Shock Metamorphism: Shatter cones rarely well-developed (Murtagh, 1976; 1972). PDF in plagioclase, quartz grains, K-feldspar, and other minerals. Maskelyniteb.
a Dating Method: K-Ar, Rb-Sr, U-Pb. U-Pb on zircons (Hodych & dunning, 1992). Triassic age confirmed by Rb-Sr mineral isochron with age of 214 +/- 5 Ma for melt rock (Jahn et al., 1978) and by Ar-Ar and K-Ar ages. b Maskelynite: A type of naturally occurring glass having the composition of plagioclase series feldspar, created by the vitrification of plagioclase by shock melting in meteorites and meteorite impacts. (Dence et al 1967)
General Area: Heavily timbered area of the Canadian Shield. The topography is generally rugged and the area has been glaciated, with north-south ice movement. The target rocks are crystalline.
Specific Features: The structure may be divided into a number of morphologic elements. The most striking is the -70 km diameter annular depression filled by the waters of the Lac Manicouagan reservoir. The annular depression is interpreted as the glacially overdeepend expression of the interior contact between the crater floor and inner blocks of the original rim . In the middle of the lake is a dissected plateau capped by -200 m of impact melt rocks and a series of uplifted peaks -5 km north of the center.
At the point of impact, the country rocks were instantaneously evaporated/melted/shattered by the energy released leaving a 200 to 600 cubic kilometre sheet of impact melt directly on basement rocks. The target rock in the vicinity of the structure is Grenville age amphibolite to granulite facies quartz and feldspar gneiss, with local anorthosites, metagabbro and metasediments overlain by Ordovician limestones, dolomites, slates and sandstones. The force of the impact exhumed and liquefied these target rocks down to as deep as 9 kilometres. The original crater became a melting pot for relatively young rocks at the surface and for much of the older minerals originally buried kilometres below the site of the impact. The heat released was so intense that it took between 1,600 and 5,000 years before the melted rocks cooled. Changes in these impactite textures toward the interior of the crater progressively increased in proportion of superheated melt and decreased in fraction of cold fragmented country rock material (Simonds 1976).
The impact did trigger a powerful seismic event as Lawrence Tanner from Bloomsburg University recently discovered in a deformed zone of the Fundy Rift Basin. “There have been previous reports attempting to link paleoseismicity, as recorded by soft-sediment deformation features, to impacts,” Tanner explained. “But this is the first instance of linking the Manicouagan impact to the stratigraphic record. The Fundy rift Basin experienced a substantial period of volcanic activity at the time of the Manicouagan impact.
Extinctions within major pelagic groups (e.g., radiolarians and conodonts) occurred in a stepwise fashion during the last 15 Myr of the Triassic. Although a marked decline in the diversity of pelagic faunas began at the end of the middle Norian, the cause of the middle Norian extinction is uncertain. Here we show a possible link between the end-middle Norian radiolarian extinction and a bolide impact. Two palaeoenvironmental events occurred during the initial phase of the radiolarian extinction interval: (1) a post-impact shutdown of primary and biogenic silica production within a time span of 104–105 yr, and (2) a sustained reduction in the sinking flux of radiolarian silica for ~0.3 Myr after the impact. The catastrophic collapse of the pelagic ecosystem at this time was probably the dominant factor responsible for the end-middle Norian conodont extinction. (Onoue 2016)
The Late Triassic (Carnian to Rhaetian Stages: ca. 237–201 Ma) has a long history of geological research, although controversy remains over the precise definition of key sub-unit boundaries, including those defining the three constituent stages. Within this context, at least five terrestrial bolide impact structures ranging from 9 to 85 km in diameter have been identified at present-day northern latitudes, the proximal remnant crater aspects of which have been studied in increasing detail over the last few decades. The more elusive distal sedimentary expressions of these multi-sized hypervelocity events remain largely unknown, although if preserved, identified and interpreted correctly, may (as precisely dateable event horizons) help to address certain existing stratigraphic uncertainties, particularly pertaining to the (longest) Norian Stage. Detailed absolute age-dating using a range of radioisotopic methods (e.g. U-Pb and 40Ar/39Ar) currently indicates that at least three of the confirmed Late Triassic