Old Red Sandstone of Shetland, Eastern outcrops
|Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.|
The Old Red Sandstone of south-east Mainland forms a narrow discontinuous outcrop which extends from Rova Head, 3.5 km N of Lerwick, southwards for nearly 40 km to Sumburgh Head (P915578). It also forms the islands of Bressay, Noss and Mousa. Fish remains, plants and ‘Estheria’ have been obtained from over 50 different localities throughout the outcrop and it is believed that the age of the beds ranges from Middle Old Red Sandstone (i.e. Eday Beds age) upwards possibly into the Upper Old Red Sandstone.
It has long been the custom to divide these beds into the following subdivisions:
- (1) Basement Breccia
- (2) Brindister Flags
- (3) Rova Head Conglomerate
- (4) Lerwick Sandstone
- (5) Bressay Flags
Although some of these divisions correspond to well-marked lithological types, others, such as the Brindister Flags, include a number of diverse rock types. The groups do not constitute a stratigraphic succession, and as is shown diagramatically in P915579, the base of the sequence is formed by the Rova Head Conglomerate in the Lerwick area, by breccia in the Quarff-Fladdabister district, and by the Brindister Flags, locally underlain by breccia, over most of the area south of Cunningsburgh. The Old Red Sandstone of south-east Shetland can be best visualised as a number of lithological facies of limited lateral extent which interdigitate with each other. They were laid down on an undulating basement of metamorphic rock, which at the time of deposition appears to have had an overall slope to the east-south-east. A north-south section at that time would have passed across an eastward-sloping scree-covered hill-shoulder in the Quarff district. This separated a deep south-eastward-flowing river valley in the north from a more open river valley, and later alluvial plain in the south. Low ground at times covered by a lake must have lain to the south-east of the present area and this lake periodically transgressed north-westward to form the relatively thin beds of fish-bearing calcareous flags which are intercalated with the terrestrial sediments.
Area south of Fladdabister
In the area south of Fladdabister (P915578) the junction between the Old Red Sandstone and the metamorphic rocks is in part a fault and in part an unconformity. A basal breccia with angular pebbles of locally derived metamorphic and igneous rocks is developed in the Spiggie area, but elsewhere, as at Little Holm (HU 380 097) in the extreme south, sandstone rests directly on the basement. A continuous sequence of strata is seen on the coast between Leven Wick and Sumburgh Head. This consists partly of cross-bedded buff sandstones with scattered pebbles and thin bands of purple or grey sun-cracked siltstone and mudstone. Interbedded with these are a number of fairly thin but widespread lenses of conglomerate and pebbly grit. All these beds were probably laid down by swift-flowing, possibly braided, rivers. In the area north-east of Exnaboe there are, in addition, some arkosic sandstones with well-rounded grains and very large-scale cross-bedding with individual sets up to 5 m thick. The latter may have been deposited by wind. Throughout the area, but particularly in the north, the cross-bedded pebbly sandstones are interdigitated with finer-grained deposits, made up of thin sandstones alternating with relatively thick beds of grey or purple mud-cracked and ripple-marked siltstone and mudstone. Sequences of the latter type form a large part of the succession of Mousa, and the Helli Ness, No Ness, Cumlewick Ness and Sumburgh Head peninsulas. They are probably the channel and overbank deposits of meandering rivers. Thinly laminated, dark grey, limy, lacustrine flags with limestone nodules and occasional fish and plant remains form a number of prominent beds within the sequence. Four such beds have been recorded in the south of the outcrop (see P915578). The most fossiliferous of these is the Exnaboe Fish Bed, exposed at Shingly Geo, 2.5 km NE of Sumburgh Airport, but the thickest and lithologically most interesting forms part of the eastern coastal cliffs of the Sumburgh Head peninsula. At Looss Laward the calcareous flags show evidence of large-scale penecontemporaneous slumping. At least two lacustrine fish beds are present on the peninsulas adjoining Sandwick and on Mousa. The following fauna has been obtained from the various fish beds of the southern area: Asterolepis thule Watson (P916223, 3) Coccosteus sp. nov., Dipterus sp., Glyptolepis ?, Microbrachius dicki Traquair, Stegotrachelus finlayi Woodward and White, Tristichopterus cf. alatus Egerton and ‘Estheria’ sp. Dr R. Miles states that this assemblage can be regarded as broadly contemporaneous with that from the Eday Beds of Orkney and the John 0’ Groats Sandstone of Caithness, as both include Microbrachius dicki and Tristichopterus. However, there are differences in that among the lung fishes and coccosteids the John O’ Groats—Eday fauna contains Pentlandia macroptera (Traquair) and Watsonosteus fletti (Watson) in place of Dipterus sp. and Coccosteus sp. nov. If these differences have any stratigraphical meaning it is that the Shetland fauna is the slightly older of the two.
