Sandwick Fish Bed

 

 

 

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:

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.