Lower Old Red (probable) Sandstone of Orkney
|Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.|
Probable Lower Old Red Sandstone
At Stromness and Graemsay (P915581) the crystalline basement rocks which form the hilly pre-Old Red Sandstone land surface of Orkney are overlain by thin layers and lenses of coarse breccia, conglomerate and sandstone. These pass laterally into sandy ﬂagstones of Stromness type. At Yesnaby, however, only the upper parts of the original hills are covered by an apron of breccia which is of the same age as the Stromness Flags. The lower parts of these hills are overlain by a series of older sandstones and subordinate conglomerates, which Fannin has termed the Yesnaby Sandstone Group, and which are themselves separated by an angular unconformity from the basal beds of the Stromness Flags.
The Yesnaby Sandstone Group contains two facies which are now separated from each other by the Garthna Geo Fault (P915581). The beds exposed to the south of the fault are termed the Harra Ebb Formation. They rest on an irregular surface of the crystalline basement which forms part of the steep western slope of an exhumed hill and, farther west, part of the ﬂattish plain at the foot of that hill. The beds are composed of up to 100 m of interbedded sandstones and siltstones with tongues and lenses of breccia and conglomerate near the base. Palaeocurrent data indicate that the beds were laid down by currents travelling directly down the hillside. It is thus possible that they originated as alluvial fan and talus deposits around the base of the hill. They are unconformably overlain by the basal sandstones and pebbly sandstones of the Stromness Flags and there is an angular discordance of 6° to 10° between the two formations. (Recent mapping by Michie and Mr N L Watts (personal communications) suggests that most of the sediments overlying the Basement hills south-east of Garthna Geo belong to the Yesnaby Sandstone Group. Michie believes that most of these rocks are older than the Harra Ebb Formation.) The rocks north of the Garthna Geo Fault have been termed the Yesnaby Sandstone Formation. They comprise two sandstone units with distinctive characters. The lower unit consists of rusty-weathering grey fine- to medium-grained well-graded sandstone with large-scale, predominantly tabular, cross-bedding. The foresets are steeply inclined and individual sets range in thickness from 1 to 3 m. At intervals of 5 to 6 m the cross-bedding is truncated by major bedding plane surfaces, which, it is believed, were originally horizontal but have since been tilted to dip at 14° to the north-west. Fannin has suggested that these beds may be of aeolian origin and that the predominant wind direction at the time was from the west or north-west. The upper unit, which is best seen in the old millstone quarry at Qui Ayre, consists of massive, ripple-marked and locally trough-cross-bedded sandstone with rare small pebbles and with traces of bioturbation. It contains some thin beds of siltstone with sun cracks. Fannin has interpreted this unit as being deposited on the shores of a body of water which advanced across the dune-field from the south. The angular diﬂerence in dip between the Yesnaby Sandstone Formation and that of the overlying Stromness Flags is 10 degrees.
Further south at Warebeth, 2 km W of Stromness, a recent borehole encountered 50 m of purple siltstone and fine-grained sandstone below the conglomerate at the base of the Stromness Flags. The purple beds overlie 11 m of breccia, which, in turn, rests on the metamorphic basement. The ‘Warebeth ‘red beds’ are lithologically similar to the ?Lower Old Red Sandstone red beds of southern and western Caithness. The purple sediments of Warebeth, the Harra Ebb Formation and the Yesnaby Sandstone Formation could all have been laid down at about the same time. They are probably local facies with a lithology determined by their position relative to the basement hills. The absence of purple sediments of possible Lower Old Red Sandstone age in the outcrops adjoining the Stromness and Graemsay inliers suggests that the latter formed hills which, in Lower Old Red Sandstone times, projected well above the plain of deposition.
This article is based to a large extent on the work of Dr N G T Fannin (Fannin 1970). In consequence the classification used in this account differs in some respects from that published by Wilson and others (1935). Information provided by Mr U McL. Michie of the Institute’s Geochemical Division has been used throughout the article.
- FANNIN, N G T. 1970. The sedimentary environment of the Old Red Sandstone of western Orkney. Ph.D. thesis, University of Reading (unpublished).
- 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.