Middle Old Red Sandstone of Orkney
|Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.|
- 1 Stromness Flags
- 2 Rousay Flags
- 3 Eday Beds
- 4 Bibliography
The Stromness Flags are composed of what appears at first sight to be a monotonous sequence of grey and black thinly bedded, in part laminated, dolomitic siltstones, shales and subordinate thin very fine-grained sandstones. These beds have been aptly named flagstones as they are particularly suitable for the production of paving flags. Most flags are rich in ferroan dolomite, which causes them to weather to an ochreous colour. They are superbly exposed along the western seaboard of Mainland and along the north shore of Hoy, and they have been studied in detail by Fannin who has shown that the succession consists of a number of well-defined rhythmic units or ‘cycles’. Fannin (1970) has used the following stratigraphic subdivisions:
|Approximate thickness (m)||Number of cycles||Average thickness of cycles (m)|
|Upper Stromness Flags||190+||20+||8.5|
|Sandwick Fish Bed Cycle (with Sandwick Fish Bed 2.5 m above base)||55–61||1||55–61|
|Lower Stromness Flags||215||24||8.1|
|Basal breccia, conglomerate and sandstone||0–20||–||–|
The Lower Stromness Flags have been taken to be of Upper Eifelian age and the beds above the Sandwick Fish Bed have been classed as Lower Givetian (Table II; Westoll 1951; Miles and Westoll 1963). Fish remains are present throughout the sequence and they are particularly abundant and well preserved in the Sandwick Fish Bed. The fauna recorded from the various subdivisions is as follows:
- Upper Stromness Flags: Coccosteus ?, Dickosteus threiplandi Miles and Westoll, Dipterus valenciennesi, Glyptolepis paucidens and Homostius milleri.
- Hoy Cycles: Dickosteus threiplandi, Dipterus sp. and Glyptolepis Sp.
- Sandwick Fish Bed Cycle: Cheiracanthus murchisoni Agassiz, Cheirolepis trailli Agassiz, Coccosteus cuspidatus, Diplacanthus striatus Agassiz, Dipterus valenciennesi, Gyroptychius agassizi, G. microlepidotus (Agassiz), Osteolepis macrolepidotus Agassiz (P916222, 3), Pterichthyodes milleri (Agassiz) and Rhadinacanthus sp.
- Lower Stromness Flags: Coccosteus cuspidatus and Dipterus sp.
Large plant fragments are relatively rare and poorly preserved and the principal genera recognised are Hostimella, Thursophyton and Protopteridium. These cannot be used for accurate dating. Spores are abundant in many of the siltstones and Richardson (1965) and Fannin (1970) have recorded a large number of species. Apart from giving general support to the dating by the fossil fish, they have not so far provided any evidence for the exact age of the Stromness Flags, but may in future form the basis for a more precise stratigraphic zonation.
Basal Breccias, Conglomerates and Sandstones
The basal beds of the Lower Stromness Flags vary greatly both in thickness and in the size of their clasts. They are well exposed on the north-western margin of the Graemsay inlier and along the shore at Ness, at the south end of the Stromness inlier (P915581). In the Yesnaby area a thin layer of basal breccia rests directly on the basement rocks which form the local hills, but along the coast thin beds of pebbly sandstone at the base of the Stromness Flags rest on the sediments of the Yesnaby Sandstone Group.
At Ness and north Graemsay the basal breccia ranges from 5 to 20 m in thickness and locally fills hollows in the old land surface. The breccia passes laterally and upward into conglomerate with small pebbles, which is interfingered with pebbly sandstones and thin lenses of sun—cracked siltstone. The breccias and conglomerates are composed of unsorted angular to subrounded boulders and cobbles of locally derived granite, granite-gneiss and, more rarely, schist, set in a matrix of arkose which in places is composed entirely of fine granite debris. At Graemsay some clasts are coated with a thin layer of interlaminated dolomite and siltstone which in places has a mammillated structure. These coatings appear to be stromatolites (Fannin 1969, p. 82). The breccias with their angular clasts are interpreted as scree and talus deposits which accumulated at the flanks of the basement hills and were later partly re-worked and rounded on the shore of the lake surrounding these hills. The stromatolite coatings of the clasts were formed in shallow water and the complete coating of pebbles suggests that the latter were being constantly rolled by waves.
