Old Red Sandstone of Shetland and its correlation

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Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.


Geological sketch-map showing the outcrops of Old Red Sandstone rocks in Shetland and their structural relationships. P915575.

The outcrops of Old Red Sandstone in the Shetland Islands fall into three groups separated from each other by two large, probably transcurrent, faults: the Melby Fault in the west and the Walls Boundary Fault in the centre of Shetland Mainland (P915575).


The three groups of Old Red Sandstone rocks of Shetland Mainland differ from each other in age, in their depositional and volcanological development, in their tectonic history and in the extent to which they have been affected by igneous intrusions. As all three appear to rest directly on the metamorphic basement, they do not represent parts of a continuous sequence but form the deposits of three geographically distinct basins. The rocks of Melby and Foula appear to have been laid down in the most southerly of these; they may in fact have been deposited along the north-western fringe of the extensive shallow and tectonically stable Orkney—Caithness basin. The Walls Sandstone and possibly also the Fair Isle sandstone may have been deposited farther north in an intermontane, tectonically and volcanologically active basin, which developed somewhat earlier than the others and was affected towards or just after the end of Middle Old Red Sandstone times by two phases of compression. The East Shetland sediments were probably laid down on the western margin of the most northerly basin which was bounded by mountainous terrain to the west, but had access to an open lake to the south-east. The present virtual juxtaposition of three such diverse groups of rocks is most readily explained by postulating subsequent dextral transcurrent movements along both the Melby and Walls Boundary faults. The extent of movement along the latter may have been in the order 60 to 80 km.


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