impact craters formed prior to commencement of the major Rhaetian Central Atlantic Magmatic Province (CAMP) volcanic episode by several million years. Impact research efforts to date have focused mainly on describing and process modeling the relatively well-preserved largest impact structure, Manicouagan (215.5 Ma; 85 km diameter) located in northeastern Quebec, Canada and, to a lesser extent, the Saint Martin (227.8 Ma; 40 km) and Rochechouart (ca. 207–201 Ma; ca. 23–50 km) structures in central Manitoba, Canada and west-central France respectively. The smaller, subsurface Red Wing structure (ca. 200 Ma; 9 km diameter, ca. 2.5 km burial depth) located in South Dakota, USA, also has attracted significant economic interest. Unlike the well-documented End Cretaceous Chicxulub impact (66 Ma; ca. 180 Km), attempts to establish a globally significant causal extinction connection between the larger impacts (e.g. Manicouagan and Rochechouart) and Late Triassic marine and terrestrial bioevents, culminating with the ‘End Triassic Extinction’ (ETE), have essentially proved unsuccessful. (David E. Brown ET AL 2018)
3. STRATIGRAPHIC RECORD IN THE FUNDY RIFT BASIN OF THE MANICOUAGAN IMPACT: BOLIDE WITH A BANG OR A WHIMPER?
The terrestrial redbeds of the Blomidon Formation were deposited in the Fundy rift basin during Norian to Hettangian time. A 10-m-thick zone of intensely deformed strata that occurs near the base of the formation is characterized by rubblization and step-wise faulting. Correlation of this zone basin-wide indicates that it is a record of a very powerful paleoseismic event. The greater thickness of the deformed zone in proximity to the border fault, the Minas Fault Zone (MFZ), indicates that movement of the MFZ was the immediate cause of this paleoseismicity. The presence in strata just above the deformed zone of quartz grains displaying features of shock metamorphism raises the intriguing possibility that reactivation of the MFZ was triggered by a bolide impact. (Tanner 2002)
The ~100 km-wide Manicouagan impact structure in northeastern Canada, 700 km from the Fundy rift basin, is one of the largest well-documented impact sites. Once considered a candidate for the cause of the end-of-Triassic extinctions, U-Pb zircon dating of the impact melt establishes the age of the impact as much earlier Norian, approximately coeval with the seismic deformation of the Blomidon Formation. Stratigraphic constraint of the age of the impact allows an accurate assessment of the effects of this event on biota. (see – David E. Brown ET AL 2018 and Onoue 2012 above)The impact did trigger a powerful seismic event as Lawrence Tanner from Bloomsburg University recently discovered in a deformed zone of the Fundy Rift Basin. “There have been previous reports attempting to link paleoseismicity, as recorded by soft-sediment deformation features, to impacts,” Tanner explained. “But this is the first instance of linking the Manicouagan impact to the stratigraphic record. I made the association between the impact and paleoseismicity in the Fundy Rift Basin, a sedimentary basin 700 km away from the impact site, and went looking for shocked quartz grains. Finding them allows us to place this impact into a stratigraphic context and look elsewhere to see if there are any significant biotic effects. So far, there don’t seem to be any.” The Fundy rift Basin experienced a substantial period of volcanic activity at the time of the Manicouagan impact. (GEOLOGICAL SOCIETY OF AMERICA 2002)
4. 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.
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)
There is a strong similarity between the profiles of the Can-Am structure and the Manicouagan Impact Crater. The coincidence between magnetic and gravity signatures strongly suggests a common source for both fields. This data documents that the Precambrian basement rocks are interrupted by anomalies that clearly outline the circular nature of the structure and provides evidence that the remnants of a complex meteorite crater is situated in the south end of Lake Huron.
The Manicouagan impact crater is one of the largest impact structures still preserved on the surface of the Earth and is classified as a complex meteorite crater. The annular moat, prominent in space images, fills a ring where impact-brecciated rock was once eroded away by glaciation. The diameter of the original crater was approximately three times the size of the circular lake taken from the space shuttle (above). Erosion has removed about a kilometre of rock from the region. The inner plateau remaining in the center of the annular moat is made up of metamorphic and igneous rock types along with melt sheet and is not as susceptible to glacial erosion.
