sand flats and sheet flooding. Hard laminated sandstones with erosive or loaded bases, rip-up clasts and occasional fish fragments may persist laterally and have been used as a marker, e.g. Lower Rousay Flagstone cycle 5. Laminated sandstones may also display slumping or ‘ball and pillow’ structures and oscillation rippled surfaces are common. Most sediments contain a proportion of mica and some sandstones contain a large proportion of muscovite. Pyrite is common in most cycles from sub-millimetre cubes to nodules of several centimetres. D2 fluvial deposits occur in the majority of cycles and are associated with beds thinning and lensing, current ripples and channel fills (Fig. 11), with currents mainly from a northerly direction (Fig. 12). Channels are typically 8‒10 m across and 2‒3 m deep, cutting into existing surfaces; the largest noted is 250 m wide by 3 m deep in Lower Rousay Flagstone cycle 16. Channels also exhibit meanders, slip-off slopes and point bars exposed, for example, in the Lower Rousay Flagstone cycle 4 on the west side of the Rapness peninsula. Occasionally a well-preserved fish is found in channel sandstones, demonstrating that fish occupied both lakes and rivers. Wave-rippled and current-rippled siltstones, and mudstones with subaerial desiccation cracks, are common throughout the sequence (Fig. 9). No aeolian sands have been detected.
Channel-fill sandstones, maximum 1.2 m thick, cut into thinly bedded siltstones and mudstones, cycle 6, Lower Rousay Flagstone Member, foreshore north of Pierowall, Westray.
Current ripples indicate input from the north (left) in Lower Rousay Flagstone Member near The Bu, Rapness, Westray. Wavelength c. 15 cm.
The Lower Rousay Flagstone Member
This unit comprises 210 m of strata, forming 17 cycles (Table 1), and covers about one third of Westray. The biozone fish Osteolepis panderi (Fig. 7c and d) is recorded in 7 of the cycles. On the west side of Tuquoy Bay [HY 4490 4305] there is a continuous succession from the Upper Stromness Flagstone Formation to the Rousay Flagstone Formation. The boundary, with a gentle dip to the west, brings the Upper Stromness Flagstone in succession with the first four cycles of the overlying Rousay Flagstone. At the junction of the two formations, cycle 1 of the Lower Rousay Flagstone preserves the first appearance of O. panderi. The 4.1 m fish bed also contains Thursius pholidotus (Fig. 7b), abundant Dipterus valenciennesi, with especially well-preserved cranial shields of this lungfish, and the tiny clam shrimp Ipsilonia orcadensis Chen and Morris, 1991, formerly Estheria. The base of the Rousay Flagstone Formation is mapped in Leather (2017, fig. 4). Cycle 5 lake bed is 15 cm thick and regarded as a very reduced lake but the cycle is included as a full cycle as other features of a complete cycle are present. In the upper part of cycle 7, in grey and grey-green siltstones, the ichnogenus Merostomites occurs (identified by Martin Whyte, formerly of Sheffield University), the trackway of a multi-limbed phyllopod, together with Rusophycus, the ‘coffee bean’ resting places and Cruziana burrowing traces (Fig. 13), all probably made by small arthropods (Trewin and Hurst 2009).
Trace fossils: (a) Merostomites, trackway of multi-limbed phyllopod, WHT 2016-4.27; (b) Rhusophycus, coffee bean resting places, WHT 2016-4.22; (c) single Rhusophycus showing lateral divisions; (d) crescent-shaped burrowing trace Cruziana, WHT 2016-4.23; (e) unidentified trackway in situ, Peterkirk, Rapness, Westray [HY 5001 4005]. (a)–(d) Lower Rousay Flagstone Member, Stancro foreshore, Rapness, Westray [HY 4995 4018].
