Middle Old Red Sandstone of Orkney, Rousay Flags
|Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.|
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 (1898b), 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 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 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.
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 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.
- FLETT, J. S. 1898b. The Old Red Sandstone of the Orkneys. Trans. R. Soc. Edinb., 39, 383-424.
- 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.
- DONOVAN, R. N., FOSTER, R. J. and WESTOLL, T. S. 1974. A stratigraphical revision of the Old Red Sandstone of North-eastern Caithness. Trans. R. Soc. Edinb., 69, 167-201.