Stac Fada Research Articles

Geochemical evidence for a chondritic impactor in altered impact glass from the Stac Fada Member impactite, NW Scotland

The Stac Fada Member (Stoer Group) is a ∼1.2 Ga melt-rich impact breccia whose source crater and, therefore, proximity remains debated. We present a detailed in situ geochemical dataset for altered impact melt clasts within Stac Fada samples from Bay of Stoer and Second Coast. These altered impact melt clasts are now predominantly composed of clinochlore. Geothermometry of this clinochlore indicates formation temperatures within the ejecta blanket at 188–231°C, in agreement with previous estimates. A positive correlation between platinum group elements Ir, Pt and Rh – apparently independent of Ni – indicates two geochemical sources for platinum group elements enrichment. We propose that these represent an Mg–Ni-rich chondritic impactor and a mafic/ultramafic-layered body within the target lithology. Spatial variation in Ni content suggests that geochemical data, in combination with field observations, may be instrumental in assessing the likely location of the impact crater.

Provenance of altered carbon phases and impact history of the Stac Fada Member, NW Scotland

AbstractThe Stac Fada Member (Stoer Group) is a ~1.2 Ga melt‐rich impact breccia preserved and intermittently exposed along the NW coast of Scotland. Using a combination of x‐ray diffraction and micro‐Raman spectroscopy, we identify potential coesite that is spatially associated with micron‐sized diamonds, as well as disordered carbon phases. Comparing the graphite G‐band of disordered carbon phases in the impact breccia to samples from underlying units indicates that most of the carbon in the Stoer Group was ultimately derived from the underlying Lewisian basement. Disordered carbon phases within the Stac Fada Member have been modified by mild heating within a hot ejecta blanket rather than shock pressure. We also report the first evidence for impact diamonds discovered within the Stac Fada Member. These diamonds have an average Raman shift of 1328.5 cm−1 and are present within both the impact breccia and the shocked gneiss clasts that are present in sandstones directly underlying the Stac Fada Member contact, and within sandstone rafts entrapped in the unit. These findings have implications for the timing of deposition of the Stac Fada Member, which must have occurred after ballistic ejection of Lewisian basement clasts during the impact event.

The Mesoproterozoic Stac Fada Member, NW Scotland: an impact origin confirmed but refined

The origin of the Stac Fada Member has been debated for decades with several early hypotheses being proposed, but all invoking some connection to volcanic activity. In 2008, the discovery of shocked quartz led to the hypothesis that the Stac Fada Member represents part the continuous ejecta blanket of a meteorite impact crater, the location of which was, and remains, unknown. In this paper, we confirm the presence of shock-metamorphosed and -melted material in the Stac Fada Member; however, we also show that its properties are unlike any other confirmed and well documented proximal impact ejecta deposits on Earth. Instead, the properties of the Stac Fada Member are most similar to the Onaping Formation of the Sudbury impact structure (Canada) and impact melt-bearing breccias from the Chicxulub impact structure (Mexico). We thus propose that, like the Sudbury and Chicxulub deposits, Melt Fuel Coolant Interactions – akin to what occur during phreatomagmatic volcanic eruptions – played a fundamental role in the origin of the Stac Fada Member. We conclude that these rocks are not primary impact ejecta but instead were deposited beyond the extent of the continuous ejecta blanket as high-energy ground-hugging sediment gravity flows.

A bigger splat: The catastrophic geology of a 1.2-b.y.-old terrestrial megaclast, northwest Scotland

Abstract Rockfalls are relatively little described from the ancient geological record, likely due to their poor preservation potential. At Clachtoll, northwest Scotland, a megaclast (100 m × 60 m × 15 m) of Neoarchean Lewisian gneiss with an estimated mass of 243 kt is associated with basal breccias of the Mesoproterozoic Stoer Group. Foliation in the megablock is misoriented by ∼90° about a subvertical axis relative to that in the underlying basement gneisses, and it is cut by fracture networks filled with Stoer Group red sandstone. Bedded clastic fissure fills on top of the megablock preserve way-up criteria consistent with passive deposition during burial. Sediment-filled fractures on the lateral flanks and base show characteristics consistent with forceful injection. Using numerical calculations, we propose that rift-related seismic shaking caused the megablock to fall no more than 15 m onto unconsolidated wet sediment. On impact, overpressure and liquefaction of the water-laden sands below the basement block were sufficient to cause hydrofracturing and upward sediment slurry injection. In addition, asymmetrically distributed structures record internal deformation of the megablock as it slowed and came to rest. The megablock is unrelated to the younger Stac Fada impact event, and represents one of the oldest known terrestrial rockfall features on Earth.