The Manicouagan multiple-arch buttress dam filled the annular moat to its present depth, creating a circular reservoir for hydro-electric power. This circular lake accentuates the contour of the eroded impact-brecciated ring area of the structure. But the diameter of the original crater was approximately three times the size of this moat.
Approximately 214 million years ago an estimated 10 kilometre wide hypervelocity meteorite impacted at between 12 and 30 kilometres per second and formed the Manicouagan Impact Crater. The resultant 100 kilometre diameter Manicouagan crater is one of the largest impact craters still preserved on the surface of our planet. For comparison, the Copernicus crater on the moon has a diameter of 93 kilometres and the annular moat of the Manicouagan structure would fit comfortably within the rim of this lunar crater.
Morphological elements of the Manicouagan structure are based on topographical expression and are:
central region – ~25 km outer diameter. (Grieve, Head 1983)
Outer Circumferential Depression, Outer Disturbed Zone and Inner Fractured Zone
The magnitude of fracturing of the country rocks in the Manicouagan structure increased towards the centre of the crater, the point of maximum shock effect. The fragmentation increased to where the energy from the impact caused the rocks to melt. These melted rocks remain today as the central peak of the crater (the island in the image). The “smaller” fragmented rocks surrounding the “melt rock” central peak were easily evacuated by glaciation and erosion. The annular moat around the Manicouagan central peak is what remains after the country rocks that experienced “maximum” fracturing were removed. This circular moat is an impact indicator.
Peripheral Trough (annular moat)
The water filled circular annular moat that is prominent in space images is only one third of the size of the original crater. The water in the annular moat fills a ring where impact-brecciated rock was eroded away by glaciation. Before flooding of the reservoir, isolated outcrops of tilted and deformed limestone, siltstone and shale were found on the inner edges of the moat (Murtaugh, 1975). This rock formation (RIGHT) is found at the extreme eastern portion of the annular moat on one of the small islands. Note the rock structure is breccia free gneiss. The central peak of the structure is visible over 10 km in the distance.
The Inner Plateau of the Manicouagan structure is bounded by the annular moat, overlain by melt sheet, underlain by shocked basement rock (Orphal, Schultz 1978). We found a “lunar landscape” here containing various breccia types. The astronauts exploring the moon found that impact-melt breccias, similar to what we found here, were the most common rock types at the Apollo highland sites (Apollos 14, 15, 16 and 17) (Haskin 1998). We documented impact breccias formed by similar and very different country rocks like those found on the moon!
The Central Region of the Manicouagan Structure is a complex zone of uplifted, shocked and metamorphosed basement rocks with small tabular bodies of impact melt and pseudotachylite veins (Orphal, Schultz 1978). Recent U-Pb zircon dating of the impact melt gave an age of 214 ± 1 million years.
The red dot represents the approximate area of the Manicouagan impact 214 million years ago in the Triassic Period. The meteorite responsible for the Manicouagan crater struck in the beginning of the Mesozoic era in the Triassic period. In that time the world climate was in a warming phase and becoming relatively dry. The continental plates, joined about 200 million years prior to this, began to break into continents. Fewer species existed than today but survived in high populations. The first dinosaurs, and possibly the first mammals, evolve. It is suggested that the Manicouagan impact triggered the extinction of terrestrial and marine organisms near the impact site but not in an open sea.
6. Aerial Exploration
The water filled circular annular moat that is prominent in space images and illustrated here from GOZooM, is only one third of the size of the original crater. This moat fills a ring where impact-brecciated rock was eroded away by glaciation.
The Manicouagan impact crater – impact melt cliff in Memory Bay. Memory Bay is prominent in the many space images of the structure as the bay that juts into the structure’s inner plateau from the east. Most of the impact melt of the impact structure is along Memory Bay.
All the previous aerial images were taken from GOZooM below 5,000 feet above ground.
7. Ground Exploration – 2006
Chuck O’Dale & Eric Kujala (1964-2017)
My ground exploration of impact structures continued in August of 2006 when Eric Kujala and I explored the east area of the Manicouagan Impact Crater by canoe.
