Oh, Canada! Part 3: Clay and Quakes

This is the third and final installment of overthinking of my family’s trip to Quebec. Allons-y!

Intro: Oh, Canada!

Part 1: Oh, Canada! Part 1: Monadnocks

Part 2: Oh, Canada! Part 2: Impact

From La Malbaie in the annular trough of the impact crater, our road trip headed northwest along the coast to St. Simeon to catch the ferry.

A small population of beluga whales hangs out in the St. Lawrence. We were lucky enough to see several of them popping up above the waves on the ferry ride!

From the ferry landing at Riviere-du-Loup we headed north to Grand Metis to see one last garden – called either Reford Garden or Jardin de Metis depending on whether you’re an anglophone or a francophone. Elsie Reford set out to build the perfect garden there in 1926, but the rolling terrain on the southern shore of the St. Lawrence wasn’t very cooperative. The local soil is a thick layer of clay with very poor drainage left behind by the Champlain Sea, an extension of the Atlantic left in the depression of the retreating glaciers between 13,000 and 10,000 years ago. The waterlogged clay hosts scenic marshes of lupins and stunted pine trees but is a significant obstacle for the ambitious gardener.

a recreation of the extent of the Champlain Sea ~11,000 years ago, courtesy of Orbitale at Wikipedia

As the Laurentian Ice Sheet retreated around 15,000 years ago its meltwater washed huge amounts of silt to the sea, which combined with marine clay deposits and the natural chemistry of the seawater created a massive layer of clay. As the crust heaved a sigh of relieve at the weight of the ice sheets being removed, that isostatic process gradually lifted the seafloor into dry land.  This garden took a lot of back-breaking work and planning to dig two- to four-foot deep beds, add in drainage systems honeycombed through the impermeable clay so the plants wouldn’t drown, and then fill in the beds with gravel and imported loamy soil. And this was in the early 1900s before Bobcat earth movers!

Mrs. Reford’s results are spectacular, though.

Eventually, though, the time came when we had to drag ourselves out of the garden and back onto the road heading south. But we had one last stop before Quebec City – my mother’s friend’s friend Yves was taking care of a Heritage Canada property at Pointe-de-Saint-Valliers and had invited us there for dinner.  Between dinner and dessert we hiked out to the point, where Yves casually mentioned that the cliff jutting out above the tidal flats was rather smaller than it had been because of that earthquake back in ’88.

Wait what, Quebec has earthquakes?!

It turns out that between the area being torn open as the Iapetus Ocean was formed after the breakup of Rodinia, a meteor punching a crater into the crust~342 million years ago, the nearby Appalachians being built as the ocean closed fully to form Pangaea 270 million years ago, the Atlantic Ocean ripping open ~175 million years ago, and the vast weight placed and released by the ice cap in the Pliocene/Pleistocene glaciation, the crust under the St. Lawrence has had a tremendously stressful life.

 

Fault map image courtesy of Tremblay et al. 2013

All this stress is exhibited as the St. Lawrence Rift System that stretches from the Ottawa-Bonnechere Graben to the mouth of the St. Lawrence. The dramatic escarpment along the northern side of the St. Lawrence Valley was formed by grabens and half-grabens dropping down due to the extensional force created around 450 million years ago, when a carbonate platform collapsed during the formation of the Iapetus ocean. After 450 million years of erosion the resistant granites, quartzite, and marble of the Canadian Shield now make up the footwall of the graben (visible as the 200 meter high escarpment), with younger rocks making up the hanging wall.  Those late Proterozoic to early Paleozoic faults were reactivated with each successive compression or decompression of the crust.

Right now, the crust is still in the process of rebounding and decompressing after the last ice age. That localized expansion of the crust creates friction along those old fault lines – when too much force builds up for friction to resist, the rocks on either side of the faults slip past each other with a violent jolt that creates an earthquake.

In the region we were traveling, these quakes are categorized within the Charlevoix Seismic Zone and the Western Quebec Seismic Zone. Pointe-de-Saint-Valliers sits on the south-western fringe of the Charlevoix Seismic zone, where the weaknesses in the crust created by the meteor impact in addition to the late Proterozoic faults release the rebounding stress after the last ice age.

Imaged edited from the EarthquakesCanada site.

Luckily the fault zones were on their best behavior while we were there.

After that leg of the road trip we spent a wonderful day-and-a-half exploring Quebec City. My mom introduced me to all her favorite haunts from her childhood there. Much ice cream was consumed, because where else can you get maple ice cream with maple sugar crunch and maple cookies?!

