Stac Fada Member

The Stac Fada Member is a significant geological deposit in northwest Scotland, recognized as evidence of a major meteorite impact that occurred approximately 1 billion years ago. This distinct layer is part of the Mesoproterozoic Bay of Stoer Formation and is characterized by its composition of impactite, a mixture of suevite and clast-poor impact melt rocks. The unit is notable for containing dark green glassy fragments of mafic composition and accretionary lapilli, which are small, rounded masses of volcanic ash or rock fragments that form around a nucleus.

Geological Context and Discovery

The Stac Fada Member's interpretation as an impactite evolved over time. Initially considered a conglomerate or a volcanic deposit, its true nature was revealed through the identification of key impact indicators. These include shocked quartz, a higher-than-expected concentration of platinum-group metals, and a non-terrestrial chromium isotope. Further evidence came from the discovery of reidite, a high-pressure mineral formed in zircon grains, which indicates pressures of at least 30 GPa, consistent with a powerful impact.

Impact Event and Crater Location

The impact event associated with the Stac Fada Member is estimated to have occurred around 990 ± 22 million years ago, based on dating of shocked zircon crystals. The impact crater itself is believed to be preserved beneath younger sedimentary layers. Current interpretations suggest the crater likely lies to the west, under the Minch waterway, or is responsible for the Lairg Gravity Low to the east. Analysis of ejecta direction and rock fabric points towards an impact location in the Minch as being more probable. The estimated diameter of the crater is approximately 13–14 kilometers (8–9 miles).

Extent and Significance

The Stac Fada Member is exposed at various localities along the coast of Wester Ross, extending for about 50 kilometers from Loch Ewe to the Stoer peninsula. The thickness of the unit varies, generally ranging from 10 to 15 meters, but can reach over 30 meters in some areas. The study of the Stac Fada Member provides valuable insights into ancient impact events on Earth, their geological consequences, and the early geological history of Scotland.