Lower Middle Devonian
(The Orkney Flagstone Group)
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:
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.
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.
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.