Above the basal breccias the Stromness Flags are made up of over 50 cycles which are thought to have been formed by the fluctuations in the level of a single large and generally shallow lake in the Orkney–Caithness area. The cycles are basically similar to those described in the Caithness Flagstones by Crampton and Carruthers (1914, pp. 89-93) and the lithological characters of such a cycle are shown in P915582. The base of the cycle is taken at the base of the black thinly laminated fine sediment which commonly contains fish remains and was laid down during a quiescent period when little or no coarse sediment entered the lake and when the water, though still shallow, was at its deepest. The sequence of sediments above this suggests that thereafter the depth of the lake gradually decreased and that eventually an influx of coarser sediment brought in by a river or stream pushed a delta out over the shallow or dried-up lake floor. It is believed that the Orcadian Basin was a rapidly sinking, tectonically controlled basin, and that the rhythmic sedimentation was regulated by an interplay of tectonic and climatic changes.
The Stromness Flags contain a number of features which can be used for local correlation. The most obvious for detailed correlation are the ‘deepest water’ fish-bearing facies of the individual cycles which are extremely persistent laterally. Individual cycles can sometimes be recognised in neighbouring sections by their total thickness, which also remains constant over long distances. There are also a number of marker horizons which maintain their character for some distance laterally. Thus in the Lower Stromness Flags two thinly laminated iron-rich beds of silty dolomite, which are rusty orange-weathering and contain large chert nodules, occur at 14 and 59 m below the Sandwick Fish Bed. A third marker horizon, just above the base of the Sandwick Fish Bed, is a 25 cm-thick calcite mudstone, which weathers to a distinctive bluish grey colour. The fourth marker is a massive 2 m-thick bed of bluish grey to black silty mudstone, which occurs at 56 m above the Sandwick Fish Bed throughout the area. In addition, several thin bands of green tuff crop out in Hoy at horizons 3 and 170 m above the Sandwick Fish Bed. These, however, thin out northwards and extend only into the southern end of Mainland.
The thickness of individual cycles commonly ranges from 5 to 10 m but the Sandwick Fish Bed cycle has a thickness of 60 m. Each cycle can be divided into two major lithological facies. The lower of these was deposited under a continuous cover of water; the upper in shallow water which at times dried up completely.
The lower facies commences with (1) up to 1 m of dark grey to black silty mudstone interlaminated with siltstone or fine sandstone. It generally has graded laminae which may have either a high bitumen and pyrite or a high carbonate (usually ferroan dolomite) content. It contains either complete or fragmentary fish remains. This deposit was laid down in relatively quiet and sometimes stagnant waters on a lake bottom undisturbed by wave action. The lake waters may, at times, have been thermally stratified and some of the graded laminae may have been deposited by turbidity currents. These ‘quiescent water’ beds grade upwards into (2) thinly interbedded bituminous silty mudstones and fine sandstones together with some discrete beds of massive siltstone and fine sandstone, some of which fill small erosional channels. This part of the sequence is characterised by the presence of numerous small sub-aqueous shrinkage (syneresis) cracks (see Donovan and Foster 1972) which were infilled by sand or silt and were then compacted and contorted. Algal stromatolite sheets and mounds are common in these beds and scattered fish fragments are generally present. Sediments of this type were laid down in water in which bottom currents were spasmodically active. In some cycles these sediments are interbedded with massive beds of calcareous siltstone which are rich in spores and comminuted plant debris.
The massive siltstones were probably formed in relatively quiet shallow water close to the lake shore which formed traps for the accumulation of ‘vegetable hash’. None of the beds in the lower facies have sun cracks, which indicates that the water of the lake never receded completely.
In most cycles the upper facies commences with (1) thinly banded sediments which are lithologically very similar to the higher beds of the lower facies, but have, in addition, well developed sand-filled desiccation polygons (P219004). In many instances stromatolite sheets and mounds cover the cracked surfaces and extend down into the sun cracks. These beds were probably laid down in quiescent waters close to the lake margin, where the shallow water periodically receded to leave coastal mud flats. In most instances they are overlain by (2) ripple-cross-laminated sandstones and siltstones with desiccation cracks. These beds appear to be alluvial flood-plain or delta-top deposits laid down by rivers which entered the lake from the north. In some cycles, particularly in the ‘Hoy Cycles’ of Hoy, such fluvial spreads are interbedded with and channelled by lenticular beds of cross-bedded sandstone which represent channel-fill deposits. In some instances the thick sandstones are interpreted as filled-up river channels, in others as filled-up delta-front channels, which may have been cut in the soft muds and sands beneath the shallow water of the lake. The fluvial sediments are invariably succeeded by thinly banded silts and sands with sun cracks (i.e. mud-flat deposits) before the abrupt change to the quiescent ‘deep-water’ facies at the base of the next cycle.