Area north of Fladdabister
In the Quarff-Fladdabister area up to 30 m of basal breccia and fanglomerate with angular to subrounded blocks and pebbles of locally derived metamorphic rock rest on an undulating basement (P218865). The breccia thins westwards and southwards and is locally completely absent. It passes upward by way of a thin series of flaggy sandstone with pebbly bands into fine-grained purplish micaceous flaggy sandstone. Farther north, in the Gulber Wick area, the purple flagstones appear to pass northwards by interdigitation into cross-bedded pebbly sandstones with conglomerate lenses and relatively thin purple mudstone partings. The pebbly sandstones are typical of the Lerwick Sandstone facies and farther north these are, in turn, interdigitated with the Rova Head Conglomerate, a coarse conglomerate with rounded pebbles of granite and quartzite which locally reach 1 m in diameter. Though to the north of Lerwick the junction between the Rova Head Conglomerate and the metamorphic basement is sheared, there are a number of exposures of basal breccia along this junction and the matrix of the most westerly exposures of conglomerate consists of fine breccia. This suggests that the conglomerate passes down, without any intervening sediment, into basal breccia, which may be thin and discontinuous. Palaeocurrent data suggest that these coarse sediments came principally from the north-west, and it is probable that the present Lerwick area lay along the course of a major intermontane valley which was being filled by the deposits of torrential rivers. On the Ness of Trebister sediments of the Lerwick Sandstone facies are interbedded with beds of black and purple siltstone and shale with abundant ‘Estheria’, and on the Ness of Sound they contain two beds of nodular limy lacustrine siltstone and mudstone with fish remains belonging to the same species as those in the southern area. These fine-grained beds appear to die out north-westwards, suggesting that at the time of deposition an open basin lay to the south-east.
In the Lerwick peninsula and on the islands of Bressay and Noss, the coarse fluvial pebbly sandstones of the Lerwick Sandstone facies give way by interdigitation upwards and probably also southwards to beds of the Bressay Flagstone facies. The latter consist of generally flaggy but locally cross-bedded sandstones interbedded with dark grey and purple siltstones and shales. Some of the rhythmic units bear a certain resemblance to the flagstone cycles of Orkney, but here the channel sandstone phases are generally thicker and with few exceptions the fine-grained beds are coarser, less calcareous and devoid of such characteristic minor structures as fine lamination and syneresis cracks. Plant remains are common in both facies of this area, and these include the conspicuous ‘Corduroy Plant’, a ribbed plant stem up to 60 cm long and nearly 10 cm wide. Fish remains found in a small number of closely adjoining localities in north-east Bressay (P915578) include the forms Asterolepis sp., Holonema ornatum Traquair and Glyptolepis cf. paucidens (Agassiz). There is no comparable assemblage either in Orkney or the Scottish mainland. Two of the genera, Asterolepis and Glyptolepis, elsewhere have a range which extends into the Upper Devonian and it is possible, but not proved, that the highest beds of Bressay may be of Upper Old Red Sandstone age. Most of the sediments ascribed to the Bressay Flagstone facies appear to be of fluvial origin.
A high proportion of the Old Red Sandstone strata of south-east Shetland dip to the east and east-south-east, with the steepest dips in the extreme south. In the northern and central parts of the area this simple dip is modified by a number of open north to north—east trending folds and by a large number of faults and shatter belts with a predominantly north—south trend. The islands of Bressay and Noss are traversed by two north—south trending belts of steeply inclined and locally inverted strata which appear to mark the positions of major shatter belts in the metamorphic basement, and which contain, or are associated with, extensive areas of tuffisitic breccia (P915591).