Reply to Discussion on ‘The Mesoproterozoic Stac Fada proximal ejecta blanket, NW Scotland: constraints on crater location from field observations, anisotropy of magnetic susceptibility, petrography and geochemistry’, Journal of the Geological Society , 176, 830–846

Simms (2019) states in his discussion: > They dismiss the Lairg Gravity Low as a possible impact crater based on palaeoenvironments, yet the cited paper specifically states ‘the Stoer Group has nothing in common with any of the Moine sequences discussed here' (Friend et al. 2003, p. 254). A direct comparison between the Stoer Group and the Moine Supergroup was never intended; instead the point we made (Amor et al. 2019, p. 844) was that earlier meta-sediments of admittedly uncertain age, as found at the Shin inlier, and over which the much later Moinian sediments were thrust, can be interpreted as marine in origin (Conclusion 1 of Friend et al. 2003), and thus quite different from the sedimentary matrix of the Stac Fada impact debris, which resembles the Stoer Group sediments. These earlier meta-sediments (now metapelites, psammites and marble) lie on a Lewisian-type crustal basement that is located to the east of the present-day outcrop of the Stoer Group, in the vicinity of Lairg, and are presumed to have been thrust an unknown distance from the east during the emplacement of the Moine thrust complex. All geological and plate reconstructions for this time place the deposition of the Stoer Group sediments in a rift valley on the eastern or southeastern margin of Laurentia (Kinnaird et al. 2007; Dalziel 2010). Furthermore, Nicholson (1993), Rainbird et al. (2001), Williams & Foden (2011), Kinnaird et al. (2007), Krabbendam & Rainbird (2012) and Lelpi et al. (2016) agree that this was a discrete and isolated rift, with significant high ground of Lewisian gneiss to the east as evidenced by deeply cut canyons and current direction towards the west (Stewart 2002), although Simms & Ernstson (2019) infer an extension of the Stoer Group deposits much further to the east. While …

Discussion on ‘The Mesoproterozoic Stac Fada proximal ejecta blanket, NW Scotland: constraints on crater location from field observations, anisotropy of magnetic susceptibility, petrography and geochemistry’, Journal of the Geological Society, London , 176, 830–846

Ken Amor's recognition that the Stac Fada Member is an impact deposit (Amor et al. 2008) proved a game-changer. I planned only to collect a piece on a holiday visit in 2011 but have since walked the entire outcrop and examined every exposure on multiple occasions. My observations extend far beyond what was outlined in my original paper (Simms 2015) and I recognize errors in the latest contribution by Amor et al. (2019). Our opinions differ on several issues: cited thicknesses for the Stac Fada Member, the unshocked nature of spallation blocks and the erosive–non-erosive contact beneath the impact deposit. I have not observed any north–south decrease in melt clast size along the outcrop. Amor et al . dismiss the Lairg Gravity Low as a possible impact crater based on palaeoenvironments, yet the cited paper specifically states ‘the Stoer Group has nothing in common with any of the Moine sequences discussed here’ (Friend et al. 2003, p. 254). I maintain that the Lairg Gravity Low is a convincing (although unproven, hence the question mark in the title of my 2015 paper) candidate for a buried impact crater. Stac Fada Member intrusions into the underlying sandstones at Stoer are critical to locating the impact. A conspicuous example (Fig. 1a) ‘appears to have travelled from a southerly direction’ (Amor et al. 2019, fig. 2b and c), yet viewed from the south (Fig. 1b) is clearly …

Discussion on ‘A reassessment of the proposed ‘Lairg impact structure’ and its potential implications for the deep structure of northern Scotland’ Journal of the Geological Society, London, 176, 817-829