A marked feature of the Lower Rousay Flagstone is the fish fauna. In cycle 1, an articulated specimen of Osteolepis panderi 12 cm in length (Fig. 7d) has associated parietal and post-parietal headshield plates of proportionate size. However, higher in the succession in cycles 7, 8 and 9, where O. panderi is present in three successive cycles with many articulated specimens, the species is typically 24 cm in length, twice the size of those in cycle 1, which could suggest an evolutionary trend. In cycle 9, in a mass mortality situation, of 119 fossils from a single bedding plain, 114 are O. panderi and 5 the larger Thursius pholidotus. Another significant species, Asterolepis orcadensis (Newman et al., 2019), occurs in six of the 17 Lower Rousay cycles, and had previously been regarded as confined to the higher part of the Upper Rousay unit in Caithness (Michie et al. 2015). The large placoderm Homostius milleri is present in eight cycles, including a large 32 cm headshield from cycle 17. East of Mae Sand in cycle 2, about 4 m above the fish bed, is the significant fossil-bearing O'Bakie bone bed, c. 5 cm thick; this extends laterally to about 2 × 4 m and reveals a rich bank of disarticulated fish bones and debris. It lies within 1.5 m of well-bedded, grey-green siltstones with muddy partings. Identification by Jan den Blaauwen of Amsterdam University, who prepared thin sections of the bone bed, revealed eight fish species, including Osteolepis panderi and the largest predator, Asperocephalus milleri Ahlberg, 1989. Also present were Glyptolepis paucidens, Homostius milleri, Rhadinacanthus longispinus Agassiz, 1844, Cheiracanthus murchisoni Agassiz, 1835, Cheirolepis sp., Dipterus valenciennesi, the clam shrimp Ipsilonia orcadensis and coprolites. The addition from other cycles of Asterolepis orcadensis, Thursius pholidotus and Diplacanthus crassisimus Duff, 1842 makes a total of 10 fish species recorded from the Lower Rousay Flagstone. The clam shrimp Ipsilonia orcadensis is not uncommon in the first four cycles.
The Middle Rousay Flagstone Member
The Middle Rousay Flagstone comprises c. 215 m of strata forming 17 cycles (Table 1), 7 of which contain the biozone fossil Millerosteus minor (Fig. 7e). This unit occupies much of the upland area of western Westray to the top of Fitty Hill (169 m) including a line of hills north to Grobust Bay, continuing NW to Noup Head (Wilson 1935, p. 92). In the east of the island, a north–south belt extends from Papa Westray to Spo Ness, through Skelwick to Twiness, with smaller outcrops in Rapness to the SE. During mapping in Papa Westray, 14 fish beds were noted, many probably fault-repeated, with 8 containing the biozone fossil M. minor. On Westray, the first two cycles of the Middle Rousay Flagstone occur near West Kirbest in the SW where they are seen in continuous sequence with, and overlying, the uppermost Osteolepis panderi cycles. Here, the Middle Rousay succession continues to the top of Skea Hill and, interrupted by a fault, to the summit of Fitty Hill, very near the boundary with Upper Rousay Flagstone. The number of cycles is suggested by a satellite image (Fig. 14) on which about 16 small scarps stand out on the south slopes of Fitty Hill, each considered to be equivalent to one cycle. This is corroborated by the total thickness and amount of dip in the sectional profile (Fig. 15).
Satellite image of Fitty Hill, Westray shows sandier beds of the Middle Rousay Flagstone stand out as minor scarps. Google imagery 2017 DigitalGlobe.
Sketch profile of the structure across Westray from west to east along line of section indicated on Figure 1. Dt; Dickosteus threiplandi; Mm, Millerosteus minor; Op, Osteolepis panderi; f, fault.
