Abstract
The only official maps to show a boundary between the Middle Devonian Stromness and Rousay Flagstones on Orkney are those published by the Geological Survey in 1932 and 1935. Since then, the difficulties of defining and locating this boundary have challenged many workers in the Orcadian Lake Basin. In 2015, the introduction of biozones by Uisdean Michie, based on fossil fish, and the recent discovery in Westray of the zone fossils Osteolepis panderi and Thursius pholidotus in a prominent fish bed across the island, has permitted the boundary for the base of the Rousay Formation to be determined in that part of Orkney. It is suggested that other characteristics of the fish bed of this rhythmic cycle of sediments, together with those in adjacent cycles, may be used as a marker for further correlation and mapping across the basin.
The Orkney group of islands lies off the NE coast of Scotland and is largely composed of Middle Devonian flagstones (Table 1). The – Eifelian to Givetian – cyclic lacustrine sequence has been divided into the Stromness and Rousay Flagstones (Wilson et al. 1935). The Stromness Flagstones are themselves separated into lower and upper series by the fossiliferous and well-mapped Sandwick Fish Bed, which crops out over the western Mainland of Orkney. The boundary forming the base of the Rousay Flagstones has been more difficult to determine.
Stratigraphic table showing the position of the Rousay Formation between the Upper Stromness Formation and the Eday Beds
Flett (1898) was the first to use the term Rousay Beds for the uppermost part of the Stromness sequence in Orkney. In 1932 he was Director of the Geological Survey when the ‘One Inch to a Mile’ (1:63 360) maps were published and the Orkney Memoir that followed (Wilson et al. 1935) with accompanying ‘Four Miles to One Inch’ map (1:253 440). This was the first and only time that a line on an official map showed a boundary separating the Stromness Flagstones from the Rousay Flagstones (Fig. 1).
Part of the ‘Quarter Inch’ geological map of the Orkney Islands. The map comes with the Orkney Memoir of the Geological Survey, 1935. This is the first and only official map that shows the boundary (arrowed) between ‘Stromness Flags’ and ‘Rousay Flags’, the latter in the paler colour. Most of the boundary is located across the Orkney Mainland. Larger scale ‘One Inch’ maps were published in 1932.
The widely recognized rhythmic sedimentation (e.g. House 1995) within the Orcadian Basin occurred throughout the Middle Devonian with many tens of cycles, each including a fish bed. The cycles reflect a system fluctuating between perennial lake and open playa in an inland basin. The climate was tropical in a latitude about 16° south of the Equator (e.g. Scotese 2013). The dark organic-rich calcareous mudstones forming the perennial lake floor commonly contain the remains of fossil fishes and are described as fish beds, although not every lake bed contains fossil fish.
The geology of Westray, the most northwesterly island of the Orcadian archipelago, is superficially very simple. The Rousay Flagstones are gently dipping, with fold axes, major faults and regional strike orientated roughly north–south. An east–west section (Fig. 2) across the island illustrates the general picture, with both a syncline and an anticline present, the ‘Westray anticline’ forming the dominant feature. In detail, there are more faults than those shown.
Sketch section a–b across Westray (Fig. 4) brings out the general geological structure from west to east across the island. As there is a small vertical exaggeration, the thicknesses of beds and amount of dip are not to scale. Key to stratigraphy: USFF, Upper Stromness Flagstone Formation; LRF, Lower Rousay Flagstone; MRF, Middle Rousay Flagstone. Length of section 7 km.
Visits to Westray took place from 1999 to 2017. The publication of Westray Flagstone, Guide to the Geology of an Island (Leather 2006) is an explanation of the geology of Westray. A collection of local fossil fishes is on permanent display at the Westray Heritage Centre (WHC). A number of cycles have been logged (Fig. 3), of which nine are shown and crop out along shore sections in the south and west of the island. Logs reveal an almost continuous run of 23 cycles from the uppermost Dickosteus threiplandi cycle (Upper Stromness Flagstone Formation) to the first cycle containing Millerosteus minor in the Middle Rousay sequence.