On the way back to Montreal to fly out we found the last remaining dinosaurs in Quebec! No dinosaur fossils have been found is this region, as any rocks that might have contained them were scoured away by the ice caps. The youngest fossils in this area date from the Ordovician. But we can still dream, right?

The author joined the intrepid herd of dinosaurs at the famous Madrid 2.0 gas station in St-Leonard-d’Aston, Quebec (exit 202 on the Trans-Canadian Highway)

Bonus: When my sister and I start hanging out, silly things happen. Like a cartoon geologic history of Laurentia.

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Oh, Canada! Part 2: Impact

Intro: Oh, Canada!

Part 1: Oh, Canada! Part 1: Monadnocks

The drive through the Charlevoix, the area on the northern bank of the St. Lawrence from Quebec City to la Malbaie, is jaw-dropping and quintessentially Canadian. Anti-moose fences running along the sides of the highway underline fantastic views of the Laurentian mountains where my cousins learned to ski. When the road winds through enough civilization for the moose fences to disappear, the civilization takes the form of adorable clapboard houses with brightly painted accents and laundry drying on the line. The entire landscape looks like it jumped out of a picture book.

However, around 342 million years ago was a really terrible span of time to live there.

It’s hard to tell why from the ground. The road from Quebec City dipped down steeply to get to the sleepy resort town of Baie-St.-Paul, climbed up a winding local road to the village of Les Eboulements perched on a cliff, and then wound back downhill to La Malbaie further north. To get to the Quatre Vents Garden, we drove uphill again along the coast north of La Malbaie. Looking at the map, the main highway that we didn’t take sweeps around in a half-circle from St. Paul to La Malbaie following the easy route through the valley.

That’s pretty odd. The rest of the valleys in the area run roughly perpendicular to the coast. Why would a valley do a u-turn and meet the coast on both ends? (valley represented by the white line on the map below)

Well, it turns out that this particular valley had some help in the form of a 1.2 mile-wide chunk of rock.

The meteor crashed into what is now the Charlevoix in the early Carboniferous period; scientists’ current best guess is that it struck 342 million year ago with a 15-million year range of error on either side. The impact razed any flora and fauna, flattened the landscape, and created dramatic shatter cone structures in the whatever underlying Ordovician limestone it didn’t pulverize completely.

Image of an excavated shatter cone, courtesy of impact-structures.com

 

Initially, the crater was as bowl-shaped as anyone would intuitively expect from bedrock that had been punched by a giant boulder from outer space. Over time, however, the center of the crater rose as a reaction to the absence of pressure after the impact. That rebound created the higher elevation where Les Eboulements stands today.

Us humans have this impact to thank for the half-circle of relatively flat terrain between Baie-St.-Paul and La Malbaie, where 70% of the population of the Charlevoix region now resides.

The crater has a much more uniform and gentle gradient, compared to the rugged highlands of the Laurentian Highlands. A RadarSat image of the Charlevoix area, courtesy of the Canadian Space Agency via impact-stuctures.com

But where is the other half of the crater? It was no match for the seismic and glacial stresses of the past 340 million years. Pangaea crashed together 40 million years later and was torn apart 125 million years after that, and between 2.5 and 0.7 million years ago an ice cap took advantage of those faults in the crust to carve away the weak points. When that ice cap shrank, all that glacial melt poured through the St. Lawrence and washed away what was left.

The view looking south from Quatre Vents Garden on the northern rim of the crater, across La Malbaie in the sunken impact ring, towards the rebounded area of Les Eboulements.

I wish I had discovered Observatoire de l’Astrobleme de Charlevoix and its awesome geological app before the trip, but if I ever go back I’ll definitely get it!

Up Next: Oh, Canada! Part 3: Clay and Quakes

 

Oh, Canada! Part 1: Monadnocks

Intro: Oh, Canada!

The drive from Montreal to Quebec City looked at first to be an unremarkable straight-line drive along the marching lines of pylons carrying hydroelectric power west and long rows of pig farms. The fertile farmland along the St. Lawrence was a perfectly uniform neon green carpet only interrupted by the farmhouses and squat concrete businesses lining the highway. Then, all of a sudden, Mont Saint-Hilaire popped up on the horizon. It looked ridiculously random, like a gargantuan pine-covered bear had curled up to rest for a moment while ambling from the Adirondacks to the Laurentians. And it wasn’t alone! As we drove past Mont Saint-Hilaire we saw another huge hill towering over the flat agricultural landscape far in the distance- Mont Rougemont.

