The Stac Fada impact ejecta deposit and the Lairg Gravity Low: evidence for a buried Precambrian impact crater in Scotland?

Michael J. Simms

Proceedings of the Geologists’ Association
Volume 126, Issue 6, December 2015, Pages 742–761
Available online 2 October 2015


“The Stac Fada Member, an impact ejecta deposit within the Mesoproterozoic Stoer Group, is represented today by just a narrow outcrop, truncated by faulting and erosion, extending for 50 km north-south along the coast of north-west Scotland. It appears to represent a non-erosive Single Layer Ejecta deposit rather than the erosively emplaced Double Layer Ejecta deposits characteristic of terrestrial impact craters and it is unique in preserving spallation debris, ejected very early in the impact process, beneath the ejecta blanket. Various sedimentary structures associated with the Stac Fada Member, from ejecta intrusions along bedding planes immediately beneath it, to erosional troughs eroded into its top, consistently indicate emplacement from the east. No surface manifestation of an impact crater has been identified but there is a remarkable correspondence between its location, as inferred from these directional data, and the position of the Lairg Gravity Low, an ∼40 km diameter geophysical anomaly centred more than 50 km east of the closest point on the Stoer Group outcrop. Proximal-distal facies changes along the outcrop of the ejecta deposit are consistent with this inferred relationship between the ejecta deposit and the gravity low. Post-impact drainage reconfiguration suggests a regional isostatic doming in response to excavation of the crater that also appears to be centred on the Lairg Gravity Low. Comparison with gravity data from impact craters elsewhere suggests that the Lairg Gravity Low represents a complex crater at least 40 km in diameter that now lies buried beneath the Moine Thrust complex.”