The presence of algal stromatolites with a wide variety of growth forms (Fannin 1969) appears to be a characteristic feature of Orkney flagstones, which has not been noted to the same extent in Old Red Sandstone sediments of other parts of Scotland. Stromatolites occur as:
- 1. sheets of limestone or dolomite ranging in thickness from a millimetre to several centimetres which cover surface irregularities such as aligned small ridges, and line small sun cracks;
- 2. mounds composed of stacked convex-upward hemispheres linked by laminated sheets;
- 3. isolated mounds.
The spectacular mounds of stromatolite made of stacked hemispheroids which are known as the ‘Horse-tooth Stone’ (P219017) are well exposed at the top of the cliff at Yesnaby [220 161]. They were first described by Heddle (1878, p. 117, plates xiv and xv) and recognised as fossil algal reefs by Anderson (1950, p. 10, fig. 6). Fannin has shown that the shape of the stromatolites of Orkney was controlled by the currents in the lake. Conversely, once the algal colony was established it formed a tough mat which was able to influence the pattern of erosion on the lake floor.
The detailed study of palaeocurrent directions has indicated that there is a dominant trend towards the south in both the major facies, with a tendency to a south-south-westward trend in the ‘fluvial’ beds, particularly within the Hoy Cycles. In the Upper Stromness Flags this trend, though still present, is less pronounced. Though the interpretation of the palaeocurrent directions must be tentative, Fannin has suggested that the Orcadian lake had a roughly east-west trending shoreline and was fed by rivers entering from the north. The periodic retreat of the shoreline was probably due to the southward advance of the delta-fronts. Within the lake the dominant current flow was offshore and there is little evidence for any long-shore currents.
The ‘typical’ cycle described above is not universally found and there is considerable variation both in time and space. The simplest cycles occur in the Lower and Upper Stromness Flags. The Hoy Cycles are considerably thicker than either of the other two groups and they have a higher proportion of lower facies sediments. They contain channel sandstones which on Hoy reach a thickness of 11 m. This indicates that during the deposition of the Hoy Cycles a large river sytem persisted over or near western Orkney. By far the thickest cycle in the Stromness Flags is that which contains the Sandwick Fish Bed. It ranges in thickness from 55 min the south to 61 m in the north, though the fish bed sensu stricto is a finely laminated carbonate-rich band only 50 cm thick and located some 2-5 m above the base of the cycle. The cycle contains up to 55 m of true lacustrine (i.e. lower facies) sediments and, as it has been confidently correlated with the Achanarras and Niandt Fish Beds of Caithness and the Melby Fish Bed of Shetland (Miles and Westoll 1963), it must represent a period when the lake was considerably deeper and more extensive than at other times during the deposition of the Stromness Flags. The maximum water depth may have been 50 m.
In the past the Sandwick Fish Bed was worked in a large number of small quarries in West Mainland for paving flags and roofing slates. At present only the quarry at Quoyloo (P915581) is still active. This still yields abundant fish remains, though modern methods of quarrying are less conducive to the preservation of good specimens. The fish bed can be recognised on the quarry face by its rusty weathering. Fish remains may be evenly scattered on some bedding planes, on others they may be so abundant as to form a matted mass. On joint faces the fish can be recognised as black coal-like lenses, sometimes up to 1 cm thick. Black bituminous matter is often present along the joint planes of the fish bed, and in the more compact carbonate—rich portions of the bed a little oil is also found.
The Rousay Flags (referred to in the Geological Survey memoir as the Rousay Beds) consist of over 1500 m of rhythmically bedded, predominantly fine-grained ‘flagstones’ which on lithological criteria alone are extremely difficult to distinguish from the underlying Stromness Flags. The ‘Rousay Beds’ were set up as a separate formation by Flett (l898b), who noted that the upper part of the Orkney flagstone succession has a less abundant fauna than the lower part and that this fauna contains the two fish species Thursius pholidotus Traquair and Coccosteus (now Millerosteus) minor (Miller) (P916223, 4) as well as the branchiopod crustacean ‘Estheria’ membranacea Pacht (now Asmussia). All these forms are unknown in the underlying Stromness Flags. Flett did not attempt a precise definition of the base of the Rousay Beds, but the Geological Survey (Wilson and others 1935, p. 18) have fixed a provisional line ‘at the base of a band of limy flags rich in fragmentary fish and plant remains that lies immediately below the lowest bed in which ‘Estheria’ membranacea has been found’. The presence of ‘Estheria’ was, in fact, taken as the main criterion for including isolated outcrops in the Rousay Flags.
