MANICOUAGAN IMPACT STRUCTURE

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.

During our drive up north to the Manicouagan Impact Crater, we stopped at various lookouts to see what we were in for. Here we can see the magnitude of the canoe trip we had ahead of us.

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.

Outside the annular moat of the Manicouagan Impact Crater some of the rock cuts along the highway change from solid granite faces to fractured walls. These rocks were fractured approximately 214 million years ago when an estimated 10 kilometre (6.2 mile) wide bolide impacted about 40 kilometres north from this spot at a velocity of between 12 and 30 kilometres (7.4 and 18.6 miles) per second. The resultant 100 kilometre (62 mile) diameter crater is one of the largest impact craters still preserved on the surface of the Earth.

Superimposed on this Manicouagan Structure map are the areas we explored. The canoe route is indicated here by the white lines and the ground explorations are indicated by the red lines. The numbers indicate locations of interest. We paddled over 80 kilometres by canoe in our three and a half days of exploration.

The Manicouagan impact crater – looking west into the annular moat. Note the rock structure that is mostly breccia free gneiss here on the eastern outside section of the annular moat at location #1.

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.

**Autochthonous** impact breccia in the Manicouagan impact crater on the inner plateau of the central peak island at location **#2.** Note the different types of rock fragments forming the breccia within the fine grained matrix mylolisthenite^*. Also note the white margin around the large breccia fragment. This white margin is a heat affected zone. The matrix material was hot enough during the formation of this breccia to produce a recrystallized band around the clast , a Heat Affected Zone, but there was not sufficient heat that flowed into the clast to melt it (Dr. Lynn B. Lundberg, PhD).

The Manicouagan impact crater – breccia on the inner plateau of the central peak island at location #3 . At the point of impact, the rocks were instantaneously evaporated/melted/shattered by the energy released. The shattered white “country” rocks shown on the image were imbedded in what is interpreted to be a fine grained matrix mylolisthenite^*. It’s possible that the extremely small size of the grains within the matrix were formed by the very high pressure of the gas generated upon impact.

^*Autochthonous breccia made of rock fragments cemented by fine-grained material produced in an impact crater larger than 4 kilometers. The mylolisthenite is precisely produced at the interface between the collapsing terrains. They were first identified in 1969 in the Charlevoix impact crater (54 km in diameter; 342 +/-15 millions of years), near Quebec City, Canada.

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).

Further west from location #3 I noticed a possible shock pseudotachylite vein within a breccia outcrop. The pseudotachylite veins associated with impacts are much larger than those associated with faults and are thought to have formed by frictional effects within the crater floor and below the crater during the initial compression phase of the impact and the subsequent formation of the central uplift.

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.

The Manicouagan impact crater – impact melt cliff on the inner plateau of the central peak island at location #4. The impact melt cliffs and talus (debris at the base of the cliff), found in the central region area of the Manicouagan Impact Crater, is composed of target rock that was made temporarily molten from the energy released during impact. There are not any detectable meteorite components in the Manicouagan structure melt rock (Palme et al., 1978). This is as close as I could get to the cliff as the steep slope of the cliff talus made footing very untenable. It was fortunate that I stopped here on the talus slope as there was a small group of shatter shatter cones at my feet to document.

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)

The Manicouagan impact crater – shatter cone. Shatter-cones form in country rock from impact pressures of typically 2-10 GPa and up to ~30 GPa, and is the only distinctive and unique impact shock-deformation feature that develops on a megascopic scale (e.g., hand sample to outcrop scale).

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).

Planar deformation features in quartz. This sample is from the Manicouagan impact crater.

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).

¹ STOKES 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:

V_s = ( 2 (ρ_p – ρ_f ) / 9 η ) g R² where:

V_s is the particles’ settling velocity (m/s) (vertically downwards if ρ_p > ρ_f, upwards if ρ_p < ρ_f ),
R is the radius of the spherical object (in metres),
g is the Earth’s gravitational acceleration (m/s²),
ρ_p is the mass density of the particles (kg/m³),
ρ_f is the mass density of the fluid (kg/m³), and
η is the fluid’s viscosity (in [kg m^-1 s^-1]).

The Manicouagan impact crater – 10 m block of mafic gneiss embedded within the impact melt cliff, north shore of Memory Bay.

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.

The Manicouagan impact crater – emergency camp at location #5. Note in the background the 18 foot long canoe relative to the size of the waves. It gives you an appreciation of the size of the waves we navigated through! ! I took this image minutes after we performed the emergency “surf”beach landing.

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!

While marooned on an island because of the high winds, at location #5, I went exploring. Following the shore to the east of our marooned island, I came to the impact melt cliffs. Note the parallel fracturing of the impact melt rock. This was as far as I could go in this direction!

Melt sheet cliff at location **#5 –** No statistically significant regional chemical variations were found as a function of vertical, lateral, or radial position in the melt sheet. A local mafic variant represented by two samples with poikilitic texture indicates that the melt is not completely chemically homogeneous (Grieve 1978)day

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.

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