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A buried Precambrian impact crater in Scotland: Or strange things spotted on holiday

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I had never actually planned to spend several years investigating a 1.2 billion year old meteorite impact deposit in Scotland. I was just going to visit a couple of sites, pay homage to a remarkable event, collect a few lumps of this extraordinary rock, and then move on to other things. It all began early in 2011 when Geoff Steel, an enthusiastic and very knowledgeable amateur geologist (an acoustic engineer by profession) and friend of many years, suggested visiting Scotland to look at some of the geology there. Geoff drew up an itinerary but I insisted on two sites; the anorthosite mountain on South Harris (that’s another story) and a recently recognised meteorite impact deposit near Ullapool. Little did I know what that holiday ultimately would lead to, and it shows that science owes as much to chance as to carefully planned investigation. This impact deposit, known to geologists as the Stac Fada Member, is a distinctive 5-10 metre thick layer sandwiched within hundreds of metres of sandstone layers known as the Stoer Group. These rocks, 1.2 billion years old, are found today as a narrow strip just a few km wide but stretching for more than 50 km along the coast to north and south of Ullapool in northwest Scotland. Beyond this area erosion and tectonic processes over countless millennia have destroyed all trace of them.

Typical Stac Fada Member impact deposit at Stattic Point, with angular fragments of green ‘impact melt’ rock mixed in with muddy red sandstone.

The Stoer Group is mostly sandstone that was deposited in rivers and lakes, but the Stac Fada Member is very different because mixed in with the sandstone are angular green fragments of what geologists recognised decades ago had once been molten rock. For years the Stac Fada Member was thought to be a volcanic mudflow, but there was a problem with this interpretation – there was nothing else like it anywhere in the Stoer Group.

This seemed odd considering how huge the volcano must have been to produce such a thick and widespread layer. Then, in 2008, a team of geologists from Oxford and Aberdeen identified microscopic shock lamellae in some of the quartz grains in the Stac Fada Member. These form only under the immense pressures generated by the impact of a giant meteorite and so their discovery instantly consigned the volcanic interpretation to the dustbin – the green fragments were rock that had been melted by the impact of a giant meteorite, perhaps several km across, that hit Earth at more than 15 km per second (33,000 mph)! The Oxford/Aberdeen team published their discovery but were unsure where the crater might be, suggesting that it was perhaps offshore to the west, beneath the sea and buried by layers of younger rocks.

Large angular blocks of Lewisian Gneiss embedded in the sand immediately beneath the Stac Fada Member impact deposit at Second Coast. These show no sign of either falling from a nearby hill nor of being swept there by a flood; they represent rock fragments blasted from the crater edge by the impact.

Exciting as it was to see this impact deposit for the first time in June 2011, I had no expectations of being able to contribute anything new or startling to the conclusions of this eminent team of geologists. However, at just the second site we visited, appropriately called Second Coast, I came across something quite unexpected that demanded an explanation. Embedded in the sandstone immediately beneath the impact layer were large angular blocks of a different, and still older, rock called Lewisian Gneiss which lies beneath much of northern Scotland.

How had these blocks got here? If they had rolled off a nearby hill they would not be as widely scattered as they are – and there was no such hill nearby anyway. If transported there by a flood then they would be aligned by the flow – but their orientations are entirely random. It looked as if they had fallen from the sky – which is exactly what I concluded! They are fragments of near-surface rock that were launched at velocities of 4-5 km/s (more than 10,000 mph) very early in the impact process. They reached their present location before the main impact deposit, itself travelling outwards from the impact at several hundred mph, arrived. This is why they are found immediately beneath the Stac Fada Member.

The spectacular exposure of the Stac Fada Member impact deposit at Stoer, looking south-west. The crags in the background are the main impact deposit while the layered rocks in the foreground are river sandstones deposited prior to the impact. A wedge-shaped mass of the impact deposit can be seen intruded along some of the layers of sandstone and indicates the direction that the impact blast was moving.

However, other geological wonders awaited us elsewhere and a day later we were heading across the water to Lewis. Nonetheless I thought it would be worth taking a closer look at these strange angular blocks sometime and perhaps write a short article about them. Returning almost four months later, in September 2011, I made another surprising discovery. At Stoer, further to the north, some of the impact deposit appears to have been pushed between some of the sandstone layers by the force of the impact blast. These ‘wedges’ thin out to the west and, crucially, show that it must have been coming from the east. The crater – if it still exists – must actually lie to the east beneath mainland Scotland! However, just 20km to the east these ancient rocks pass beneath the Moine Thrust, where a thick layer of younger Moinian rocks (~1 billion years old) have been pushed westwards across northern Scotland for tens of kilometres by tectonic movements around 450 million years ago. Any Precambrian impact crater that lies further east will now be deeply buried beneath these Moinian rocks. So, assuming it still exists, how might the crater be detected?

An annotated section of the UK gravity map published by the British Geological Survey. Red and orange indicates a stronger gravitational pull than average; blue and purple is weaker. Gravity lows in The Minch and the Moray Firth are due to thick sedimentary rocks; the Cairngorm gravity low is due to the granite there. The Lairg Gravity Low had defied any easy explanation until now.

Impact craters often have a distinctive geophysical signature. Excavating a large hole in dense rock and filling it with rubble and sediment creates an area of lower density that can be detected as a ‘gravity low’. Returning to Belfast after this second trip I consulted the museum’s rather crumpled copy of the British Geological Survey’s gravity map of Britain, and I was astonished to discover a large and roughly circular gravity anomaly centred on the town of Lairg. This is more than 50km east of the remaining patches of impact deposit, but in location that is remarkably consistent with the directional data from the Stac Fada Member. The size of the gravity anomaly suggests it could represent a crater at least 40 km across.

Until now geologists have explained the Lairg Gravity Low as due solely to the effects of tectonic processes. Although it is similar to the gravity signatures of other well-documented impact craters, it would be quite unrealistic to suggest that the Lairg Gravity Low is a buried impact crater on this tenuous evidence alone. What makes this a much more plausible explanation is the existence of a thick and extensive impact deposit, the Stac Fada Member, just a few tens of kilometres to the west and the evidence within the deposit of its emplacement from the east. Taking all of the evidence together, the impact hypothesis appears to explain both the cause and origin of the gravity low, and indeed its specific location, as the consequences of a single event for which the Stac Fada Member impact deposit provides substantial surface evidence. When Geoff and I set off to Scotland in 2011 we intended only to pay homage to an unusual rock and collect a few samples. The subsequent journey has been remarkable, culminating with the discovery of Britain’s first impact crater that – at more than 40km across – is among the fifteen largest on Earth. But the story did not end there. In February 2016 I was approached by Dr Tori Herridge, a palaeobiologist who has fronted several science documentaries (e.g. Woolly Mammoth: The Autopsy), with a view to making a documentary about the search for this Scottish impact crater. And so in May of this year I spent a week on location, in an unseasonably sunny Scotland, filming for the Channel 4 documentary ‘Scotland’s Lost Asteroid’. Anyone wishing to read the original article can download a copy at Researchgate or download a copy here.