The Rousay Flags form the bedrock of about a third of the total area of the Orkney Islands (P915567). They form the greater part of the northern islands except Eday, the eastern quarter of West Mainland and large portions of East Mainland, Burray, South Ronaldsay, Flotta and east Hoy. They are believed to be the stratigraphical equivalents of the Mey and Ham—Scarfskerry subgroups of the Caithness Middle Old Red Sandstone (Donovan and others 1974) and have been classed as Givetian. In addition to the diagnostic fossils mentioned above the Rousay Flags have yielded the following forms:Asterolepis orcadensis Watson (P916223, 5), Cheirolepis sp., Glyptolepis paucidens, Gyroptychius sp., Homostius milleri, Mesacantlzus peachi Egerton, Osteolepis panderi Jarvik, and Thursius sp.
Though rare fish remains are to be found in the ‘quiescent water’ facies of most cycles in the Rousay Flags the Geological Survey noted a number of ‘fish beds’ which contain certain species in abundance. Thus in Rousay three bands with abundant remains of Millerosteus minor occur at horizons which are said to be about 360, 450 and 456 m above the base of the group (P915583). Similar fish beds with M. minor occur in Westray and Papa Westray. In Eday, Stronsay and on the east coast of Deerness fish remains with abundant specimens of a small form of Dipterus valenciennesi and rare samples of Asterolepis orcadensis are present in the flags immediately below the top of the group.
Examples of ‘Estheria’ are abundant in a number of the ochreous-weathering, fish-bearing ‘quiescent water’ facies beds within the lower part of the Rousay sequence, particularly in the north—east corner of West Mainland (between Evie and Wide Firth) and in southern Rousay. Scattered ‘Estheria’ have been recorded on most islands and they occur throughout the sequence. In South Walls (Hoy) they are present in a bed thought to be only 60 m below the top of the group. Plant remains are fairly abundant but they appear to be even more fragmentary and more poorly preserved than in the Stromness Flags. Only fossil wood, Hostimella and Thursaphyton have been recognised. Stromatolites have now been recorded throughout the Rousay Flags and they are particularly common on Sanday.
The cyclic units of the Rousay Flags are closely similar to those of the Stromness Flags, but the following minor differences have been used to distinguish them:
- 1. The Rousay Flags commonly weather to a grey colour which contrasts with the predominant ochreous-weathering of the Stromness Flags and indicates that their carbonate content is mainly calcite, rather than ferroan dolomite.
- 2. The fish-bearing ‘quiescent water’ facies of the Rousay cycles weathers in places to a purplish colour and the fish beds are commonly impure limestones.
- 3. The effects of differential weathering of the hard and soft members of individual cycles are more pronounced in the Rousay Flags than in the Stromness Flags and terrace features are prominent on the hillsides of Westray, Rousay and West Mainland. Terraces are, however, much less obvious in the Rousay outcrops further east and south.
Though sandstone does not, as a rule, form an appreciably greater proportion of the cyles of the Rousay Flags than the Stromness Flags, thick and, in places pebbly, sandstones are present in the higher beds of the group on the island of Rousay (P915583). On this island the 90 m of strata immediately below the highest exposed fish bed contain several bands of pebbly sandstone. The pebbles are up to 5 cm in diameter and are composed mainly of quartz, granite and schistose rocks. The pebbly sandstones thin out and decrease in grain size in a north-westerly direction and they do not reappear at the probable equivalent horizon in Westray. Other prominent sandstones can be seen at the north-east end of North Ronaldsay where they are fine- to medium-grained, reddish in colour, and can be traced laterally for 800 m.
The rhythmic sequences and sedimentary structures found in the Stromness Flags (pp. 74-76) are equally well developed in the Rousay Flags. Slump structures and vertical collapse structures such as ‘ball and pillow’ are common in the upper (fluvial) facies (p. 75) within the upper part of the sequence, suggesting a correlation with the Mey Subgroup of Caithness, where these structures are particularly well developed.
Within the topmost beds of the Rousay Flags exposed on both shores of Long Hope in eastern Hoy, a fish-bearing bed is associated with about 45 cm of black shale which is rich in carbonaceous matter and resembles impure oil shale. On the north-east shore of Stronsay (P915584) a 5 cm-thick bed of bright coaly matter has been recorded in the Rousay Flags. This material, which is brittle and burns with a bright smoky flame, resembles albertite in composition.
In certain areas of Orkney (e.g. East Mainland, South Ronaldsay) the proportion of siliceous, ripple-marked sandstone within the cycles increases towards the top of the group. The Rousay Flags proper are overlain by a highly variable sequence of thin sandstones interbedded with red and purple marls and, in places, calcareous flags. These ‘passage beds’ have characteristics which are intermediate between those of the Rousay Flags and Lower Eday Sandstones and on the Geological Survey maps beds of this type have been included in the Lower Eday Sandstone in South Walls (Hoy) and South Ronaldsay, whereas in Deerness, Eday and Sanday they have been classed with the Rousay Flags. As there appear to be no lithological marker beds and as there is no close palaeontological control, there is, however, no real justification for assuming these passage beds to be diachronous. In the present account the passage beds are for convenience treated as a separate lithostratigraphical unit within the Eday Beds (p. 81).