A succession of nine logs mapped along the foreshore between Mae Sand and Ness of Tuquoy in the SW of Westray (see Fig. 5), showing details of the continuous sequence of cycles across the boundary between the Upper Stromness Flagstone Formation (a–e) and the Rousay Formation (1–4). The thickness is shown in metres and the logs provide an example of the cyclicity of these thicker than average lacustrine beds. Fish species are: Dv, Dipterus valenciennesi; Gp, Glyptolepis paucidens; Rl, Rhadinacanthus longispinus; Op, Osteolepis panderi; Tp, Thursius pholidotus; Gm, Gyroptychius milleri. The sketch section c–d below is along the same foreshore and the black lines mark the position of the fish bed at the base of each cycle. RF, Rousay Flagstones; USF, Upper Stromness Flagstones.
The Rousay Formation
Between the late 1920s and 2015 the Rousay Formation was mapped by Wilson & Jones (1932). In 1990, Astin published a lithostratigraphical survey correlating cycles in several parts of Orkney. In 2015, Michie et al. (2015) published a survey based on the fish biostratigraphy of the Rousay sequence, dividing it into three units, Lower, Middle and Upper Rousay Flagstones.
Fauna
Orkney flagstones have commonly been described as monotonous, although intervals containing well-preserved fossil fishes have been known for 175 years (e.g. Miller 1841; Agassiz 1844–45). The 1935 Orkney Memoir defined the base of the ‘Rousay Beds’ by the presence of Estheria, a small bivalved crustacean or clam shrimp, strictly Estheria membranacea, only 2‒3 mm across and now named Ipsilonia orkneyensis (Chen & Morris, 1991), having previously also been attributed to Asmussia murchisoniana. The 1932 maps are scattered with notes such as ‘limy flags with Estheria’, or dotted with ‘Estheria’. This crustacean has since been found to occur lower in the succession so it is possible that the boundary shown on the 1932 maps may include some of the Stromness beds as Rousay Flagstones. The Orkney Memoir (Wilson et al. 1935, p. 167) has a chapter on the ‘Fossil Fishes of the Orcadian Old Red Sandstone’ by D. M. S. Watson, in which he notes that
Coccosteus (Millerosteus) minor and Thursius pholidotus are common, easily recognised and restricted to the zone [here, of Wilson et al.’s ‘Rousay Beds’]. Osteolepis microlepidotus (O. panderi) is restricted to this zone, but is difficult to identify except in well preserved specimens.
Michie et al. (2015) used the same fish species to divide the newly named ‘Rousay Formation’ into three biozones. This is a useful framework and shows the Lower Rousay unit marked by Osteolepis panderi Pander, 1860, and the Middle Rousay by Millerosteus minor Miller, 1858. Thursius pholidotus Traquair, 1888, occurs in both units. The Upper Rousay has no fish species as zone fossils as yet.
The biozone fossil for the Upper Stromness Flagstone Formation, Dickosteus threiplandi Miles & Westoll, 1963, occurs in most of the fish beds of the Upper Stromness Flagstone Formation in Orkney and was identified in Westray at Ness of Tuquoy five cycles below the top of the sequence. The base of the Rousay Formation here is marked by the first appearance of O. panderi, which is restricted to the Lower Rousay unit and identification is only possible by the presence of at least parietal or post-parietal head plates. At Brough of Birsay in Mainland Orkney, four cycles have been identified above the last containing D. threiplandi that are devoid of this species. From these four cycles, no fish remains have been identified (J. F. Brown, pers. comm.). The fourth cycle appears to be cut off abruptly at the top of the island. Previously, Speed (1999) had attempted to identify the boundary between the Upper Stromness Flagstone Formation and the Rousay Formation, making use of these barren cycles by recording the lithostratigraphy and measurements of shore sections of several cycles in order to correlate them.