Fair Isle is made up of sedimentary rocks of Middle and possibly also Lower Old Red Sandstone age, which are for the most part steeply inclined to the east-south-east. At least 2700 m of strata are present and these can be divided into four lithostratigraphical units (P915580). All four groups contain a high proportion of grey to buff, locally red-stained, arkosic sandstone and along the north-west coast of the island as well as in Bu Ness and Vaasetter in the east there are a number of beds of pebbly grit and conglomerate. The two lower groups contain beds of dolomitic mudstone and siltstone which are relatively thin and widely spaced in the lowest part of the sequence, but form a high proportion of the Observatory Group in which bands of predominantly fine-grained sediment are up to 200 m thick. In the northern part of the Bu Ness Peninsula the sequence has a strongly rhythmic character with beds of sandstone, up to 9 m thick, alternating with much thinner beds of siltstone and mudstone. The fine-grained rocks of the Observatory Group have yielded remains of the plants Dawsonites roskiliensis Chaloner, Hostimella sp. and T hursophyton milleri (Salter); the branchiopod crustacean ‘Estheria’; and unidentifiable cycloid fish scales. The Bu Ness Group has yielded Hostimella sp., cf. Prototaxites sp., Svalbardia scotica Chaloner, Zosterophyllum? and ‘Estheria’, as well as scales of dipnoan fish and a plate of a coccosteid arthrodire. The flora from the Observatory Group could be of Lower Devonian age, but both the fish and plants from the Bu Ness Group have a Middle Devonian aspect.
Palaeocurrent indicators suggest that the sediments were deposited by currents moving in an easterly to north-easterly direction. The pebbly sandstones and conglomerates were probably deposited by braided rivers in alluvial fans close to the western margin of a lake basin. The fine-grained dolomitic beds were probably laid down during periods when the lake waters encroached westwards across the distal edges of these fans. The rhythmic sequences of Bu Ness may be the deposits of a river or rivers meandering over an alluvial plain.
Over the greater part of Fair Isle the strata are steeply inclined to the east-south-east, but in the south-west corner of the island they are flexured into open, eastward-plunging folds. In this area the fine-grained sediments have a slaty cleavage and a lineation, the geometry of which reflects that of the major folds. Fair Isle is cut by a number of near-vertical west-north-west trending faults, along which the deep geos on the west coast of the island have been excavated. Most of these faults do not have large throws, but one may have a dextral transcurrent displacement of 550 m. Some of the fault planes have basic or acid dykes emplaced along them and many contain, or are associated with, a network of scapolite-carbonate veins. The sedimentary rocks cropping out in the south-west corner of the island are indurated either as a result of the tectonic deformation of the area or by the thermal metamorphism produced by a granitic mass which may crop out beneath the sea just south-west of Fair Isle.
The three groups of Old Red Sandstone rocks of Shetland Mainland differ from each other in age, in their depositional and volcanological development, in their tectonic history and in the extent to which they have been affected by igneous intrusions. As all three appear to rest directly on the metamorphic basement, they do not represent parts of a continuous sequence but form the deposits of three geographically distinct basins. The rocks of Melby and F oula appear to have been laid down in the most southerly of these; they may in fact have been deposited along the north-western fringe of the extensive shallow and tectonically stable Orkney-Caithness basin. The Walls Sandstone and possibly also the Fair Isle sandstone may have been deposited farther north in an intermontane, tectonically and volcanologically active basin, which developed somewhat earlier than the others and was affected towards or just after the end of Middle Old Red Sandstone times by two phases of compression. The East Shetland sediments were probably laid down on the western margin of the most northerly basin which was bounded by mountainous terrain to the west, but had access to an open lake to the south-east. The present virtual juxtaposition of three such diverse groups of rocks is most readily explained by postulating subsequent dextral transcurrent movements along both the Melby and Walls Boundary faults. The extent of movement along the latter may have been in the order 60 to 80 km.