Skerry of Ness, Warebeth, Skara Brae, Hill Of Cruaday, Orkney – an excursion
From: Fannin, Nigel G.T. Edinburgh Geological Society field excursion Orkney May 1991 (unpublished)
Locality 1 Skerry of Ness (HY 257 078) to Warebeth (HY 236 086)
This section displays the Lower Stromness Flagstones resting on a thin basal sequence which overlies metamorphic and granitic basement rocks. The top of the Lower Stromness Flagstones is marked by the abnormally thick Sandwick Fish Bed, the Orcadian equivalent of the Achanarras Fish Bed in Caithness. The lower most beds of the Upper Stromness Flagstones are exposed at Pulse Skerry (HY 235 083) but these are cut by the Warebeth Fault which down throws the Lower Stromness Flagstones. The Lower Stromness Flagstones sequence is repeated in the shore section at Stenigar and The Sandwick Fish Bed is exposed in cliff top quarries near the farm of Breckness.
At the Skerry of Ness the basal beds of the Lower Stromness Flagstones rest on granites and gneiss which form part of an exhumed hill and which slopes gently to the south. Up to 20 m of coarse breccio-conglomerates rest on, and fill hollows, on the basement surfaces. These pass up into conglomerates and arkosic pebbly sandstones with suncracked siltstone lenses. The sequence fines rapidly upwards and 25m above the base pebbles and coarse sands have all but disappeared. Some pebbles and cobbles show thin algal stromatolite coatings and above the conglomerates are well developed algal stromatolitic mounds, which show strong similarites to those found at present in Shark Bay, South Australia.
Above, and away from the influence of the coarser basement sediments the more typical fluvio-lacustrine cycle is developed. The deepest water sediments of the lake deposits, formed under anoxic conditions, form the prominent massive black rhythmite beds which are taken as the base of each cyclic unit. These pass upwards into interbedded sands and silts, often showing evidence of soft sediment slumping, which were deposited under oxygenated shallow water with well developed wave and current ripples. Above these shallow water lake sediments evidence of exposure, in the form of desiccation cracks, as the lake margin migrated under climatic influences, becomes increasingly common. In some sections channel sandstones may be developed. The top of the shallowing sequence is then cut abruptly with the appearance of the massive black rhythmites representing the transgression as lake levels rose again to begin the next cycle of gradual shallowing.
Correlation between faulted sections is greatly aided by the development of these rhythmites, any of which may contain fish. The development also, however of two distinctive rhythmite beds containing large chert nodules, respectively 14 m and 59 m below the Sandwick Fish Bed, make useful marker horizons which further aid correlation (HY 246 078). These chert rich beds also provide clear evidence of the peculiar water chemistry of the hypolimnion. The rhythmite laminae, which can be traced through the chert nodules, indicate compaction of up to 80% and confirm the very early diagenetic origin of the chert.
At the Warebeth the remains of a lead mine, which was worked around 1775, are still to be seen, and along the foreshore there are good exposures of NE trending breccia veins in the fault shatter zone with baryte, strontianite and galena.
Locality 2 Skara Brae (HY 231 188)
A brief stop can be made at the Bay of Skaill to visit the stone age village of Skara Brae. The Sandwick Fish Bed also outcrops in the bay but is poorly exposed.
Locality 3 Hill of Cruaday (HY 246 217)
The quarry at Cruaday has been a collecting site for fossil fish since the early 19th Century. The quarry is now only occasionally worked but still provides many excellent specimens of fossil fish and the spoil tips are well worth picking over.
The high quality of flagstones, for which the quarry was opened, are derived from the Sandwick Fish Bed which, on the basis of the faunas is correlated with the fish beds at Achanarras in Caithness and Melby in Shetland as well as several localities around Inverness. The Orkney sequences show that the Fish Bed represents the deep water facies of an unusually thick fluvio-lacustrine cycle which is repeated by faulting in at least ten localities along the west coast of Orkney. Stratigraphically the cycle is identified by its thickness (between 55 and 61 m) and its position relative to the two chert marker horizons. Near the base of the cycle however a 30 cm thick calcium carbonate rich horizon weathers pale blue, contrasting with the normally ochreous weathering of the ferroan dolomite cements of the flagstones, and forms a distinctive marker which can be traced throughout the west coast area. A thin ash layer about 3.5m above the base of the cycle occurs in the southern localities. The main fish bearing horizon occurs about 2.5m above the base of the cycle.
More significantly however the cycle is seen to split into two parts towards the north.This suggests that the shoreline, which presumable lay to the north of Orkney migrated southwards but never reached the most northerly localities before a rise in lake level pushed the shoreline northwards again. Thus in the southern localities the deep water sediments deposited in the de-oxygenated conditions of the hypolimnion of the lake formed rythmites and black silty mudstones between 15-20m thick. This unit thins northwards and passes into slumped and thinly laminated siltstones representing shallower, better oxygenated conditions. The reappearance of massive black rhythmites at the northern localities heralds the temporary return of the deeper water facies before the southwards shoreline migration eventually reached the southernmost localities.
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