The Orcadian Basin formed in response to extensional tectonic forces and consequent major growth faults with the sediments deposited in a series of tilted “half grabens”. The Orkney Flagstone group is 752m thick, and consists of grey and black thinly bedded flagstones clearly laid down in an extensive freshwater lake. Thicknesses vary, but where given below the figure represent the actual measured value in type sections. The Orkney Flagstone Group, principally Eifelian in age, .is sub-divided into three parts:
Lower Stromness Flagstone formation:
A basal breccia beach deposit fringing the Granite Gneiss Complex in the Stromness area and reworked Yesnaby Sandstone at Yesnaby, pass upwards into 277m (43 cycles) of lake cycle flagstones. The top of the formation is marked by the distinctive Sandwick Fish Bed 20m thick which is exposed on the coast and in several quarries in the west Mainland.
Upper Stromness Flagstone Formation:
285m (25 cycles) of lake cycle deposits with a higher content of fluvial river sand and sheet flood deposits coming from the northwest in the normal lake cycle. The top of the formation is less distinct than the base. The presence of fossil Asmusia murchisoni, was used by the Geological Survey to define the beginning of the Rousay Flags. Recent stratigraphy puts the top at the transition from a series of very sand rich and thick cycles to a distinct set of much thinner cycles with current ripple marks in the sheet flood sands indicating flow to the south and southwest.
Rousay Flagstone Formation:
the basal fish bed of the formation is relatively thick and rich in fossil fish and stromatolites followed by 170-190m (18 cycles) of lake cycle deposits similar to the underlying Upper Stromness Flagstone Formation. Near the top of the Formation, at Saquoy Head, is a distinctive pebbly sandstone known as the Saquoy Sandstone Member thinning from 17m in Rousay to 4m in Eday. It seems to have been deposited from an eastward flowing river carrying quartzite, psammite and dolomitic limestone pebbles from an exposed basement source in the west.
During the Devonian Period, North America and Northern Europe were joined together forming Euramerica, one of the three major continental masses on the Devonian globe, with Orkney placed south of the equator. Most of Britain formed part of this landmass with mountains in the northwest and the open marine Devonian Sea covering southwest England.
The Petroleum System consists of four main Geologic components, source, reservoir, seal and trap plus additional processes necessary to generate and store hydrocarbons in the subsurface. Their presence is required to generate a viable EXPLORATION TARGET.
The Quaternary deposits found on the East Shetland Platform are thin (Johnson et al., 1993). The Quaternary thickens above the Viking Graben to as much as 300m and provides a long and relatively detailed record of glaciation.
Rifting in the Northern North Sea commenced during the early Triassic, peaked during the late Jurassic, and terminated by the late Cretaceous. The Viking Graben can be seen as a failed arm of proto-Atlantic rift. Almost throughout this period, the Shetland area formed an area of positive relief.
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