Middle Old Red Sandstone of Orkney, Eday Beds

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Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.

Introduction[edit]

Middle Old Red Sandstone Fish. P916222.
Geological sketch-map of Orkney. P915567.
Geological map of Eday and parts of adjoining islands, showing structure of the Eday Syncline (b) Section A—B across the Eday Syncline. P915584.
Hypothetical section showing the possible lithological variations within the Eday Beds from north to south. P915585.
Measured section showing cyclic sequence in the Eday Flags, Rushacloust, west coast of Eday. P915586.

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 (P916222, 2) Pentlandia macroptera (P916222, 1), Tristichopterus 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.

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.

Passage Beds[edit]

The thickness of the passage beds underlying the Lower Eday Sandstone 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[edit]

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 550 m, 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 (ND 476 856) and Wind Wick (ND 457 870), fish-bearing rhythmic units similar to those in the Eday Flags 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 (ND 261 027) on the north coast and beneath the Upper Old Red Sandstone tuffs along the west coast between Rack Wick (ND 197 990) and Rora Head (ND 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.

Eday Flags[edit]

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 (HY 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 50 m 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[edit]

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 (HY 595 020). The igneous rocks were interpreted as lava flows by Wilson and others (1935)[1], but Kellock (1969)[2] has upheld Flett’s original concept (Flett 1898b)[3] 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[edit]

The Middle Eday Sandstone ranges in thickness from possibly 400 m 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 (HY 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. 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’.

Eday Marls[edit]

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[edit]

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.

Conditions of deposition[edit]

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)[4] 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)[1]. Information provided by Mr U. McL. Michie of the Institute’s Geochemical Division has been used throughout article.

Bibliography[edit]

Full bibliography list

  1. 1.0 1.1 WILSON, G. V., EDWARDS, W., KNOX, J., JONES, R. C. B. and STEPHENS, J. V. 1935. The Geology of the Orkneys. Mem. geol. Surv. Gt Br.
  2. KELLOCK, E. 1969. Alkaline basic igneous rocks in the Orkneys. Scott. Jnl Geol., 5, 140-53.
  3. FLETT, J. S. 1898b. The Old Red Sandstone of the Orkneys. Trans. R. Soc. Edinb., 39, 383-424.
  4. FANNIN, N. G. T. 1970. The sedimentary environment of the Old Red Sandstone of western Orkney. Ph.D. thesis, University of Reading (unpublished).