Geoscientist Online

Conclusive evidence of the biggest meteorite strike ever to hit the UK has been found by British scientists, writes Ted Nield, from the 72nd Annual Meeting of the Meteoritical Society, Nancy, France.

Geoscientist Online 15 July 2009

It was once suggested that the Precambrian Torridonian sediments of North West Scotland had fallen off the Moon¹. Now it appears that at least part of that massive Supergroup, the 1.2 billion year-old Stac Fada Member of the Stoer Group, may indeed have had an extraterrestrial origin.

For many years the origin of the Stac Fada Member had been somewhat unconvincingly ascribed to volcanic agencies. But a self-funded team from the Universities of Aberdeen and Oxford has found evidence that it represents a proximal ejecta blanket (“impactite”) thrown up by a massive strike². Material ejected by the impact was scattered over an area about 50 kilometres across, roughly centred on the town of Ullapool.

Ken Amor, of Oxford University's Department of Earth Sciences, speaking at the MetSoc meeting in Nancy today, described how chemical testing of the rocks found the characteristic signature of meteoritic material in the Stac Fada Member, notably high levels of the key element iridium, normally only found in low concentrations in surface rocks on Earth, but present here at 20 parts per billion (ppb). This value is considerably higher than surrounding sediments, with typical values in the range 0.9 to 6ppb.

“We found more evidence when we examined the rocks under a microscope” says Amor: “tell-tale microscopic parallel fractures also imply a meteorite strike”.

The proposed volcanic origin for the Stac Fada Member (which can be up to 20m thick in places) was always problematic, as no volcanic vents or other volcanic sediments can be found in the same region. The team sampled the formation during fieldwork in 2006 and published preliminary findings in the journal Geology in April 2008³. Shocked quartz and biotite, as well as abundant devitrified melt clasts together provided conclusive evidence of high-pressure shock metamorphism. Chromium isotope values and elevated levels of platinum group metals and siderophile elements clearly indicate the addition of meteoritic material, the authors say. The shocked quartz typically displays two sets of planar deformation, but can show as many as four; while the grains themselves are angular, their faces reflecting crystallographic cleavages, and appear dusty in transmitted light.

The basal suevite includes clasts of Lewisian gneiss, torn from the topography over which the Torridonian alluvial fan sediments were deposited. Also present are melt clasts, now devitrified to chlorite, and some as large as 17cm in diameter. At Enard Bay, the Stac Fada Member also contains distinctive accretionary lapilli, which Amor and colleagues interpret as an air-fall deposit. Cross-cutting the rock are sinuous track-like features, containing crystals of feldspar that grew in place (authigenic). These have long been interpreted as water-escape structures, but the presence of feldspar infilling suggests to Amor that the emplacement was hot.

The bedded sediments below the Stac Fada Member may be folded, overturned, or completely delaminated and re-incorporated within the overlying sequence. These features give directional information about the blast wave. This suggests to the authors that the precise impact site – and the place to look for any surviving crater with some geophysical surveying – would now be just offshore. Amor believes that such a crater could have been (depending on the thickness of the sediment cover into which the meteorite fell) may be six to eight kilometres across.

The impactor itself was probably a stony meteorite. Typically chondritic ratios of nickel and chromium, as well as distinctive chromium isotopes, clearly confirm the presence of a meteoritic component in the impact layer. Field evidence also suggests that the impact occurred in what was then a continental setting, in groundwater-bearing sediments.

Professor John Parnell, Head of Geology & Petroleum Geology at the University of Aberdeen, co-author of the paper presented at Nancy, said: "These rocks are superbly displayed on the west coast of Scotland, and visited by numerous student parties each year. We're very lucky to have them available for study, as they can tell us much about how planetary surfaces, including Mars, become modified by large meteorite strikes.” The sedimentary environment of the Torridonian bore many similarities to that of Mars, which suggests to the researchers that the Stac Fada member may provide a useful analogue for Martian fluidised crater ejecta.

Scott Thackrey, a research student at Aberdeen, added: "The type of ejected deposit discovered in North West Scotland is only observed on planets and satellites that possess a volatile rich subsurface, like Venus, Mars and Earth. Due to the rare nature of these deposits, each new discovery provides revelations in terms of the atmospheric and surface processes that occur round craters just after impact."

"If there had been human observers in Scotland 1.2 billion years ago they would have seen quite a show” Amor told Geoscientist Online. "The massive impact would have melted rocks and thrown up an enormous cloud of vapour that scattered material over a large part of the region around Ullapool. It is very unusual to find ejecta blankets on Earth because they tend to be quickly eroded away. In this case rapid sedimentation, a feature of much of the Torridonian, caused the blanket to be rapidly buried, and thus preserved.

Amor told delegates: “It is the most spectacular evidence for a meteorite impact within the British Isles to date".

The research was conducted by Ken Amor, Professor Stephen Hesselbo and Dr Don Porcelli of Oxford University's Department of Earth Sciences, and Professor John Parnell and Scott Thackrey of the Department of Geology and Petroleum Geology at the University of Aberdeen.

References:

1. Selley, R C 1970 Ancient Sedimentary Environments – a brief survey (1st Edn) 240pp Chapman and Hall.
2. Field observations and geoschemistry of the Stac Fada ,e,ber (Torridonian Supergroup, Scotland); a mesoproterozoic proximal impact ejecta blanket. K Amor, S Helsselbo, D Porcelli, S Thackrey and J Parnell. Abstract # 5312, 72nd Annual Meeting of the Meteoritical Society, Nancy France.
3. A report of the research, ^{'A Precambrian proximal ejecta blanket from Scotland}' is published online in the journal Geology: http://geology.gsapubs.org/content/36/4/303.abstract.
4. Field descriptions, photos, thin sections at: http://www.earth.ox.ac.uk/~oesis/nws/nws-a98-st1.html#field