In the NW of the island, a survey from Narr Ness in Grobust Bay to Noup Bay, revealed 15 further Middle Rousay Flagstone cycles, 6 containing the biozone fossil Millerosteus minor (Fig. 7e). The first of these (cycle 3) yielded M. minor and Thursius pholidotus among stromatolites. Cycle 8 at HY 4299 4947 in the middle of Grobust Bay contains a large fossiliferous slab c. 25 m2 dipping SW at 14°, covered in fish fragments including M. minor, Homostius milleri, Glyptolepis paucidens Agassiz and Dipterus valenciennesi. Cycle 11, at HY 4216 4964, below a ruined sheepfold, has well-developed 5 cm bun-shaped stromatolites in the fish bed where, at low tide, large slabs carry well-preserved plates of H. milleri and T. pholidotus together with M. minor, G. paucidens, spines of Rhadinacanthus longispinus and small teeth. Cycle 17, HY 4125 4905 east of Noup Bay, is the highest cycle recorded in the survey, where large slabs contain scattered M. minor debris on several bedding planes. The beds dip into Noup Bay, the core of the syncline that may be traced south to the shore below Backarass. Cycles 16 and 17 are the only examples where the biozone fossil M. minor occurs in two successive fish beds. The run of 15 cycles confirms Speed's (1999) description of 14 cycles on the northern limb of the syncline, down the succession from Noup Head. Speed indicated a thin Saquoy Sandstone conglomerate in the 4th cycle from the top, halfway down the cliff from Noup Lighthouse. The Middle Rousay Flagstone is also recorded in the south of the island at Neven on the east side of Rapness peninsula and, in the east, on the promontory of Spo Ness, where two adjacent fossiliferous cycles yielded M. minor. The Westray map (Fig. 1) shows localities of M. minor.
The Upper Rousay Flagstone Member
The uppermost unit of the Rousay Flagstone is only partly exposed on Westray where it comprises c. 175 m, mainly around Weather Ness in the SE where 8 cycles were mapped (Table 1), with evidence of two more cycles near Stangar Head to the north. Of these 10 cycles, 3 contain Thursius pholidotus, known previously only in the Lower and Middle Rousay flagstones. Logging has shown that Upper Rousay Flagstone cycles differ from those in underlying sequences in that they are thicker and sandier, some displaying the tripartite fish bed particularly well, with a core of almost pure limestone interbedded between siltier laminites. A few cycles have no fish bed, merely a transition zone, apparently signifying a lake bed that never quite became established. In one case (cycle ‘2’) a thick transition zone contains two closely spaced intervals of black lake laminite of 10 and 5 cm, both containing fish fragments.
Discussion
The thick series of fine-grained deposits that filled the Orcadian Basin during the Rousay Flagstone Formation over a period of c. 5 Ma suggests a tranquil period of minimum vulcanicity and Earth movements, and the regularity of deposition reflects a gradual and uniform post-rift subsidence. The fine-grained deposits suggest rivers issued from a stable surrounding topography of mature and subdued relief. Quartz, muscovite and decomposed feldspars suggest a hinterland of crystalline metamorphic rocks (Bluck 2000). The dominant current direction at this time, from the north, may indicate provenance within the Caledonian mountains of Scandinavia or Greenland. The absence of riparian vegetation may have led to a fluvial input in the form of a constantly changing braid-plain across broad low-lying fan deltas. Evidence of a shallow lake or lakes is common and includes features such as stromatolites and desiccation cracks that emphasize a widespread low slope of the lake floor and intermittent exposure to the atmosphere.
Transition zones C1 and C2 mark a change from playa to permanent lake of phase B, and vice versa and occur below and above lake bed laminites, respectively. Laminae of 2 to 10 mm are about 10 times the thickness of adjacent lake laminae and comprise density flow (sheetflood) deposits with distinct sandstone and mudstone components. To account for the greater thickness and the grading of laminae within transition zones, it might be suggested that heavy monsoon precipitation produced a seasonal incursion of meteoric water from braided rivers on to extensive plains of sand and salt flats, as sheetflood water with a potentially high transporting power picked up mud, silt and fine sand held largely in suspension. With a slackening of flow as energy decreased, coarser material settled first, producing thicker layers, then thinner layers of silts and muds in succession, possibly in shallow lake waters where velocity was checked. As the flow ceased, some water remained in the seasonal extension of the lake, forming oscillation ripples in sandy laminae. This accounts for the grading and thinning of laminae through space and time in an annual deposition adjacent to and within the lake margin.