The interpretation of the Stac Fada Member of the Stoer Group of NW Scotland as a proximal ejecta blanket of late Mesoproterozoic age (Amor et al. 2008) has led to the search for the impact crater from which it originated. One prominent suggestion has been that this crater lies buried beneath east-central Sutherland, broadly centred on the village of Lairg (Fig. 1; Simms 2015). Simms & Ernstson (2019) develop this suggestion by interpreting the well-known negative anomaly that dominates the Bouguer gravity map of northern Scotland to represent the roots of this crater. However, they qualify their interpretation, noting that the geology at the Stac Fada outcrops implies a more distant crater location, and go on to propose hitherto unreported ‘crustal shortening structures' to explain this discrepancy. In doing so, they challenge current understanding of structural evolution in the Moine Thrust Belt and for crustal structure in NW Scotland. The purpose of this discussion is to demonstrate not only that the Simms & Ernstson (2019) proposals are incompatible with the known structural geology of northern Scotland, but that the geology of the Moine Thrust Belt renders the Lairg site to be one of the very few places that can be regarded as a highly unlikely source for the Stac Fada impact rocks. Internal Earth processes, rather than extra-terrestrial interventions, should be sought as explanations for the ‘Lairg gravity low'. A more general aim is to show, through this discussion, that making small changes to tectonic models commonly carries unintended consequences for geological interpretations that can go unrecognized if only illustrated on unscaled sketches. Fig. 1. Simplified geological map of northern Scotland, with locations discussed in the text, together with the location of the contested ‘Lairg impact crater’. X–Y is the location of the section line for Figure 2a. As recognized …

Reply to Discussion on ‘A reassessment of the proposed ‘Lairg Impact Structure’ and its potential implications for the deep structure of northern Scotland’. Journal of the Geological Society, London , 176, 817-829

Butler and Alsop provide a clear and thorough explanation as to why our suggestion (Simms & Ernstson 2019), that the Lairg Gravity Low might be a buried impact crater translocated from further east, is incompatible with what is known of the structure of northern Scotland, and I concede that in this particular scenario they are probably right. My original paper (Simms 2015) was based largely on field observations that, amongst other things, indicated that the Stac Fada Member impact ejecta preserved at outcrop had come from the east (despite claims to the contrary by others; Amor et al. 2008, 2019). When subsequently I became aware of the LGL I realized that its …

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

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.

A reassessment of the proposed ‘Lairg Impact Structure’ and its potential implications for the deep structure of northern Scotland

The Lairg Gravity Low may represent a buried impact crater c . 40 km across that was the source of the 1.2 Ga Stac Fada Member ejecta deposit but the gravity anomaly is too large to represent a simple crater and there is no evidence of a central peak. Reanalysis of the point Bouguer gravity data reveals a ring of positive anomalies around the central low, suggesting that it might represent the eroded central part of a larger complex crater. The inner or peak rings of complex craters show a broadly consistent 2:1 relationship between ring diameter and total crater diameter, implying that the putative Lairg crater may be as much as 100 km across. This would place the crater rim within a few kilometres of the Stac Fada Member outcrop, a location inconsistent with the thickness and clast size of the ejecta deposit. We propose that the putative impact crater originally lay further east, substantially further from the Stac Fada Member than today, and was translocated westwards to its present location beneath Lairg during the Caledonian Orogeny. This model requires that a deep-seated thrust fault, analogous to the Flannan and Outer Isles thrusts, exists beneath the Moine Thrust in north–central Scotland.

On the track of a Scottish impact structure: a detrital zircon and apatite provenance study of the Stac Fada Member and wider Stoer Group, NW Scotland

AbstractThe Stac Fada Member of the Stoer Group, within the Torridonian succession of NW Scotland, is a melt-rich, impact-related deposit that has not been conclusively correlated with any known impact structure. However, a gravity low approximately 50 km east of the preserved Stac Fada Member outcrops has recently been proposed as the associated impact site. We investigate the location of the impact structure through a provenance study of detrital zircon and apatite in five samples from the Stoer Group. Our zircon U–Pb data are dominated by Archaean grains (> 2.5 Ga), consistent with earlier interpretations that the detritus was largely derived from local Lewisian Gneiss Complex, whereas the apatite data (the first for the Stoer Group) display a single major peak atc.1.7 Ga, consistent with regional Laxfordian metamorphism. The almost complete absence of Archaean-aged apatite is best explained by later heating of the > 2.5 Ga Lewisian basement (the likely source region) above the closure temperature of the apatite U–Pb system (c.375–450°C). The U–Pb age distributions for zircon and apatite show no significant variation with stratigraphic height. This may be interpreted as evidence that there was no major change in provenance during the course of deposition of the Stoer Group or, if there was any significant change, the different source regions were characterized by similar apatite and zircon U–Pb age populations. Consequently, the new data do not provide independent constraints on the location of the structure associated with the Stac Fada Member impact event.

