The oldest sedimentary rocks in Orkney are probably of Lower Devonian age. They outcrop to the north and east of the Yesnaby exposure of basement rock.

Garthna Geo

Yesnaby Sandstone formation

Schematic Lower Devonian Environment

Schematic Lower Devonian Environment

Harra Ebb formation

The Harra Ebb formation (60-100m) consists of breccias, sands muds, and conglomerates deposited on an uneven surface of an eroded basement gneiss hill. Paleocurrents show that current movement is to the west and the deposit was an alluvial fan. probably flowing down slope.

Interesting solution/erosion hollows are are formed at several spots on the Harra Ebb unconformity surface. They were subsequently filled with Yesnaby Sandstone. The origin of these erosion features on the unconformity surface is unknown. The cylindrical shape of the one (1m x3m) argues for a karst like solution surface.

The breccias and beach sands overlying the basement rocks on the West Shore, Stromness, interleave with the lake sediments and are therefore contemporaneous and of Middle Devonian age. The breccias on the east side of the Stromness Basement Hill / Island are more similar to the Harra Ebb lithologies while the breccia overlying the Quoyelsh Felsite consists mainly of Felsite clasts and boulders.

A borehole at Warebeth (a mile west of Stromness) revealed 200 ft (61m) of purple siltstone, sandstone and breccia below the flagstones. The lower flagstone succession in the borehole directly correlates with the West Shore succession.

From Rodgers 1987

The Yesnaby Sandstone formation

The Yesnaby Sandstone formation consists of up to 200m of desert sandstone (dune bedding clearly shown in the sea stack of the Yesnaby Castle). The Harra Ebb formation was  tilted and eroded before the dune sands  were deposited on  the uneven surface . The sand thus varies in thickness over the area.

On the south side of Garthna Geo the Yesnaby Sandstone filled in a fossil 20m cliff cut into both the Basement Complex and the Harra Ebb. The sand overflows the cliff top and thins to a “Zero Edge” punch-out to the south at the head of Inganess Geo.

The dune sands pass upwards into reworked fluvial sands with trace fossils of burrowing organisms observed in the “millstone quarry” southwest corner..

These strata were finally tilted about 10 degrees to the west before being overlaid by the Middle Devonian lake sediments. The contact is characterised by lag deposits with white quartzite  pebbles 1-3cm across.

This porous desert sand is occasionally blackened with shiny black bitumen (dead residue of crude oil) showing that it had once been a reservoir for hydrocarbon liquids. This reservoir was breached recently when the rocks came to near the surface. It is estimated that this OIL RESERVOIR could have held between 350 and 1,000 Million Barrels of Oil in place.

From Rodgers 1987

  • 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.

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  • The age of the Orkney dykes is 252+/-10 million years, placing the time of intrusion in the late Permian, a period of renewed tectonic movement in the sedimentary basins to the west of Orkney.

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  • 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.

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  • At the opening of the Tertiary chalk sedimentation continued on the northern North Sea. Starting around 63 Ma, the East Shetland Platform was uplifted and tilted towards the south-east. ...

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  • 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.

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  • 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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