As I’ve written before, the last 30 years has seen a big change in the way Geologists think about the Earth; this planet of ours does not sit in isolation in the Universe but is frequently hit and changed by meteorites (and other bits and pieces). When I received by Geological education in the 90′s there was little mention of such things. Even more recently, after the evidence had piled up, it all felt a bit remote, nothing to do with the rocks I personally knew. Then in 2008 a paper came out that really brought the influence of extra-terrestrial events home to me.
In my parent’s garage is a pile of rocks collected when I was a lad. One sample was from the Proterozoic Torridonian sedimentary rocks of Assynt, Scotland, from a unit called the ‘Stac Fada member’. The guide-book described this as a lahar, a volcanic mud-flow deposit, but there was an air of doubt in the description, (there is no other sign of vulcanism in the entire sequence). Let have a look at the sample:
The green blobs are of devitrified glass (now chlorite), the pink a feldspar vein. The matrix is very poorly sorted and contains chunks of more normal rock-types. Calling this a lahar is not an unreasonable interpretation: rapidly cooled lava provides the glass and subsequent rain mixes it up together with whatever else is lying around and deposits it all in an untidy heap downstream.
However a paper in Geology presents a much more interesting interpretation: the Stac Fada member is a layer of debris formed close to a major meteorite impact. So, the glass was produced by the heat of the impact and the force of the impact threw debris over a wide distance, leaving the unsorted bits lying in an untidy heap on the ground.
The evidence is pretty compelling: the deposit contains shocked quartz and enriched levels of Iridium. Underlying layers are disrupted and locally ripped up into the impact deposit and it can be traced as a single deposit across the entire basin. The feldspar veins, and other evidence, shows that the deposit was hot when it was emplaced.
Suddenly, my little sample changes from run-of-the-mill lahar into glamorous suevite. Even better, the authors are able to infer some things about the impact. The geochemistry suggests that the impactor was a stony meteorite, which after being vapourised on impact is now distributed through the deposit. The patterns of overturning of layers underneath the deposit give evidence of the direction of impact and suggest the impact site is from an area now just offshore. They estimate the crater would be 6-8 km across.
Another buried crater?
Offshore from these rocks is a sedimentary basin, the Minch, so maybe a trace of the crater remains, buried underneath Mesozoic sediments? Well, maybe, but the Torridonian sediments that contain the impact layer show evidence of being deposited in a narrow fault-bounded basin, so the impact site might have been on the now-eroded flanks. Also between 1.2Ga and the start of the formation of the Minch basin Scotland enjoyed (from memory) the Grenvillian, Grampian, Scandian and Acadian orogenies. The area in question is in the ‘foreland’ of these events, but we are rather far from a stable cratonic situation here. So, the crater may be visible, still preserved deep under the Minch, but I wouldn’t bet on it.
The identification of the Stac Fada member as a trace of an impact is down to detailed field Geology. There is nothing startling about the deposit, it can’t be sensed remotely and the clinching evidence is based on microscopic and geochemical analysis. There must be a lot more of these subtle traces of impacts yet to be found in the Geological record.
Following its ‘transformation’ from lahar, my sample of suevite is no longer in my parent’s garage, but instead has been promoted to my office desk, next to some ichthyosaur bones, waiting to catch the attention and perhaps the imagination of passing colleagues.
A field guide to the deposit from Oxford Uni. Lots of nice pictures: http://www.earth.ox.ac.uk/~oesis/nws/nws-a98-st1.html
A write-up of the research from the Geological Society: http://www.geolsoc.org.uk/gsl/geoscientist/geonews/page6069.html