STAC FADA

STAC  FADA IMPACT STRUCTURE

by: Charles O’Dale

  • Type: Complexa
  • Location: N 58° 01.3’ W 04° 55.6’ (approximate)
  • Diameter: ~40 Kmb
  • Age Ma: 1177c ±  5      CRETACEOUS
  • Shock Metamorphism: authigenic K-feldspars precipitated in degassing structures . (Parnell 2011)

aComparison 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. (Simms 2015)

bNo 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. (Simms 2015)

c 40Ar/39Ar dating of authigenic K-feldspars precipitated in degassing structures within the Stac Fada Member. (Parnell 2011)

The Stac Fada structure is in Northern Scotland in the area within the red circle.

The Mesoproterozoic Stac Fada proximal ejecta blanket, NW Scotland: constraints on crater location from field observations, anisotropy of magnetic susceptibility, petrography and geochemistry

Kenneth Amor, Stephen P. Hesselbo, Don Porcelli, Adam Price, Naomi Saunders, Martin Sykes, Jennifer Stevanović and Conal MacNiocaill

Journal of the Geological Society, 176, 830-846, 10 June 2019

Abstract

The Stac Fada Member of the Mesoproterozoic Stoer Group (Torridon Supergroup) in NW Scotland is a proximal ejecta blanket surrounding an unidentified asteroid impact crater. A combination of field observations of the ejecta deposit and underlying strata, the geographical distribution of terrane-identified basement clasts found embedded in the impactite, and anisotropy of magnetic susceptibility of the impact melt rocks at different locations can constrain the crater location to be about 15–20 km WNW of Enard Bay and thus buried under Mesozoic sediments in The Minch. Syncompressional structures within the suevite at Stattic Point give a clear indication of a southeasterly direction of mass motion. The signatures of two different terranes within the Lewisian gneiss help identify the origin of clasts found in the impactite at three locations. These clasts are unshocked and interpreted as having been swept up by the density current post-impact; their geographical distribution provides an important clue to ejecta emplacement pathways crossing the Assynt and Gruinard terranes. Anisotropy of magnetic susceptibility is used to measure flow direction in pyroclastic density current deposits and is applied here to derive a direction of motion for the impactoclastic density current. It provides good agreement with the other independent methods.

Simplified geological map of NW Scotland (credit: British Geological Survey

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

Michael J. Simms
Department of Natural Sciences, National Museums Northern Ireland, Cultra, BT18 0EU Northern Ireland, UK 2015

A B S T R A C T
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.

Regional geology of  northern Scotland showing the out crop of the Stoer Group and its relationship to the residual gravity field (contoured at 2m Galintervals)  and gravity lineaments for the Lairg Gravity Low (from Rollin, 2009). Arrows indicate directional azimuths within the Stac Fada Member; for details see Fig. 6. Rosediagrams show palaeo current data for the Meall Dearg Formation (partlyfrom Stewart, 2002, Fig.17) and  long axis  orientations of accretionary lapilli. The fine dotted lines are radii projected from the centre of the Lairg Gravity Low through four key exposures of the Stoer Group. (Simms 2015)

A major Precambrian impact in Scotland

The age of the Stac Fada member is around 1200 Ma, determined by Ar-Ar dating of K-feldspar formed by sedimentary processes. Geochemistry of Lewisian gneiss clasts compared with in situ basement rocks, magnetic data from the matrix of the deposit, and evidence of compressional forces restricted to it suggest that the debris emanated from a site to the WNW of the midpoint of the member’s outcrop. Rather than being a deposit from a distant source, carried in an ejecta curtain, the Stac Fada material is more akin to that transported by a volcanic pyroclastic flow. That is, a dense, incandescent debris cloud moving near to the surface under gravity from the crater as ejected material collapsed back to the surface. On less definite grounds, the authors suggest that a crater some 13 to 14 km across penetrating about 3 km into the crust may have been involved.

 

The age of the Mesoproterozoic Stoer Group sedimentary and impact deposits, NW Scotland
JOHN PARNELL, DARREN MARK, ANTHONY E. FALLICK, ADRIAN BOYCE2 & SCOTT THACKREY
School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK 2011

Abstract:
The Mesoproterozoic Stoer Group of the ‘Torridonian’ has been dated at 1177  5 Ma (2), based on 40Ar/39Ar dating of authigenic K-feldspars precipitated in degassing structures within the Stac Fada Member, interpreted previously as an impact ejecta deposit. Fluid inclusion analysis of the K-feldspar shows that it precipitated from hot fluids, and so must immediately post-date emplacement of the ejecta blanket. The oxygen isotopic composition of K-feldspar is remarkably constant at 9.7  0.2‰ (V-SMOW) and is consistent with derivation from evaporated lake water. The new age supports previous determinations of 1199  70 Ma from Pb–Pb dating of limestone and 1180 Ma from palaeomagnetic data, but provides much higher precision.

While serving in the Royal Canadian Navy, I sailed on a  Royal Navy minesweeper in the vicinity of the Stac Fada structure without realizing it. Illustrated here is Ailsa Craig, an island the outer Firth of Clyde, 16 km west of mainland Scotland, imaged by yours truly from HMS Humber, 1990. Formed from the volcanic plug of an extinct volcano, blue hone granite is quarried to make curling stones.