No detailed account of the lithology and palaeogeography of the Rousay Flags has as yet been published. The lithology of this formation is, however, so similar to that of the Stromness Flags that the general conclusions concerning the environment of deposition of the latter (see pp. 74-77) must also apply. Without a detailed knowledge of the lithological variations within the cycles in space and time and without palaeocurrent data, nothing can be said at this stage about the shape and extent of the Orcadian lake or lakes in Rousay times or of the directions from which rivers entered the area.
The Eday Beds consist of massive yellow or red sandstones which contain two major units in which sandstones alternate with either flags or marls. The subdivisions of the group are as follows [Maximum thickness in Eday, as measured by Miss J. M. Ridgway, are as follows: L.E.S. 220 m, E.F. 33 m, M.E.S. 395 m, E.M. 103 m, U.E.S. 315 m. ]
- Upper Eday Sandstone
- Eday Marls
- Middle Eday Sandstone
- Eday Flags (with Eday Volcanic Rocks)
- Lower Eday Sandstone
- Passage Beds
The total thickness of the Eday Beds generally exceeds 1000 m. The lower half of the group may be equivalent to the John 0’ Groats Sandstone of Caithness and the entire group has been included in the upper half of the Givetian stage of the topmost Middle Devonian.
Fish remains are rare in the sandstones, but the Eday Flags have yielded a highly characteristic fish fauna which includes Microbrachius dicki (P916223, 2) Pentlandia macroptera (P916222, 1), T ristichopterus alatus (P916222, 2) and Watsonosteus fletti. This fauna has now been recorded in a number of localities in Orkney. It closely corresponds to that obtained from the fish beds at John O’ Groats. It also has some species in common with the Old Red Sandstone of south-east Shetland (p. 64).
The Eday Beds form the greater part of the island of Eday, where they crop out in the relatively unfaulted core of the Eday Syncline (P915584). They also give rise to considerable, though somewhat faulted, outcrops on Sanday, Stronsay, Shapinsay, East Mainland, Burray and South Ronaldsay. The outcrops extend over a distance of 58 km from north to south, and this makes it possible to assess the lithological variations within the group in this direction.
The thickness of the passage beds underlying the Lower Eday Sandstone (p. 81) varies greatly. In Eday the transition from Rousay to Lower Eday lithology is fairly rapid and obvious but in the southern isles and on Mainland there is an apparent northward and westward thinning out of the Passage Beds from South Ronaldsay (possible maximum 260 m) and Deerness towards the western part of East Mainland, the south coast of West Mainland and north Hoy.
In the northern isles the Passage Beds consist of thin, in places reddish, sandstones interbedded with beds of marl and with calcareous flags. The latter have, in several islands, yielded Dipterus valenciennesi. On the east coast of Deerness (East Mainland) the beds consist of thin units (up to 2 m thick) of thinly bedded purplish- or yellowish-weathering siltstone and sandstone alternating with thicker beds and lenses of massive sandstone which become more closely spaced upwards. In South Ronaldsay the Passage Beds form a thinly bedded sequence of yellow and reddish marls, flags and sandstones and on the shores of Long Hope in south-east Hoy they are made up of thin beds of hard fine-grained sandstone alternating with layers of green ‘marly’ flags with thin sandstone ribs.
The variation in the thickness and lithology of the Passage Beds suggests that they pass laterally by intercalation into either the Rousay flagstone or Eday sandstone facies. Such an interdigitation suggests an environment of deposition along the margin of a lake flat which was crossed by or impinged upon by the channels, flood plains and deltas of rivers. At first these distributaries were confined to small belts within the area, but later they advanced and spread out laterally to cover the entire district.
