Middle ORS palaeogeography of Shetland and Orkney.
Old Red Sandstone
ORS deposition was in a variety of continental settings but in ‘Scotland’, part of the continental interior, sedimentation was into intermontane ‘pull apart’ basins and lakes. Erosion eventually breached the mountain chain between ‘Scotland’ and ‘Shetland’, so that by middle ORS times the metamorphic rocks were once more buried to a depth of several kilometres beneath sediment.
Three distinct ORS sedimentary basins are recognised in Shetland and were brought to their present relative positions by strike-slip fault movement. Although the development of these basins overlapped in time the oldest is the Walls Basin (Lower to Middle Devonian Walls Sandstone) has a thickness in the order of 10km.
The lower formation of the Walls Sandstone, The Sandness Formation, varies from conglomerates and breccia at its unconformable base with Grenvellian basement rocks through increasingly finer-grained sandstones from NE to SW along the strike of the beds. The sediments in the NE have been interpreted as those of alluvial fans and braided rivers and those in the SW may have been laid down on flood plains of meandering rivers. Basic and intermediate lavas, acid tuffs, agglomerates, ignimbrites and felsite intrusions interdigitate with the sandstones at the top of this formation. The relationship of these volcanics with the sedimentary sequences suggest the lavas and pyroclastic rocks were extruded onto river plains, the drainage patterns of which were being modified by growing volcanic highlands.
The upper formation, The Walls Formation, is mainly a dark grey, fine grained sandstone interbedded with siltstone, shale and rarely limestone. Variously this formation has been interpreted as having been laid down in a large rapidly deepening lake; in a shallow lake; entirely in an alluvial environment. Unlike the rocks of the other two Shetland basins the rocks of the Walls Basin have been affected by two episodes of folding due to the intrusion of the c. 360 Ma Sandsting Granite complex.
The rocks of the Melby Basin outcrop west of the Melby-St Magnus Bay Fault (Fig. 2) and on the island of Foula to the west. On Foula the succession is about 1800m thick comprising pebbly and cross-bedded sandstones of fast flowing rivers and shales and siltstones of meandering rivers. On the Melby peninsula the lower exposures of the Melby Formation comprise about 600m thickness of alluvial buff and red sandstones, pebbly sandstones and sandy siltstones laid down by rivers flowing from the WNW. These intercalate with two lacustrine siltstone and shale beds containing both fish and plant remains. A probable correlation of the Melby fauna with the Achannaras fish bed of Caithness and the Sandwick fish bed of Orkney suggests that the large Lake Orcadie (Fig. 8) may at times have transgressed the area. The upper part of the Melby Formation comprises thick beds of pink feldspathic sandstone containing plant remains, volcanic clasts and thick beds of bioturbated sandy siltstone deposited by currents flowing from the ENE. The change of current direction was probably due to the formation of volcanic highlands to the NE.
Intercalated with and overlying the sedimentary sequence in the Melby Formation are two thick flows of rhyolite. It is probable that these are equivalent to the volcanic rocks of the island of Papa Stour. The Eshaness peninsula also lies within the Melby Basin and comprises a suite of volcanic rocks that may be a continuation of the Papa Stour suite. An ignimbrite from the Eshaness suite has yielded a Rb-Sr isochron age of 365 +/- 2 Ma. Geochemically the lavas of the Melby Basin have been shown to be transitional between calc-alkaline and tholeiitic and have the characteristics of a suite formed by extensive fractional crystallisation. Magma generation resulting in the Shetland volcanic province may have been due to subduction beneath Shetland (and the Scottish Highlands) of a remaining fraction of Iapetus oceanic crust.
The South-east Shetland Basin is probably the youngest and (at the time of its formation) the most northerly. The range of depositional environments and the exhumed landscape of the ORS of Shetland are best exposed in what was once the western margin of this basin. Here deposition of sediments was into a NNW-SSE elongated basin that was perhaps a failed rift valley containing small lakes joined by rivers flowing SSE. These lakes had shallow shoaling margins and expanded and contracted (seasonally?) producing symmetrical cycles of lacustrine sediments.
Fault movement may have controlled sedimentation early in the life of the basin when a fluvial regime dominated but it is most likely that most of the sediments represent quiescent basin infill. The source rocks for most of the sediments were the schists and gneisses of the Caledonian mountain chain that now form the deeply eroded basement to the ORS. The Basin may have been situated on the eastern margin of a mountain range with an N-S trend (Fig. 8); erosion of this range exposed granites of the type now seen in the Northmaven area of Shetland.
The sandstones of the South-east Shetland Basin mostly now dip gently to the east. However the sediments have been locally disrupted to produce two narrow belts of steeply dipping, and in places inverted, strata. These ‘steep belts’ are thought to lie above N-S trending fracture belts in the metamorphic rocks beneath. Uplift and fracturing of the metamorphic basement is likely to have been caused by the emplacement of magmas into the basement rocks some time after the deposition of the sediments, possibly during Carboniferous or Permian times (345 to 225 Ma). These magmas induced some melting of the basement rocks, particularly the abundant limestone and magnesite, which contaminated the magma producing large volumes of gas and carbonate-rich hydrothermal solutions. The high pressure gases, hydrothermal solutions and contaminated magma then found their way upwards through fractured metamorphic rocks into the sedimentary rock above brecciating these and forming veins and dykes of ‘tuffsite’ and carbonate. (‘Tuffsite’ here refers to fine-grained material consisting of sandstone fragments and lava particles that were intruded in a fluidised form).
Hydrothermal fluids driven up through the ‘steep belts’ carried metals derived from the metamorphic basement beneath which may have hosted stratiform sulphide deposits such as those seen at Garths Ness in south Mainland. Copper and iron mineralisation is associated with the steep belts and two lodes of copper were mined commercially in Sandwick on SE Mainland. No copper deposits of this size occur in Scotland or western Norway and this zone of mineralisation may have been a source of economic prosperity in Shetland over 2000 years ago. A recent archaeological dig on an Iron Age broch site at Scatness on south Mainland has shown that Shetland may have once been a major centre for bronze and iron production (presumably with tin imported from Cornwall, the nearest source?).