Geology of Orkney

The unconformity at the base of the overlying Devonian rocks is highly irregular, with the exposed basement forming hills in this buried land surface.

The stratigraphic breakdown of the Devonian sequence has traditionally been into Lower, Middle and Upper ORS due to a general lack of biostratigraphic control.

The two flagstones sequences are divided by the Sandwick fish bed, equivalent to the Achanarras formation of Caithness, representing an unusually persistent, deep and widespread lake that filled much of the Orcadian Basin.

Traditionally the presence of an unconformity at the base of the lavas beneath these sandstones on southwestern Hoy has been interpreted to mark a major time break.

[2][8] It has been proposed that the Hoy sandstones are the lateral equivalents of the Eday Group, the thickness and associated facies changes being caused by active extensional faulting during deposition.

Hydrocarbon exploration wells drilled in the Inner and Outer Moray Firth basins show similar sequences continuing without break into the Lower Carboniferous.

This syncline is parallel to neighbouring normal faults and thickness variations in the lower Eday Sandstone indicate that this structure was active during the middle Devonian.

The permanent lake laminites from the flagstone sequences are moderate to good source rocks and are currently marginally mature at the surface as a result of burial after the Devonian.