Old Red Sandstone of Shetland, Central outcrops
| Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office. |
Walls Peninsula
The greater part of the Walls Peninsula of Shetland Mainland is composed of the Walls Sandstone (Finlay 1930)[1], which is of possibly Lower and Middle Devonian age. The Walls Sandstone consists of two major stratigraphic units which are separated from each other by the east-north-east trending Sulma Water Fault (P915598).
Sandness Formation
The lower unit, called the Sandness Formation (P915597) crops out north of the fault. It rests unconformably on the metamorphic rocks of the Walls Peninsula, though the junction is in most places obscured by small-scale faulting. Its thickness ranges from 1350 m to about 3000 m and the lower part of the formation consists mainly of sandstones and conglomerates, which appear to cover an undulating metamorphic basement and which were derived from a north-easterly source. There is a marked decrease in the overall grain-size and in the proportion of conglomerate from north-east to south- west along the strike, and the sedimentary structures suggest that whereas the sediments in the north-east were laid down in alluvial fans and by braided rivers, those in the south-east may have been formed in the flood plains of meandering rivers. The upper part of the formation contains the Clousta Volcanic Rocks, which consist of a number of basaltic and andesitic lava flows, ignimbrites, tuffs and several cones of predominantly acid agglomerate. There are also a number of concordant intrusions of felsite. Some of these appear to have been intruded into loosely consolidated sediments, suggesting that the age of intrusion was penecontemporaneous with the deposition of the sediments. The sedimentary rocks associated with the lavas and agglomerates include some thick beds of mudstone and calcareous siltstone which may have been laid down in lakes ponded up by the newly erupted volcanic rocks.
Walls Formation
The upper unit of the Walls Sandstone, termed the Walls Formation, crops out south of the Sulma Water Fault. It consists of up to 9000 m of highly folded, generally dark grey, fine-grained evenly bedded sandstone, which is in many instances interbedded with siltstone, shale and, locally, limestone. In thin section the sediments have some of the characteristics of greywackes, such as a high proportion of clastic matrix, and on a larger scale they have certain features in common with flysch deposits, though there are also a number of significant differences. The mode of deposition of these beds is not yet certain; the author believes that they may have been laid down in a fairly deep and extensive lake.
Poorly preserved fish remains determined by Dr R. S. Miles as acanthodian fish aff. Cheiracanthus sp. and crossopterigian and dipnoan scales have been found at a number of localities in the Walls Formation, and poorly preserved plant remains, principally of Hostimellid type occur in both formations of the Walls Sandstone. The fish remains suggest that the Walls Formation may be of Middle Devonian age, whereas the plant remains from the Sandness Formation, which include Psilophyton sp., do not rule out a Lower Devonian age for the latter.
Structure
The rocks of the Sandness and Walls formations have been involved in two phases of intense folding. The first phase produced tight folds with east-north-east trending axes, as well as some east-north-east trending movement planes. The major structures ascribed to this period are the complex Walls Syncline and the Watsness-Browland Anticline (P915598). Within a 3 km- wide zone on either side of the Walls Syncline the fine-grained sediments have taken on a slaty cleavage and a lineation with an orientation which reflects the geometry of the major folds. Some areas are also affected by complex minor folding. The second period of deformation produced north-north-east to north trending folds, and in the area extending from Gruting Voe to Brindister Voe minor folds and cleavages developed during this period have deformed the earlier structures. In the more intensely folded areas the sediments have a mineral assemblage characteristic of the zeolite facies and, locally, the greenschist facies of regional metamorphism.
The rocks of the Walls Formation have been intruded by the Sandsting Complex (P915574) which contains several large and many smaller enclaves of hornfelsed sediments. Thermal metamorphism has given rise to mineral assemblages characteristic of the hornblende-hornfels facies near the contact and alteration of a lower grade extends in places as far as 1.5 km from the granite. Both periods of folding in the Walls Sandstone are later than the emplacement of the complex which is dated at 360 to 370 million years. The deformation most probably took place during Upper Devonian time and is thus the latest phase of intense localised late-Caledonian folding recorded within the British Isles.
North Mainland
A number of small outliers of folded and faulted, predominantly sandy sediments crop out along and close to the north-east coast of North Roe (P915575). A rather larger outcrop forms most of the island of Gruney, 1.5 km N of the Point of Fethaland. This outcrop consists of a basal breccia overlain by conglomerate and arkose with thin beds of mudstone. These rocks may have been laid down first as scree, then as alluvial fan deposits and finally as the channel and overbank deposits of swift-flowing rivers. They are to some extent comparable with some of the sediments of south-east Shetland, but if Pringle’s (1970, p.166)[2] contention that they are older than the Ronas Hill Granite is correct, they may be of roughly the same age as the Walls Sandstone.