At this point I want to strongly recommend AGAINST exploring this body of water by canoe. The weather in this area changes within minutes creating dangerous waves in the annular moat (see below), and being in a canoe that is full of rocks in those conditions is not a good idea!
Driving north to the Manicouagan structure I specifically watched the changing rock faces along the road as we entered the area of the 100 kilometre diameter structure. In the inner fracture zone of the structure, some of the rock faces along the highway changed from solid granite faces to fractured walls.
We had a short wind delay here before we made our dash for the main island of the structure that is just visible in the image over 10 km away. At the time this picture was taken, the wind was subsiding with barely a whitecap to be seen and we started our canoe trip across the moat. We got half way across before the wind increased again! Fortunately by that time we were in the lee of the couple of islands visible in the distance. Paddling against the wind took us most of the day to finally reach our main island camp location.
These photos illustrate the typical breccia outcrops found within the central peak area of the Manicouagan Impact Crater located in Quebec, Canada.
Impact breccias: country geology melted, mixed, crushed and compressed by shock waves at various stages in the cratering process. Even within the brief formation time of an impact crater, it is possible for the multiple generations of breccia to develop and to produce distinctive differences, even though the time between one breccia generation and the next may be measured in seconds or minutes (French 1998).
Impact related pseudotachylite was first recognized at the Vredefort crater in Africa and are common within the Sudbury impact structure.
The colour of the breccia filled rocks around Memory Bay varied from white to copper to dark-mafic.
From location #3 we had a hard uphill slog against the wind to the shock impact melt cliff on the south shore. We eventually made it to the south shore where I walked to the impact melt cliffs shown here at location #4.
Shatter cones at Manicouagan formed over varying shock pressure ranges. A comparison of the shat-ter cones collected along a radial tracsect from 27 to 12 km from the centre of the structure reveal a systematic in-crease in the intensity of shock metamorphism recorded by the quartz and oligoclase indicating an increase in shock pressure from ~5 GPa to ~30 GPa. The most shocked oligoclase sample preserves textures indicative of dynamo-thermal conditions and not just static high pressure (ductile and melt textures). (L. M. Thompson, et al 2016)
I feel very fortunate to document this specimen as I do not know of another shatter-cone from Manicouagan in any other collection. Unfortunately, weight constraints with the loading of the canoe prevented me from returning with shatter-cones (rocks don’t float).
The passage of the shock wave through the rock changes the structure of some of the enclosed minerals. IE: change is possible in the feldspar mineral plagioclase. The shock wave can break down the structure of the mineral, changing parts of it into a diapletic glass (glass formed at high-pressure in the solid-state) which is isotropic, or uniform in all directions. This photograph of a thin slice of plagioclase, 0.03 millimetre thick, is seen here in cross-polarised light, with a ‘sensitive tint’ plate. The original plagioclase is coloured yellow and the shock-changed mineral is purple. This sample is from the Manicouagan impact crater. (Courtesy Denis W. Roy & MIAC).
This series of cartoons by Denis W. Roy illustrates the sequence of events that formed the Manicouagan impact crater, Québec, Canada. (Courtesy of MAIC)
There is a very odd feature in one of the impact melt cliffs in Memory Bay. A 10m block of mafic gneiss is suspended about 20m above the base of the melt sheet. Such a block is 0.3g/cm³ denser than the melt and should settle at a minimum of 5 cm/sec (Stokes Law1) through a Manicouagan composition melt with 2% H2O (water) if it were still liquid at 1000°C. In order for that block to remain suspended, the melt must have begun to crystallize rapidly enough to trap the block before it settled to the bottom of the sheet (Simonds 1976).
1STOKES LAW: If the particles are falling in the viscous fluid by their own weight due to the Earth’s gravity, then a terminal velocity, also known as the settling velocity, is reached when this frictional force combined with the buoyant force exactly balance the gravitational force. The resulting settling velocity (or terminal velocity) is given by:
Vs = ( 2 (ρp – ρf ) / 9 η ) g R2 where:
Vs is the particles’ settling velocity (m/s) (vertically downwards if ρp > ρf, upwards if ρp < ρf ),
R is the radius of the spherical object (in metres),
g is the Earth’s gravitational acceleration (m/s2),
ρp is the mass density of the particles (kg/m3),
ρf is the mass density of the fluid (kg/m3), and
η is the fluid’s viscosity (in [kg m-1 s-1]).