This image of Mont Saint-Hilaire courtesy of the PlentyofNothing blog, as mine didn’t turn out. It looks more bear-like from ground level, I promise. http://lintel.typepad.com/plentyofnothing/2010/04/les-mont%C3%A9r%C3%A9giennes.html

These isolated hills have the fun-to-say moniker of “monadnocks”, named after the Abenaki name of the type example in New Hampshire.

Mont Saint-Hilaire, Mont-Royal in the center of Montreal, and their fellow Monteregian Hills in Quebec are actually parts of a much larger family of igneous intrusions stretching from Triassic kimberlites in Nunavut to the Miocene Great Meteor Seamount in the Northern Atlantic- all children of the Great Meteor Hot Spot. David W. Eaton & Andrew Frederiksen reconstructed a map of its path in a 2007 article published in Nature. I’ve shown a figure from it below to show the extent of the extended family of plutons, which are igneous intrusions that formed as buoyant magma rose through cooler overlying layers of rock.

It’s a diverse family. Pangaea started to break apart 200 million years ago, and the earliest of the plutons formed approximately 178 to 147 million years ago  as the magma rising off of the hot spot created the Churchill Kimberlite deposits in Nunavut and Ontario. Unlike the famous diamond-bearing kimberlites of Africa or even those further west in Canada, these kimberlite tubes ferried up spinels and olivine from deep within the mantle to the surface. Little topographic relief remains from these intrusions, as they are located under what was ground zero of the Laurentian ice sheet during the Pleistocene.

As the North American continent floated northwest during the breakup of Pangaea, the stationary hot-spot within the mantle created a south-east-trending series of volcanic necks punching up through the overlying sedimentary rock. Next up were the Monteregian hills around 129 million years ago. These hills are mainly composed of syenite, which is an alkali-rich magma with a composition similar to granite but impoverished in quartz. (citation)The cool thing about a hot spot is that the magma that forms the intrusions is influenced by the diversity of rocks that it melts through. Mont Saint-Hilaire is a mineralogist’s dream – over 277 varieties of minerals have been cracked out of the geodes of Mont-Saint-Hilaire, many new to science!

The rare mineral Serandite, found at Mont Saint-Hilaire.

(http://www.dakotamatrix.com/mineralpedia/8304/serandite)

 The sturdy igneous rocks of these monadnocks are much more resistant to weathering than the surrounding sedimentary rocks and so stand proudly above the surrounding area that has been washed away by rivers, cracked by frosts, and bulldozed by ice sheets.

As the steady expansion from the mid-Atlantic ridge North America northwest away from Europe and Africa, the hotspot came to rest under New England and form the White Mountains of New Hampshire that give monadnocks their name around 100 million years ago. After that, the hot spot ran out of North America above sea level and started to create the New England seamounts between 80 and 100 million years ago. All was quiet for a few million years while the Great Meteor hot spot took a well-deserved break from its creative streak, ducked under the mid-Atlantic ridge, and then belched out the Seewarte Seamounts on the African plate around 15 million years ago.

Despite the diversity of forms of the intrusions and the changing composition of the magmas depending on the overlying rocks within the Great Meteor Hotspot Track, studies such as Zurevinski, Heaman, and Creaser 2011 showed that the isotopic composition of strontium and neodymium in the rocks are similar enough for them to all to be driven by the same hot spot deep beneath the crust.

Hot spots are tricky beasts to study, especially when their handiwork lies too deep beneath the sea to get a rock sample to analyze for mineral similarities to suspected igneous relatives. The Hawaiian hot spot steals the spotlight, of course, since everybody loves a good explosion from a distance. There’s no proof that the Monteregian hills ever went ballistic like that, as the remaining rocks look more like volcanic necks that cooled calmly far beneath the surface. On the other hand, this slow geologic process gave us all the explosion of colorful minerals found in Mont Saint-Hilaire!

Twenty minutes passed by, and Mont Saint-Hilaire and its comrades faded into the rear-view mirror, and we reached Drummondville.From there to Quebec City the pig farms were once more the highlights out the window, with the exception of the melodramatic billboards admonishing us all to avoid the perils of texting while driving. Very Quebecois.

Up next: from the Great Meteor Hot Spot Track to the actual meteor that hit the Charlevoix! Oh, Canada! Part 2: Impact