Cyclicity
The outstanding feature of the Orkney flagstones is their cyclicity, reflecting the influence of Milankovitch cycles (House 1995) on the Devonian climate. Regular sedimentary cycles in the Rousay Formation of Westray average 10‒12 m in thickness. Since the lamination in the lake bed is interpreted as annual varves (Rayner 1963), the number of years can be counted. About 25 lake bed sequences 2–5 m thick have been logged in Westray and, using the widely accepted rate of deposition of 0.3 mm a-1, the average thickness gives a duration of about 10 ka in each cycle. Above the dark calcareous mudstones of the lake bed, the sediments that make up the rest of the cycle are mainly pale grey or greenish-grey siltstones, mudstones and fine-grained sandstones, interpreted as reflecting a transition from perennial lake to open playa and ephemeral lake. Whereas the deposits of perennial lakes (broadly referred to as fish beds) can be traced and correlated over a wide area (Donovan 1980), the upper parts of the cycles vary in thickness, even over distances of a few hundred metres. They also have time gaps, representing periods when there was no sediment added or when dust was blown from the desert surface, and laminae or bedding cannot be used to establish length of time.
The biozones identified by Michie et al. (2015) have enabled a new boundary to be established for the base of the Rousay Formation on Westray (Fig. 4).
Sketch map of Westray (Orkney) showing the boundary of the base of the Rousay Formation (RF) The pecked line to the east follows the possible much faulted boundary on the east side of the anticline. The map also shows the lines of sections (Fig. 2 and 3) and the distribution of biozone fishes: Mm, Millerosteus minor (Middle Rousay), Op, Osteolepis panderi (Lower Rousay) and Dt, Dickosteus threiplandi (Upper Stromness). Based on British Geological Survey (1999) 1:100,000 map and reproduced with the permission of the British Geological Survey ©NERC. All rights Reserved.
Boundary biostratigraphy
The Upper Stromness Flagstone Formation in Westray occupies the north–south axis of a faulted anticline in the centre of the island, and it is the western border of this belt of older strata that is the focus of this paper (Fig. 4). Below the base of the Rousay Flagstones, the fifth cycle from the top of the Upper Stromness Flagstone Formation, logged at Ness of Tuquoy, includes the biozone species D. threiplandi. In 2006, several median dorsal and nuchal plates of D. threiplandi (WHC 2016.4.10 and WHC 2016.4.12) were found at the Ness of Tuquoy in Westray (Figs 3 and 5) together with a lower jaw of Gyroptychius milleri Jarvik, 1948, identifying the Upper Stromness Flagstone Formation in Westray for the first time. From the Ness of Tuquoy, the succession continues with a gentle westward dip to include the last four cycles of the Upper Stromness Flagstone Formation and the first three or four cycles of the Lower Rousay Flagstones, before disappearing beneath Mae Sand. Gyroptychius milleri has been identified in four of the uppermost five cycles of the Stromness Formation. The last four cycles contain, in addition to disarticulated remains of Dipterus valenciennesi Sedgwick & Murchison, 1829, Glyptolepis, Homostius and Diplacanthus longispinus (now Rhadinacanthus longispinus Burrow et al., 2016). The second cycle from the top is a well-developed lake bed of 6.5 m with three intervals characterized by stromatolites, Dipterus and coprolites.
Sketch map of part of southwestern Westray (Orkney), Mae Sand to Ness of Tuquoy (see Fig. 3) showing the locations of fish beds and line of section c–d in a continuous sequence from the top five cycles of the Upper Stromness Flagstone Formation (USF) and the first four cycles of the Rousay Formation (RF). Biozone fishes are Op, Osteolepis panderi (Lower Rousay) and Dt, Dickosteus threiplandi (Upper Stromness). Based on British Geological Survey (1999) 1:100,000 map of Orkney and reproduced with the permission of the British Geological Survey ©NERC. All rights Reserved.