I mentioned earlier that it is not recommended to do this trip by canoe (as we did). The waves on the annular moat and in Memory Bay can reach dangerous heights very quickly. On our final day in the impact structure we were returning to our starting point in Kauashapishkau Bay in a semi-calm wind. But within 20 minutes the wind had increased from under 10 kts to greater than 30 kts! We had to perform an emergency beaching on an island 1/3 of the way across the annular moat.
We made it onto the beach by surfing the waves! It was fortunate that Eric was an experienced white water canoe operator, as we may have otherwise been dumped. Here, we had just started a fire to dry ourselves off. We will be marooned here for 22 hours waiting for the winds to subside. So, what to do until the wind subsides? Why, explore of course!
We eventually made it off the island at 5AM the next morning. Within 15 minutes of waking up during a wind lull, we had struck camp and were paddling for dear life for the eastern shore of the annular moat! We made it back across the reservoir without incident in under three hours. We had a great tail wind. What an adventure!! It was surreal later that morning eating our hot egg and bacon breakfast in a restaurant and realizing that just hours ago we were marooned without any idea of how long it would be until we could get back to the main land.
Components of the RASC Sesquicentennial Logo:
The aurora borealis is a quintessentially Canadian space-weather phenomenon, one shared with other high latitude cultures. RASC members have contributed to the scientific, historical, and artistic investigation of the northern lights, and have promoted their recreational enjoyment.
The spiral galaxy represents both the work of Canadian observational cosmologists (e.g., Sidney van den Bergh‘s classification of Galaxy morphology, Laura Ferrarese‘s work on the morphology & dynamics of early type galaxies), as well as the efforts of amateur Canadian observers of deep-sky objects (DSOs), and imagers.
from (AstroNotes October 2010😉 – I included the geology from our exploration of the Manicouagan Impact Crater as examples for “Identifying Impact Craters/Structures”.
~214 Ma – LATE TRIASSIC EXTINCTION
Scientists reported in the journal Nature today (March 13, 1998) that they had found evidence of a chain of five craters formed 214 million years ago that was likely due to pieces of a comet crashing into the Earth’s surface, similar to the Comet Shoemaker-Levy 9 impact on Jupiter in 1994. The craters no longer appear to be in a straight line due the shifting of the Earth’s continents due to plate tectonics. Two of the craters, Manicouagan and Saint Martin, are in Canada (Quebec and Manitoba, respectively). The other three craters are Rochechouart in Europe, Obolon in the Ukraine and Red Wing in Minnesota. The impacts appeared to occur at the Norian stage of the Triassic period, about six million years after a mass extinction that wiped out 80% of all the species on Earth, but the ages of all the craters are uncertain enough to include this extinction (from ScienceWeb Daily).
The 34-million-year (My) interval of the Late Triassic is marked by the formation of several large impact structures on Earth. Late Triassic impact events have been considered a factor in biotic extinction events in the Late Triassic (e.g., end-Triassic extinction event), but this scenario remains controversial because of a lack of stratigraphic records of ejecta deposits. Here, we report evidence for an impact event (platinum group elements anomaly with nickel-rich magnetite and microspherules) from the middle Norian (Upper Triassic) deep-sea sediment in Japan. This includes anomalously high abundances of iridium, up to 41.5 parts per billion (ppb), in the ejecta deposit, which suggests that the iridiumenriched ejecta layers of the Late Triassic may be found on a global scale. The ejecta deposit is constrained by microfossils that suggest correlation with the 215.5-Mya, 100-km-wide Manicouagan impact crater in Canada. Our analysis of radiolarians shows no evidence of a mass extinction event across the impact event horizon, and no contemporaneous faunal turnover is seen in other marine planktons. However, such an event has been reported among marine faunas and terrestrial tetrapods and floras in North America. We, therefore, suggest that the Manicouagan impact triggered the extinction of terrestrial and marine organisms near the impact site but not within the pelagic marine realm (Onoue, Tetsuji, October 2012).
Summary of impact structures in the Late Triassic.