Gypsum pseudomorphs (Fig. 10) are common in transition zone C and starry encrusted surfaces are a consistent feature. It is suggested that during the dry season when lake waters had retreated, gypsum precipitated subaerially in moist silts in a vadose setting close to the water table (Warren 2006). The small regular size of pseudomorphs suggests they grew during a short period of time in response to intense evaporation and upward percolation of groundwater. The return of meteoric waters engulfed the salt flats, ending the growth of gypsum and promoting the growth of dolomite, reversing the evaporite sequence and leaving durable pseudomorphs. Gypsum pseudomorphs are present in beds of hard siltstone 1.0–1.5 m thick and, in mapping, their occurrence makes it easier to locate lake beds and fish-bearing strata.
Structural geology
Late Devonian rifting in the Orcadian Basin, where major faults run roughly north–south, facilitating an east–west extension, is considered to be related to sinistral transtensional movements parallel to the Great Glen Fault, with reactivation of principal strike-slip faults during crustal shortening related to Late Carboniferous to early Permian Variscan inversion (Woodcock and Strachan 2000). Post-Carboniferous crustal shortening and basin inversion in Caithness and Orkney has recently been determined as mid-Permian c. 267 Ma (Dichiarante et al. 2016), coinciding with the intrusion of a suite of igneous dykes (Brown 1975) across Orkney. Shortening resulted in gentle folding across Westray with dips of 5° to 12° and two major folds, a north–south anticline across the centre of the island and a syncline to the west. Incompetent lake bed mudstones have been prone to low-angle overthrusting, and horizontal movement has produced zones of intensely deformed ‘décollement’. Rucking and bedding plane slip in less competent beds has also taken place, from which the direction of movement mainly to the east is indicated.
The East Scapa Fault cuts south to north across Westray opening up a splay of smaller faults in an apparent positive flower structure, as described by Woodcock and Fischer (1986). This formed on the releasing bend of a major strike-slip fault, in this case the transcurrent movement north along the curve of the East Scapa Fault from Kirkwall Bay through Egilsay to Westray. This sinistral movement has enabled the opening up of the splay in a number of steep imbricate faults. (Figs 1 and 16). The structural pattern only became apparent following a survey of 120 faults along 100 km of coastline in Westray and Papa Westray. Movement along strike-slip faults produced disturbances of near-vertical to vertical strata close to the faults (Fig. 17), the wider the outcrop of the disturbed zone, the greater the movement along the fault. Five groups of faults were identified according to width of outcrop of near-vertical beds. Major faults are over 20 m wide (the largest in the Twiness peninsula is over 200 m (Fig. 17)), large faults are from 5 to 20 m wide, intermediate faults from 2.5 to 5.0 m and minor faults from 0.5 to 2.5 m, the latter sometimes showing a mare's tail splay. Faults were plotted according to orientation on a 1:25 000 map (Ordnance Survey 2003) and magnitude and, where possible, the downthrow side noted. The results revealed four major strike-slip faults: East Scapa Fault, Skelwick, Twiness and Rapness faults, which together take on a flower structure pattern with an origin south of Westray. A minor subsidence pattern of curved strike-slip faults across Papa Westray defines a symmetrical graben, mapped by Wilson (1935). Although only seen in map view, a sectional profile after Woodcock and Fischer (1986), combined with Westray geology illustrates how convex-upward faults may coalesce to form a Y-shaped flower structure (Fig. 18). A few groups of minor faults demonstrate horse-tail splays, e.g. Point of Huro on the southern point of the Rapness peninsula and in the far north of the island at Bow Head. Crustal shortening across the flower structure resulted in older rocks appearing in the core and younger strata on each flank. Thus, the East Scapa Fault at Narr Ness, north of Pierowall, marking the west boundary of the flower structure, reflects a reverse movement of 45 m cutting out about 160 m of the sequence. The skewed beds immediately west of the fault indicate a southerly (sinistral) movement. On the eastern margin of the flower structure near Weather Ness Point, reverse movement along twin faults cuts out 345 m of strata, i.e. two-thirds of the Lower Rousay and all of the Middle Rousay flagstones leaving younger Upper Rousay Flagstone to the east.