Of the cycles logged in Westray between 2005 and 2015, only one appeared again elsewhere in the island. The remaining cycles have their own characteristics of thicknesses, faunas and features, such as décollement, subaqueous shrinkage cracks, subaerial shrinkage cracks, stromatolites and pseudomorphs after gypsum. The term décollement is used here to describe a band of intensely disturbed sediment within a fish bed, possibly due to oil generation and escape. Such bands are commonly from 20 to 50 cm thick in which there has been directional movement and imbrication (Fig. 6). The rock appears shiny due to the presence of slickencrysts. These bands are easily recognized and are only found in some fish beds, thus helping to distinguish one cycle from another (Astin 1990). The same cycle, with fish beds identical to those mentioned above, occurs in two areas 8 km apart, one at the eastern end of Mae Sand (Fig. 5) and the other at Vere Point (Fig. 7). In 2015, intervening outcrops of the same cycle were also identified, on the west side of the Bay of Tuquoy and on each side of the Bay of Pierowall, completing a line of five outcrops along strike. The discovery in 2016 of the biozone fish O. panderi (Fig. 8a) in three of these outcrops and T. pholidotus in one of them, confirmed this to be Cycle 1 of the Lower Rousay sequence.
Cliff at West Kirbest, Westray, looking north, showing a band of décollement within a fish bed. Arrows show hammer (for scale) and imbrication.
Sketch map of the northern part of the island of Westray, Orkney (Bay of Pierowall) showing the continuation of the base of the Rousay Formation and cycles 1-4, lying above cycles b to e of the Upper Stromness. RF, Rousay Formation; USF, Upper Stromness Flagstone Formation. Based on British Geological Survey (1999) 1:100,000 map and reproduced with the permission of the British Geological Survey ©NERC. All rights Reserved.
Fossils from Cycle 1 of the Rousay Formation: (a) articulated specimen of O. panderi ‒ NMS G 2017.9.3. (b) well-preserved cranial shield of lung fish Dipterus - NMS G.2017.9.4.
From south to north the outcrops of Cycle 1 are:
HY 4490 4305, shore section alongside an old roofing slate quarry (Midhouse Quarry) at the south end of the track from Tifter farmhouse (Tufter on OS maps), east of Mae Sand (Fig. 5). The 4.1 m lake bed sequence has yielded a single well-preserved but partly disarticulated specimen of T. pholidotus (NMS G.2017.9.1) together with disarticulated Dipterus including several 3–5 cm well-preserved cranial shields (Fig. 8b, NMS G.2017.9.4). Recently, disarticulated O. panderi (NMS G.2017.9.2) have also been found here with jaws, parietal and post-parietal plates. The fish fragments are concentrated in the lowest 2 m of the bed, associated with bi-coloured flaser bedding, syneresis cracks and, 1 m from the base, a 25 cm contorted décollement band. Dark grey bituminous silty mudstones continue in the upper 2.1 m of the lake bed laminites and form a shallow anticline. Above the fish bed, 2.4 m of very thinly bedded dark grey to grey alternating mudstones and siltstones with occasional pseudomorphs after gypsum and mud pellets, reflect a transition from ‘permanent’ lake to playa, lithological association ‘C’ of Donovan (1980).
HY 4533 4438. The next appearance of Cycle 1 is a 50 m long outcrop on the west side of the Bay of Tuquoy (Fig. 5), where a fully articulated specimen of O. panderi was collected in 2016 (Fig. 8a, NMS G.2017.9.3). On the same slab, there are also parietal and post-parietal plates and, on a different bedding plane, four jaws and scattered rhomb-shaped scales of 2–3 mm.
HY 4513 4805, c. 2.5 km to the north, Cycle 1 lies along strike of outcrops on the south side of the Bay of Pierowall at Ness of Brough (Fig. 7). Here a medium-sized articulated 32 cm specimen of Dipterus was found, together with coprolites and groups of the clam shrimp I. orkneyensis. Details of the cycle beginning at HY 4504 4812 are similar to those of locality 4.
HY 4530 4926, on the north side of the Bay of Pierowall (Fig. 7), and east of Gill Pier, Cycle 1 is prominent next to the ruined storehouse, and contains occasional cranial shields of Dipterus and groups of the clam shrimp I. orkneyensis. The fish bed is 4.9 m thick here and there are signs of quarrying in the hard easily split ‘slates’ forming its upper part. The cycle is repeated by a fault near Burnt Mound.
HY 4592 5060, at Vere Point (Fig. 8), on the south side of The Ouse, the fish bed is identical to that of the Tifter locality with well-preserved Dipterus cranial shields. A small anticlinal fold is present in the lake bed deposits. However, the upper part of the cycle is not visible as it dips westward beneath the sands of The Ouse. Further north, towards Bow Head, the cycle is faulted out.
