MANICOUAGAN IMPACT STRUCTURE – IMPACT EJECTA

MANICOUAGAN IMPACT EJECTA

The fireball generated by the impact probably expanded as far as the present location of New York City. The impact also triggered powerful seismic events and ejected material out of the atmosphere. The ejected material was sent on a ballistic trajectory around the earth. Like the Chicxulub impact, the Manicouagan impact left behind a global geochemical signature in the rock record.

As a result of the Manicouagan impact, molten rock and dust from this bedrock left a thin layer of glass beads and shattered mineral grains in a rock deposit in the United Kingdom.

Dating of this UK deposit connected it to Manicouagan precluding most of the European impact sites nearby as the source of the deposit layer.

 

The documented late Triassic spherule layer of SW England deposit (illustrated here) contains an abundance of spherules, common shocked quartz and a suite of accessory minerals believed to have been derived direct from the impact site. These include garnets, ilmenites, zircons and biotites. Garnets and ilmenites are highly fractured, and biotites show prominent kink bands indicative of shock.
The Late Triassic ejecta deposit of SW Britain where impact melt spherules have been completely altered to clay. Radiogenic dating of this deposit  shocked biotites (observed exclusively in this Late Triassic ejecta deposit) yielded ages consistent with the Grenvillian target rocks at Manicouagan (Thackrey 2009).
 In an article published November 14, 2002 in the journal Science researchers led by Dr. Gordon Walkden of Aberdeen University have reported the discovery of a 214 million year old impact layer in the rocks of the west of England. The 2 cm thick layer consists of millimetre-sized green spherules that were formed as molten droplets of rock in the impact of a large asteroid or comet with the Earth. The droplets formed by condensation from gasses generated by vaporisation of rocks at enormous temperatures and were scattered over the entire Earth surface.Julian Parker, from Aberdeen, studied the spherule layer with Walkden and discovered quartz grains that had been deformed by intense pressures. “The orientation of the distorted planes through the grains showed they had been shocked,” said Walkden, “and proves the layer was formed as debris thrown out from a giant collision.” Dr. Simon Kelly, from the Open University, measured the age of the spherule layer using the decay of radioactive potassium that is found in all potassium-bearing minerals. The age of 214 million years is the same as the 100 km wide Manicouagan impact crater in Canada which is, therefore, the likely source of the impact layer. Kelly, however, suspects that a number of craters that have similar ages may have formed the same time as a string of impacts.

 

European Space Agency: [SELECTED QUOTES]The documented late Triassic spherule layer of SW England deposit (illustrated here) contains an abundance of spherules, common shocked quartz and a suite of accessory minerals believed to have been derived direct from the impact site. These include garnets, ilmenites, zircons and biotites. Garnets and ilmenites are highly fractured, and biotites show prominent kink bands indicative of shock.

The time of the deposit was determined by noting the decay of radioactive potassium in the spherule layer and dates at 214 ± 2.5Ma. This is comfortably within the date of the late Triassic Manicouagan impact event.
Single grain K-Ar ages for the biotites are in the range 700-1100Ma which are superficially similar to the Grenville rocks around the Manicouagan impact site. Given the geometry of the contemporaneous Triassic continental assembly, the contact point of the impactor was only 20 crater diameters from the site of deposition of this ejecta layer. (Thackrey et al 2006) [SELECTED QUOTES]

A distal impact deposit has recently been reported from a location near Bristol, England. The 0-15 cm thick, discontinuous layer is of Late Triassic age and occurs in a sequence of red calcareous mudstones deposited unconformably on Carboniferous limestone, in a semi-arid, continental environment. It consists of closely packed, millimetre sized green spherules, composed of a green clay surrounding a calcitic or hollow core. They have been inferred to be diagenetically altered type I spherules, formed from quenched impact-melt droplets, deposited aerially from an expanding impact ejecta curtain. The stratum, with a reported 39Ar – 40Ar age of approximately 214 Ma, is coincident with the age of two known impact craters from the Late Triassic, Manicouagan in Canada and Rochechouart in France. The focus of the present study is to determine if a meteoritic component and projectile type can be identified from this impactoclastic air-fall bed and adjacent strata, using a combination of geochemical analyses – chromium isotope systematics, siderophile and platinum group elemental ratios. In particular, we have developed a new method for the isolation and concentration of chromium, following sample dissolution. Chromium isotope ratios are determined using a multiple collector, inductively coupled plasma, mass spectrometer (MC-ICP-MS) for which the conditions have been established to obtain high precision ratio measurements. ( AMORet al 2005)

A Late Triassic Impact Ejecta Layer in Southwestern Britain

Gordon Walkden 1*, Julian Parker 1, Simon Kelley 2

1 Department of Geology and Petroleum Geology, Kings College, University of Aberdeen, Aberdeen AB24 3UD, UK.
2 Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, UK.

* To whom correspondence should be addressed. E-mail: spherules@abdn.ac.uk.

Despite the 160 or so known terrestrial impact craters of Phanerozoic age, equivalent ejecta deposits within distal sedimentary successions are rare. We have recognized a Triassic deposit in southwestern Britain that contains spherules and shocked quartz, characteristic of an impact ejecta layer. Inter- and intragranular potassium feldspar from the deposit yields an Ar-Ar age of 214 ± 2.5 million years old. This is within the age range of several known Triassic impact craters, the two closest of which, both in age and location, are Manicouagan in northeastern Canada and Rochechouart in central France. The ejecta deposit provides an important sedimentary record of an extraterrestrial impact in the Mesozoic that will help to decipher the number and effect of impact events, the source and dynamics of the event that left this distinctive sedimentary marker, and the relation of this ejecta layer to the timing of extinctions in the fossil record.

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Asteroid impacts may not always cause mass extinctions, say geologists in the US. They believe that the the impact that created the 100-kilometre Manicouagan crater in Quebec did not coincide with any extinction event.

Some scientists had claimed the Manicouagan impact occurred at the same time as the mass extinctions which ended the Triassic period about 202 million years ago. But Joseph Hodych and G. Dunning of Memorial University of Newfoundland in St John’s have now dated zircon crystals from the site. They find them to be 214 million years old.

Evidence that the impact of an asteroid caused the mass extinctions at the end of the Cretaceous period 65 million years ago made many geologists suspect that a similar impact at the end of the Triassic period caused the mass extinctions then. The Manicouagan crater, one of the largest impact structures on the Earth’s surface, was an obvious candidate for the impact site. Previous estimates of its date came with errors that were large enough to overlap with the boundary between the Triassic and Jurassic periods.

The best evidence for an impact at the end of the Triassic was reported by David Bice of Carleton College in Northfield, Minnesota, in late 1990. At the Triassic-Jurassic boundary in Italy, he found ‘shocked’ quartz grains, which are considered evidence of an impact. Bice and others believed that the extinctions occurred rapidly.

Hodych and Dunning’s date, obtained by analysing uranium and lead isotopes, is the most precise yet (Geology, January, p 51). Bice remains unconvinced.

From issue 1807 of New Scientist magazine, 08 February 1992, page 23

Compilation of selected terrestrial meteorite impacts during the Triassic and the postulated Late Triassic multiple impact theory, modified after Spray et al.(1998). Lucas et al.(2012)suggested an age of ∼220 Ma for the Carnian/Norian boundary, which has an age of ∼227Ma in the current International Stratigraphic Chart (Cohen et al., 2013). Impact age data from Koeberl et al.(1996), Ramezani et al.(2005), Schmieder and Buchner (2008), Schmieder et al.(2010)and this study.

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