A Precambrian proximal ejecta blanket from Scotland

Abstract

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 occurring during emplacement of ejecta. The Stac Fada Member of the Precambrian Stoer Group of Scotland has previously been described as volcanic in origin. However, shocked quartz and biotite provide evidence for high-pressure shock metamorphism, whilst chromium isotope values and elevated abundances of the platinum group metals and siderophile elements, indicate addition of meteoritic material. Thus the unit is reinterpreted as of 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, formed by an impact into water-saturated sedimentary strata. The continental geological setting and presence of groundwater, makes this deposit an analogue for Martian fluidized ejecta blankets.

Origin of small pits in martian impact craters

“…Although conduction is the dominant mechanism for heat loss from the solid clasts in impact melt-rich breccia, water is lost mainly by diffusion through and between grains, and fluid flow through vent pipes, like those found at Stac Fada, Scotland (Amor et al, 2008;Branney and Brown, 2011) and in the Ries fall-out suevite (Newsom et al, 1986;Engelhardt et al, 1995;Osinski, 2004). As with volcanic vents and vent pipes in pyroclastic deposits (e.g., Engelhardt, 1972;Pohl et al, 1977;Newsom et al, 1986), such conduits in impact melt-rich breccia are the most efficient means of releasing large volumes of water vapor from a particulate deposit.…”

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

“…It is distinctively full of melt fragments, and has been variably interpreted as an ash flow (Lawson 1972), a peperitic mudflow (Sanders & Johnston 1989), a volcanic debris flow above basaltic magma (Young 2002) and as an ejecta deposit from meteorite impact (Amor et al 2008). We assume the most recent interpretation of an ejecta deposit, which is based upon robust geochemical and mineralogical criteria, including an iridium anomaly, an extraterrestrial chromium isotope signature and planar deformation features in quartz (Amor et al 2008). All models involve emplacement of a hot melt-bearing sandy deposit onto wet sediment.…”

“…It is, therefore, important as an archive of information about the Mesoproterozoic environment. In particular, it contains (1) well-preserved microfossils and stromatolites (Cloud & Germs 1971;Stewart 2002), representing the oldest-known fossiliferous beds in the British Isles, (2) finely laminated deposits of a permanent lake, including a record of climate cyclicity (Andrews 2008), and (3) a rare deposit of proximal ejecta from a large meteorite impact event (Amor et al 2008). Studies of these deposits require an accurate and precise date, but this is difficult in Mesoproterozoic rocks because of paucity of successions and suitable mineral phases for isotopic dating.…”

High Molybdenum availability for evolution in a Mesoproterozoic lacustrine environment

“…The high sedimentation rate of the Stoer Group, characteristic of a continental basin, means that the carbon burial rate in lake sediments was much higher than in contemporary marine sediments 32 . A single marker horizon consisting of fine red sediment mixed with melt fragments (Stac Fada Member) is interpreted as an impact ejecta deposit 33 . Ar-Ar analysis of authigenic K-feldspar in the impact deposit yields a date of 1.18 Ga (ref.…”