The beds in all of these localities show a gentle westerly dip of about 10° and, by the use of strike lines over intervening land, it was possible to define a boundary for the base of the Rousay Formation across Westray. See the map in Figure 4.
The last three outcrops (Figs 4 and 7) coincide with those marked on the 1932 Survey map as ‘purple flags with fish’, and shown on the map as a blue line. In the Memoir, Wilson et al. (1935), p. 92, stated
From the band of hard dark flags, which can be traced from Vere Point to the coast south of Berriedale and also across Pierowall bay to the Ness of Brough, the following forms have been obtained: Dipterus platycephalus, Dipterus cf. valenciennesi, Glyptolepis paucidens, and an osteolepid.
Wilson et al. (1935) described the same (Cycle 1) fish bed, and the osteolepid could be O. panderi. It is now accepted that D. platycephalus was simply a larger specimen of D. valenciennesi.
Cycle 2 of the Lower Rousay sequence averages 17 m thick and is seen in three localities: to the east of Mae Sand (Fig. 5), on both sides of the Bay of Pierowall near Ness of Brough, and near Gill Pier (Fig. 7):
At the Mae Sand locality, the fish bed is highly deformed along a strike-slip fault, and is almost vertical. It is only 1.5 m thick. However, 7.0 m above the fish bed, and within the same cycle, the sediment is locally enriched in disarticulated fish remains that form a ‘bone bed’. The fishes have been identified by Jan Den Blaauwen of Amsterdam University (pers. comm.), using numerous thin sections that demonstrate that there are eight genera present, including small osteolepid scales probably of O. panderi. Other genera recognized are: Glyptolepis, Homostius, Rhadinacanthus, Cheiracanthus, Cheirolepis, Dipterus, Ipsilonia and coprolites.
On the south side of the Bay of Pierowall (Fig. 7) the fish bed is 4.3 m thick and includes black silty mudstones with occasional laminae of 2–3 mm. At 1.6 m from the base, there is a 5 cm thick limestone containing stromatolites. Fragments of Dipterus, Homostius, Cheiracanthus and coprolites are present here. Subaqueous shrinkage cracks and single pseudomorphs after gypsum are also seen.
Near Gill Pier (Fig. 7), Cycle 2 again includes a 4.3 m fish bed. This is overlain by a 0.6 m band of mudstone with subaqueous shrinkage cracks and silty layers with pseudomorphs after gypsum, overlain by 2.0 m of grey siltstones and 0.5 m of black mudstone, also with shrinkage cracks and wavy bedding. Above this 3.1 m interval, where the lake appears to have been drying up, the fish bed reappears with 1.2 m of black mudstone containing scales of Glyptolepis, coprolites and shrinkage cracks. Thus, the total thickness of the two-part fish bed here is 5.5 m (4.3 + 1.2 m).
Cycle 3 of the Lower Rousay sequence outcrops at the springs below ‘Laird's Hut’, near Mae Sand (Fig. 5). The fish bed is 3.5 m thick, consisting of black bituminous mudstones, silty in the lower 50 cm, with thin beds containing subaqueous shrinkage cracks and pseudomorphs after gypsum. Fish present include Glyptolepis and Homostius; small coprolites are common. The same cycle appears near the Cornhouse, Broughton, on the south side of the Bay of Pierowall where Glyptolepis and coprolites have been found. The cycle is repeated at the warehouse at Gill Pier on the north side of the Bay of Pierowall.
Cycle 4 of the Lower Rousay sequence contains a richer fauna and shows subaerial desiccation cracks. Outcrops include the east end of Mae Sand and Scarfhall Point on the south side of the Bay of Pierowall. Fishes identified are Glyptolepis, Homostius, Dipterus, Cheiracanthus and Rhadinacanthus, together with coprolites and plant remains.
