Harrow and Hillingdon Geological Society
The Orcadian Basin, then and now
Orkney comprises 70+ islands 10 miles from mainland Scotland and has a population of about 20,000. It was originally one big island which was inundated due to sea-level rise. The geology is almost entirely Devonian on a metamorphic basement. Orkney was below the equator at the start of the Devonian and just at the edge of Caledonian mountain-building. Devonian sediments of the Lower and Upper Stromness Flagstones, the Rousay Flagstones and the Eday Group comprise water-laid sediments with some wind and biological input.
The Orcadian Basin ranged between a wet phase with seasonally active rivers to playa lakes, sand dunes etc of an arid desert. The basement, of which there are 3 outliers to the west at Yasnaby, consists of amphibolite gneisses and foliated granites with minor late undeformed granite and aplite veins. There is no Silurian, the basement being overlain by Devonian lake deposits, with some Permo-Carboniferous basic dykes and Recent glacial and peat deposits.
With a total thickness of 3,500m, rhythmic deposition of the Stromness Flagstones includes over 100 cyccles of deposition 5-10m thick. It is well seen at Marwick Head. In the Yasnaby Castle area the strata is loaded with mud flakes and stromatolites. Multi-layering is caused by sand, bacteria/algae and sedimentary activity.
The flagstones have been extensively quarried and saw marks and pecker marks result from this extraction.
Sedimentary structures in the Stromness Flagstones include raindrop impressions in the flagstones, which indicate the wind direction, pyrite crystals on bedding planes, stromatolite patches and bitumen, indicative of a lot of biological activity. Syneresis cracks are due to water removal and there are algal ridges, algal multi-layers, fine layering and bitumen, channelling and ripple marks. Rolled algal mats occur in the channels, including an algal mat vertical to the bedding.
There are occasional slumps or folding, dune bedding and mud cracks plus fish parts and a grazing trail. There is no bioturbation because of the anoxic conditions at the lake bed. The Sandwick Fish Bed is exposed at Quayloo and Sandwick Quay. Some flagstones show indentation due to wedge-shaped gypsum crystals.