Map of the northern part of Orkney showing principal faults and how the flower fault splay across Westray originates from sinistral transtensional movement along the slightly curved northern extension of the East Scapa Fault.
Vertical strata, Twiness, Westray, mark a major strike-slip fault that runs north to cliffs of vertical bedding on the NE coast near Gestaquoy and south to The Flood Skerry immediately NW of Wart Holm. Near-vertical strata extend for 230 m across the Twiness peninsula. Distant figure for scale.
Sketch of sectional profile of a positive flower structure adapted to Westray surface geology along an approximate line from near Pierowall [HY 437 485] to Weather Ness [HY 524 403] to the SSE, illustrating how convex-upward faults may coalesce to form a Y-shaped structure in a strike-slip situation. For simplicity, four principal faults (named) are included and three minor ones.
Hydrocarbon source
The organic-rich laminites of the Middle Devonian of the Orcadian Basin have proved to be source rocks for the oil in the Beatrice Field (Marshall et al. 1996). Rarely, beads of bitumen have been found in fish beds, e.g. Lower Rousay Flagstone cycle 8, and in Rapness quarry in cycle 4 of the Lower Rousay, where the disturbed core of the fish bed forms an oil shale, flakes of which can be ignited by a strong flame. For sediments to have reached the oil window, an overburden of 2.4 km was required (Owen 1994).
Igneous activity and mineralization
About 250 Permian lamprophyre dykes (Brown 1975) have been recorded in Orkney, and Wilson (1935) recorded a single 0.5 m wide dyke on Westray. Recent surveys have revealed two additional, small, deeply weathered dykes along the same foreshore in Westray's Westside (Fig. 1).
Uranium is marked in two localities on the Geological Survey map of Westray. Uisdean Michie (pers. comm. 2008) described patches of fish bed with scintillometer readings of up to 1100 counts per second, about 10 times the normal reading on the flags, and suggested that bone fragments such as those of the large armoured fish Homostius are particularly radioactive where bone phosphate has absorbed uranium and thorium. The largest mineral vein noted in Westray is of barite, exposed on the north shore of The Ouse, c. 0.5 m wide and running for a few metres in an ENE direction. Minor calcite and dolomite veins, commonly pink, occur in highly distorted strata at Skel Wick on the east coast and on the east side of the Rapness peninsula.
Conclusion
Following the subdivision of the Rousay Formation on Orkney by Michie et al. (2015) into three biostratigraphic units, based on biozone fish fossils, it became possible to construct an outline sketch map of the geology of Westray. Continued identification of fossil fish alongside detailed logging of 42 cycles with structural mapping have led to the production of the new map of Westray presented here, the first of its kind to portray the new Rousay units. Future work on the Rousay Flagstone Formation in other parts of Orkney and Caithness would extend our knowledge of this important and less studied part of the Middle Devonian of Scotland.
Acknowledgements
Thanks are extended to John Flett Brown for encouragement and guidance over the years and the suggestion of a fault survey, to constant friend Jan den Blaauwen for his identification and thin sections of the many fish fossils, to the late Uisdean Michie for voluminous emails and to Westray Heritage Centre for warmly embracing their local geology. Sincere thanks to reviewers Mike Newman and Steven Andrews whose experienced advice led to useful changes and additions to the manuscript, and to most helpful Editor Colin Braithwaite.
Author contributions
DL: conceptualization (lead), data curation (lead), investigation (lead), writing – original draft (lead), writing – review & editing (lead)
Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Data availability
All data generated or analysed during this study are included in this published article.
Scientific editing by Colin Braithwaite
- © 2021 The Author(s). Published by The Geological Society of London for EGS and GSG. All rights reserved
References
A new geological map and review of the Middle Devonian rocks of Westray and Papa Westray, Orkney, Scotland