Discussion
In order to map the base of the Rousay Formation, the top of the Upper Stromness Formation needs to be established. At Ness of Tuquoy in SW Westray (Fig. 5) the biozone fossil for the Upper Stromness Flagstones is found only in the fifth cycle from the top. Here, there are four cycles between the last cycle with D. threiplandi and the first Rousay Formation cycle with O. panderi. These five cycles of the Upper Stromness Flagstone Formation are all thick, varying from 16 to 24 m, about 100 m in total for all five. They dip gently to the west at about 10°, and the outcrop can occupy several hundred metres of foreshore. Thus, assuming no major faults, they form a difficult sequence to identify and are not the best way to determine the Lower Rousay boundary. The occurrence of D. threiplandi five cycles from the top is recorded only in Westray. There is no proof that this is a general feature and the position of D. threiplandi in the sequence has yet to be determined in other parts of Orkney.
The cycle with the most outstanding characteristics is Cycle 1 of the Lower Rousay Flagstones, which averages 15‒17 m in thickness and contains a fish bed of 4.1 m. It is the lowest bed in which the two biozone fossils O. panderi and T. pholidotus occur, and marks their first appearance in the succession. But the fish bed has several other useful characters. It tends to form prominent outcrops with a thick band of hard, quality ‘roofing slate’, calcareous mudstones and one or two bands of two-tone wavy flaser bedding – forming buff patches within the dark laminite. There is a 25 cm band of intense deformation (décollement) and occasional well-preserved cranial shields of the lungfish Dipterus (Fig. 8b) and numbers of the bivalved crustacean I. orkneyensis. This bed may prove useful in identifying the base of the Rousay Formation across the basin. However, if, in future, either of the biozone species O. panderi or T. pholidotus is found in one of the four cycles below the Rousay Formation base as now defined, then the position of the boundary would have to be revised.
Applying the characteristics of Cycle 1 in Westray to other parts of Orkney, there is, so far, only one locality in north Mainland, on the top of Vinquin Hill where O. panderi is reported from roofing slates quarried from a 2 m thick bed (Wilson et al. 1935, p. 61). Cycle 1 is also similar to that described at Mid Howe in the island of Rousay by Michie et al. (2015).
Changes of lithology across the boundary are slight, and rates of deposition immediately above and below it were similar, generating cycles of similar thickness. Sandy beds are more prominent higher in the sequence as a whole, as might be expected from a slowly filling lake basin. Also, in the longer term, there are signs of shallowing higher in the succession, with sporadic increases in the appearance of stromatolites, plant remains, desiccation cracks and pseudomorphs after gypsum. However, the more frequent appearance of these features is not consistent, and not all occur in the same fish bed. The principal changes in the cycles across the boundary, and ultimately that which determines its position, is the presence of three biozone fish species.
Conclusions
The discovery of the first appearance of the biozone species O. panderi and T. pholidotus in a prominent fish bed (forming the base of Cycle 1 described here) in Westray has allowed the base of the Rousay Formation to be recognized across the island from south to north (Fig. 4). So far, only three fish species have been found in the basal 4.1 m fish bed, the two biozone species and the lungfish Dipterus, together with the clam shrimp I. orkneyensis. Dipterus is present as disarticulated remains including well-preserved cranial shields. Osteolepis panderi is less common and difficult to see, especially on wet rock surfaces. Thursius pholidotus is very rare in this Cycle 1 fish bed. However, rocks of the cycle have other useful characteristics that aid recognition, including a band of décollement, wavy flaser bedding with light-coloured patches, and a thick, hard silty mudstone band of ‘roofing slate’. Thus, Cycles 1–4, with O. panderi present in one of them, and the uppermost cycles of the Upper Stromness Flagstone Formation, including one with D. threiplandi, form a series of fish beds that may provide a marker for further work across the Orcadian Basin.
Acknowledgements
I would like to thank Jan Den Blaauwen and John Flett Brown for their generous support, and to acknowledge the invaluable help of the late Uisdean Michie and fossil fish specialist Mike Newman. Thanks, too, to Editor Colin Braithwaite. Full colour maps of this paper are published online.
- © 2017 The Author(s)
References
Demarcation of the boundary between Middle Devonian Upper Stromness Flagstone and Rousay Flagstone formations in Westray, Orkney