Lower Eday Sandstone
Over most of its southern outcrop the Lower Eday Sandstone consists of a bright yellow medium- to fine-grained cross-bedded sandstone, with relatively few small scattered pebbles, and with individual sets that rarely exceed 60 cm in thickness. Complete, unfaulted, sections of the sandstone are seen in Eday, where it ranges in thickness from 100 to 220 m and consists of two distinct portions. The lower of these, which forms from half to three-quarters of the sequence, is a medium- to coarse-grained predominantly reddish purple trough-cross-bedded sandstone. It contains scattered pebbles and lenses of conglomerate which commonly form lag deposits in the bottoms of the troughs. The pebbles are up to 7 cm in diameter, usually subrounded, and consist of pink granite and pegmatite, granitic gneiss, fine-grained quartzite, beige and red chert, clear and milky vein quartz, and lesser amounts of sandstone and hypabyssal igneous rock fragments. The upper part is a yellow, medium-grained sandstone which is generally devoid of pebbles and is characterised by large-scale planar and trough-cross-bedding. In the west of Eday this sandstone is resistant to weathering and, as it splits into large rectangular blocks, it has been quarried for freestone at Fersness Bay in Eday and on Fara. Palaeocurrent data indicate that in Eday the Lower Eday Sandstone was laid down by swift currents which entered the area from the south and south-west. On Sanday the lower facies consists of pebbly coarse- to medium-grained, yellow, orange and green sandstones. The pebbles have a maximum diameter of 13 cm and are either isolated or form lags in the basal layers of cross-bedded units. The upper facies consists of medium-grained white and pale green, often mottled, sandstones with some small isolated pebbles.
As the sandstone is traced south-eastward through Stronsay into Shapinsay it becomes progressively less pebbly and predominantly yellow in colour. On Mainland good sections are seen on the south and south-east coasts of Deerness and there are extensive exposures along the north shore of Scapa Flow (P915567). In Deerness the sandstone is 200 to 215 m thick, and consists of massive, predominantly yellow, medium-grained sandstone with generally small- to medium-scale cross-bedding and only rare scattered pebbles. A feature of the sandstone exposed in Deerness and many other outcrops is the high proportion of sets with some form of convolute or slumped bedding. The thickness of sandstone exposed along the north shore of Scapa Flow, on the downthrow side of the North Scapa Fault, is estimated at 550m, which is much greater than is usual in this group. It is possible that the sandstone here includes not only the Lower Eday Sandstone but also higher members of the Eday succession. This would imply that the horizon of the Eday Flags is entirely represented by sandstone. Alternatively, the increased thickness of sediment could be due to vertical movements along the fault taking place contemporaneously with the deposition of the Lower Eday Sandstone. Tectonic activity of this kind would account for the abundance of disturbed bedding structures in the sandstone in this part of Orkney.
In South Ronaldsay the Lower Eday Sandstone consists of soft, medium- to fine-grained yellow cross-bedded sandstone with some bands of grey or yellow marly shale and siltstone and very rare pebbly lenses. The bands of fine calcareous sediment are most abundant in the southern outcrops, and on the east coast. Between Halcro Head [476 856] and Wind Wick [457 870], fish-bearing rhythmic units similar to those in the Eday Flags (p. 85) are common in the upper part of the sandstone, suggesting that this part of the sequence is perhaps better placed in the lower part of the Eday Flags. Similar rhythmic units with calcareous flags containing fish remains (usually Dipterus valenciennesi, and Pentlandia sp.) have been recorded within the lower strata of the Lower Eday Sandstone of Flotta and these may therefore be considered as Passage Beds. In east Hoy the group consists of soft, yellow and reddish cross-bedded sandstone, and in north-west Hoy, where there are small outcrops of the sandstone at Sea Geo [261 027] on the north coast and beneath the Upper Old Red Sandstone tuffs along the west coast between Rack Wick [197 990] and Rora Head [182 992], it is characteristically soft, sulphur-yellow, fine- to medium-grained, with thin cross-bedded sets, thin bands of marly flags and rare pebbly lenses. The Lower Eday Sandstone status of the Rack Wick outcrop is, however, open to question.
The Eday Flags are a group of mixed sediments of very variable thickness, which appear to pass laterally by interdigitation into the Middle and possibly also the Lower Eday Sandstone (P915585). They consist of rhythmic units which range in thickness from two metres to several tens of metres. Most of these units have the following two phases:
- 1. A fining-upward phase of buff, yellow or, more rarely, red sandstone and sandy siltstone. This phase ranges in thickness from less than 1 m to over 25 m.
- 2. A coarsening-upward phase of grey, black, and locally purple ‘flagstone’ which generally has a finely laminated fish-bearing ‘quiescent water’ facies at the base succeeded by most of the other units and lithological features found in the lacustrine parts of the cycles in the Stromness and Rousay Flags.
A measured section of three Eday Flagstone cycles in Eday is shown in P915586. The cycles of this formation differ in several ways from the rhythmic units of the Stromness and Rousay Flags. The sandstone phases are in many instances much thicker and the exposed sections of many cycles have one or more thick, often red, channel-fill sandstones. These sandstones in places contain scattered pebbles and they are generally coarser-grained than the sandstones in the Stromness and Rousay Flags. Within the dark calcareous finely laminated fish-bearing siltstone-shale sequences near the base of the flagstone phases, one commonly finds bands of non-laminated pale grey silty mudstones which are 10 to 30 cm thick. These non-laminated beds may be the distal deposits of intermittent turbidity currents. Sun cracks and syneresis cracks are common in most siltstones and fine sandstones, but stromatolites have not been recorded. The thin sandstone ribs within the flagstone phase very commonly have load-cast bases, many are slumped, and some exhibit graded bedding.