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

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 50km 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 ∼40km diameter geophysical anomaly centred more than 50km 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 40km in diameter that now lies buried beneath the Moine Thrust complex.

Precambrian reidite discovered in shocked zircon from the Stac Fada impactite, Scotland

Terrestrial impact events have had a profound influence on Earth’s geological, geochemical, and biological evolution. However, the record of Precambrian impacts is poorly constrained due to the dynamic nature of plate tectonics, erosion, and deposition of younger rocks that may destroy or cover the evidence. Here we report the first Precambrian occurrence of the rare mineral reidite (ZrSiO4) within grains of shocked zircon in the ca. 1.18 Ga Stac Fada Member (Stoer Group), northwestern Scotland. The reidite, preserved as <2-µm-wide lamellae, is unambiguous evidence of shock pressures in excess of ∼30 GPa and confirms the impact origin for the Stac Fada deposit. The reidite lamellae are locally deformed, and sites of deformation record its decomposition to baddeleyite (ZrO2) and amorphous silica, the first natural example of this transformation. The findings demonstrate that reidite and baddeleyite may form and be transported in high-energy ejecta without physical or chemical breakdown and are stable during sedimentary diagenesis and low-grade metamorphism. Thus, reidite may be preserved over time scales exceeding 1 b.y., establishing the use of reidite within detrital shocked zircon from Precambrian strata as a viable and valuable means of recognizing and characterizing ancient terrestrial impact events.

Impactoclastic Density Current Emplacement of Terrestrial Meteorite-Impact Ejecta and the Formation of Dust Pellets and Accretionary Lapilli: Evidence from Stac Fada, Scotland

We present an impactoclastic density current model for terrestrial meteorite impact ejecta emplacement that is based on a hitherto-unreported threefold sequence of lithofacies in the recently identified Stac Fada impact ejecta outflow blanket in Scotland. Massive suevite (division A) grades seamlessly up into massive suevite containing whole and broken accretionary lapilli (division B), and this is overlain by a thin layer of clast-supported dust pellets (division C). The sequence closely resembles sequences in accretionary lapilli-bearing ignimbrites emplaced by pyroclastic density currents at volcanoes, and we deduce similar emplacement mechanisms. We propose that ≥99% of the thickness of the Stac Fada ejecta blanket as seen at outcrop was deposited from a single depletive (spatially decelerating), granular fluid–based density current that rapidly waxed and then waned. A previously unreported thin layer of pellets (division C) records direct fallout from a residual atmospheric dust plume. We propose that the impactoclastic density current was density stratified and comprised a ground-hugging component of granular fluid passing up into less concentrated, more turbulent levels and a buoyant, dilute upper component. Larger rock fragments rolled and saltated within the basal granular fluid. In the buoyant plume, dust agglomerated into pellets, and these dropped into lower, more vigorous levels of the density current, where successive concentric layers of fine dust accreted to them as they circulated and hardened. The resultant accretionary lapilli sedimented through the granular fluid into lower levels of the current and were deposited from its base. Dust pellets continued to form in the residual atmospheric dust plume after passage of the density current and fell out to form the thin layer (division C) of framework-supported pellets at the top of the ejecta blanket. The impactoclastic density current model may have applications to finer-grained parts of some continuous impact ejecta blankets elsewhere on Earth as well as on Mars and Venus, where features indicating flow processes are evident.

The age of the Mesoproterozoic Stoer Group sedimentary and impact deposits, NW Scotland

Abstract: The Mesoproterozoic Stoer Group of the ‘Torridonian' has been dated at 1177 ± 5 Ma (2σ), based on 40 Ar/ 39 Ar 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. Supplementary material: The raw 40 Ar/ 39 Ar data, J-parameters, correction factors, air calibrations and blank data are available at http://www.geolsoc.org.uk/SUP18453 .

Crystal tuff in the Stoer Group, Torridonian Supergroup, NW Scotland

Synopsis One thin bed in sandstones belonging to the Stoer Group (Torridonian Supergroup) contains feldspar and quartz grains showing deep concave embayments which are interpreted as corrosion features, and are all embedded in a fine-grained illitic or sericitic matrix. Its bulk chemical composition is similar to that of Phanerozoic metabentonites but it differs from the overlying Stac Fada Member which is believed to represent either a volcanogenic deposit or an impact ejecta blanket. Field relationships, petrographic and chemical characteristics point to this bed having a volcanic origin. This thin bed represents a crystal tuff and is the earliest example of a volcanogenic deposit in the Torridonian Supergroup in Scotland.