The Eday Flags are thickest and best developed in and near Deerness (south-east Mainland), where they are up to 150 m thick and have a volcanic horizon near their base. Excellent exposures are seen on the west shore of Newark Bay [567 036]. In South Ronaldsay the thickness of the Eday Flags is also about 150 m, but the group contains a progressively higher proportion of sandstone as it is traced southward and westward from the north shore. In this island the flagstone phases of the cycles become more red, green and marly towards the south. Traced northwards from Deerness the formation diminishes rapidly in thickness to 100 m in Shapinsay, less than 50m in Stronsay and south-west Eday, until in the Calf of Eday and Sanday the Lower and Middle Eday Sandstones are separated by only 10 m or so of flaggy sediments. This apparent thinning appears to be a facies change resulting from the progressive northward thickening and coarsening of the sandstone phases and the corresponding thinning out of the flagstone phases in the higher cycles of the group. This process of interdigitation can be most readily demonstrated on Eday. Thus at Rushacloust on the south-west shore of the island, the formation consists of a lower, 18 m-thick, section which consists of three cycles with relatively thin sandstones and well-developed grey fish-bearing flags, and an upper section, about 25 m thick, composed largely of red, cross-bedded pebbly sandstone but containing three thin beds of siltstone and flagstone. The number of cycles with fish beds and thin sandstones decreases northwards to two at Fersness Bay in the centre of the island and to one on the shores of Calf Sound and on Sanday. The thin beds of flagstone in the higher cycles die out completely northwards, but on the shore of Fersness Bay an isolated fish bed crops out some 110 m above the highest remaining cycle with a flagstone phase.
Eday volcanic rocks
A number of isolated outcrops of fine- to medium-grained basic igneous rocks associated with thin beds of tuff have been recorded in the basal beds of the Eday Flags at Deerness, Shapinsay, and Copinsay [595 020]. The igneous rocks were interpreted as lava flows by Wilson and others (1935), but Kellock (1969) has upheld Flett’s original concept (Flett 1898b) that the rocks exposed at Muckle Castle (Deerness) and the Black Holm of Copinsay are intrusions. The former is probably a volcanic plug and the latter a sill. The basalts of Haco’s Ness (Shapinsay), Point of Ayre (Deerness), and the inland exposures in Deerness are lava flows with scoriaceous tops, pipe-amygdales and sandfilled fissures. At least two flows are present and though one is very thin the other is at least 7 m thick. The intrusive rocks and the lavas are alkaline olivine-dolerites and basalts which contain interstitial analcime, natrolite and alkaline feldspar. They are petrographically similar to the Carboniferous teschenites and basanites of the Midland Valley of Scotland. The field and petrographic evidence, however, leaves no doubt that the lavas and intrusions are coeval and that both are of Middle Old Red Sandstone age.
Middle Eday Sandstone
The Middle Eday Sandstone ranges in thickness from possibly 400m in Eday and Sanday to only about 90 min East Mainland and South Ronaldsay. This southward decrease in thickness appears to be due to the lateral passage of the sandstone facies into both Eday Flag and Eday Marl type lithology (P915585). Within the beds mapped as Middle Eday Sandstone there is a marked reduction in overall grain-size and an increase in the proportion of marly sediment from both the north and the south towards Deerness. In Eday and south-west Sanday the sandstone consists essentially of reddish purple, principally trough-cross-bedded, medium- to coarse-grained gritty sandstone with scattered pebbles and conglomerate lenses. Conglomerates are particularly well developed at Hegglie Ber on the south-west coast of Sanday. The pebbles consist predominantly of coarse granite, quartzite and Vein quartz but throughout the entire thickness of the sandstone and in the overlying groups there are also an appreciable number of rounded clasts of porphyritic and spherulitic rhyolite and scoriaceous basic lava. A few silty and marly beds are present in the lower part of the sequence.
In Stronsay the formation is composed of red and yellow sandstones, with only a few scattered pebbles and with marly beds, which are particularly prevalent towards the base. In Deerness the only undoubted outcrop of the Middle Eday Sandstone is that exposed at Newark Bay [569 040] where the strata immediately above the Eday Flags consist of a series of fining-upward cycles, each made up of a sandstone unit 50 cm to 2 m thick overlain by deep purple poorly laminated sandy siltstone and siltstone with thin ribs of commonly convoluted sandstone. Apart from their duller purple colour these beds are similar to the typical sediments of the Eday Marls (p. 88). The sequence exposed at Newark Bay appears to pass upwards into more massive sandstones. West and south of Deerness, at Scapa Bay and Burray, the higher sediments of the formation are exposed, these being red, white, pink and yellow sandstones with thin marly bands. Still further south, in South Ronaldsay, the sediments ascribed to this formation consist largely of red and yellow cross-bedded sandstones with only rare pebbles and with thin bands of grey, red and yellow ‘marl’.
This formation consists principally of thick beds of bright red and pale green calcareous sandy siltstone and pale green siltstone, alternating with thinner beds of hard, cross-bedded red and yellow sandstone. The sandstone bands and lenses become thicker and more closely spaced in the upper part of the sequence. The thickness of the group is over 200 m in South Ronaldsay, at least 150 min East Mainland and then decreases northwards to about 100 m in Eday and even less in Sanday. Again, much of this thinning may be due to the interfingering of the marl facies with the adjoining sandstone facies (P915585).
The diagnostic feature of the formation is the bright red colour of the fine-grained sediments. The rhythmic units with their high proportion of marly siltstones are fining-upward cycles of the type generally thought to be formed by meandering streams with shifting channels and extensive overbank deposits. In Eday the channel sandstones in the lower part of the sequence are less than 2 m thick and are separated from each other by up to 15 m of red sandy micaceous siltstone which is poorly sorted and has some ripple marks, sun cracks and traces of bioturbation. There are also bands and zones with calcareous concretions. As the sequence is followed upward the ratio of sandstone to marl increases and there is a gradual passage into the overlying Upper Eday Sandstone. The Eday Marls also crop out in East Mainland where they form the Head of Holland Peninsula [490 120] just east of Kirkwall, as well as in Burray, Hunda [432 969] and the north-west of South Ronaldsay. In these areas the beds form cyclic units which are similar to those in the lower part of the Eday Marls sequence of Eday.
Upper Eday Sandstone
The Upper Eday Sandstone crops out in two areas: a northern area which comprises the outcrops in Eday and south-west Sanday, and a southern area around the south shore of Scapa Flow, which includes the outcrops on Burray, north-westem South Ronaldsay and Flotta. In the southern area the group consists of beds of soft, cross-bedded, red, pink and yellow sandstone alternating with bands of red and green ‘marl’. The latter are most common and thickest at the base of the formation. In Eday, where the group reaches a thickness of more than 300 m, it falls into two lithological divisions. The lower division consists of beds of red and yellow sandstones with scattered pebbles, which pass upwards into more massive trough-cross-bedded pebbly sandstones alternating with beds of marl. The pebbles and sedimentary structures in the latter are closely comparable to those found in the Middle Eday Sandstones. The upper division shows a reversion to finer-grained sediments and consists of bright red and purple sandstones with thin bands of suncracked sandy marl.
5=== Conditions of deposition === No modern account of the lithology and palaeogeography of the Eday Beds has as yet been published, and the following suggestions as to the environment of deposition and direction of palaeocurrents are based on observations made by the author and Dr N. G. T. Fannin during a brief spell of field work in 1972.
The sandstones and red ‘marls’ of the Eday Beds were deposited in the channels, alluvial fans and flood plains of fairly large rivers which entered the area from the south-west and either filled up the Orcadian lake or at least encroached upon its margin. The presence within the Eday Flags of grey thinly bedded lacustrine flagstones, which alternate with channel sandstones and lake delta deposits, indicates that there was a time when the waters of a lake again covered the greater part of the Orkney area. This lake was fed by swifter and more active rivers than those which entered the lake at the time the Stromness and Rousay flags were laid down. They regularly built out deltas into the lake and these pushed its margin north-eastwards beyond the limits of the present area. The lithology of the higher beds of the Eday group indicates that a fluvial regime was soon re-established, first in the north, but then over the whole area. The Eday Sandstones were probably laid down by swift, braided and straight rivers which formed alluvial fans, but the Eday Marls were formed in the channels and alluvial plains of slower meandering streams. The appearance of swift-flowing rivers is probably the result of differential vertical movements along faults which were responsible for the tectonic uplift of the source area or areas.
This article is based to a large extent on the work of Dr N. G. T. Fannin (Fannin 1970) and the section on the Eday Beds incorporates data supplied by Miss J. M. Ridgway. In consequence the classiﬁcation used in this account differs in some respects from that published by Wilson and others (1935). Information provided by Mr U. McL. Michie of the Institute’s Geochemical Division has been used throughout article.
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