A Precambrian proximal ejecta blanket from Scotland

Ejecta blankets around impact craters are rarely preserved on Earth. Although impact craters are ubiquitous on solid bodies throughout the solar system, on Earth they are rapidly effaced, and few records exist of the processes that occur during emplacement of ejecta. The Stac Fada Member of the Precambrian Stoer Group in Scotland has previously been described as volcanic in origin. However, shocked quartz and biotite provide evidence for high-pressure shock metamorphism, while chromium isotope values and elevated abundances of platinum group metals and siderophile elements indicate addition of meteoritic material. Thus, the unit is reinterpreted here as having an impact origin. The ejecta blanket reaches >20 m in thickness and contains abundant dark green, vesicular, devitrified glass fragments. Field observations suggest that the deposit was emplaced as a single fluidized flow that formed as a result of an impact into water-saturated sedimentary strata. The continental geological setting and presence of groundwater make this deposit an analogue for Martian fluidized ejecta blankets.

Palaeomagnetism of the (Late Mesoproterozoic) Stoer Group, northwest Scotland: implications for diagenesis, age and relationship to the Grenville Orogeny

The Stoer is the lowest of three groups comprising the Torridonian Supergroup, a clastic succession deposited late in Mesoproterozoic times on the (Lewisian) metamorphic foreland to the Caledonian Orogen in northwest Scotland. This study reports a palaeomagnetic, rock magnetic and magnetic fabric investigation through the full stratigraphic thickness of the succession. A primary magnetic fabric of sedimentary origin defines current flow from a westerly veering to northwesterly source. Rock magnetic studies identify the presence of both magnetite and hematite in these sediments. Magnetite is apparently of primary detrital origin whilst the hematite probably results mostly from early diagenesis in an environment of restricted chemical weathering. Palaeomagnetic study of sedimentary slumps shows that magnetic remanence post-dates deposition but was probably fixed by early dewatering and lithification because slumped blocks of Stoer in basal Torridon Group sediments preserve a primary remanence. Tilt adjustment, although inconclusive, also implies that magnetic remanence is older than pre-Torridian Group tectonic deformation. The lower part of the Stoer succession shows a progressive increase of magnetic inclination with shallower components resident in magnetite and steeper components in hematite. The succession above the Stac Fada Member has the steepest magnetic inclination and shows no significant difference between magnetite and hematite component directions. The inferred time sequence of palaeopoles coincides with the Gardar palaeomagnetic track (∼ 1250–1130 Ma) at 1180 Ma, conforming to a Pb–Pb determination of 1199±70 Ma. The Stoer Group was fully lithified and deformed before deposition of the Torridon Group at ∼ 1030 Ma because it contains no vestige of the range of Caledonian and later overprints found extensively in the latter. Sedimentation and lithification of the Stoer Group are therefore linked with a phase of extensional tectonism at 1200–1150 Ma and deformation is attributed to a culminating phase of deformation in the nearby Grenville Belt at ∼ 1100 Ma.

Stratigraphy and geochemistry of volcanic mass flows in the Stac Fada Member of the Stoer Group, Torridonian, NW Scotland

ABSTRACTThe Stac Fada Member, which forms part of the Stoer Group, contains the only definitive evidence of volcanic activity within the thick Torridonian succession. At the type area in the Bay of Stoer, four volcanic-rich sandy mudstone units are recognised in the Stac Fada. An irregular erosional contact at the base of the main Stac Fada unit truncates two thinner layers of similar volcanic-rich material that were formerly interpreted as intrusions. Directional sedimentary structures associated with the Stac Fada mass flows, and the distribution and thickness of an accretionary or armoured lapilli tuff, support the existence of at least two volcanic sources. Geochemical evidence suggests that the thick Stac Fada unit at the Bay of Stoer and comparable units to the SW were derived from the same volcanic centre. The chemical composititon of volcanic fragments in the main Stac Fada unit indicates that their potassic nature is not a primary feature but is due to alteration and subsequent metasomatism.

Stratigraphy and geochemistry of volcanic mass flows in the Stac Fada Member of the Stoer Group, Torridonian, NW Scotland

Stratigraphy and geochemistry of volcanic mass flows in the Stac Fada Member of the Stoer Group, Torridonian, NW Scotland