Limestone reefs in southern Yukon

Part 8 of Your Backyard Geology

Within Whitehorse city limits, you can see Grey Mountain from almost anywhere. It is one of the first landmarks tourists see flying into the city. Eight thousand kilometres away, the residents around Salzburg, Austria, can look up from their morning coffee and see the rugged, beautiful grey-white peaks of the Northern Alps. In both cases, you’re looking at limestone. The two were formed in similar environments about 215 million years ago, during the Triassic period.

Limestone is easy to recognize. It usually has a distinct colour when compared to surrounding rocks. It can be bright white, grey or even black. It tends to stand out in any landscape. Though the mountain east of Whitehorse was called Canyon Mountain by the first Europeans, today it is better known as Grey Mountain. The Yukon Geographical Place Names Board gives the Southern Tutchone name for Grey Mountain as dazhän dhäl kay tthe dägay ghàkwänä̀ch’in kàkwäni which translates to “The May” or Grey Rock Mountain.

To understand how Grey Mountain formed, we need to go back to the Triassic period. At the time, the continents we know today were joined together into one big supercontinent known as Pangea. The Atlantic Ocean did not exist. North America was joined to Europe and South America snuggled into Africa. Globally, there was one large body of water. It was known in the east as the Tethys Sea and in the west as the Panthalassic Ocean.

The earth was going through a warm cycle at this time. There was no polar ice. Around the edges of Pangea, especially near the equator in shallow waters, conditions were right for marine life to thrive.

Limestone is composed mainly of calcium carbonate. Most of it has biological origins. The shells of clams, oysters, snails and the outer layer of corals are all made of calcium carbonate. Some marine plants, including algae, secrete calcium carbonate.

Reef-building animals grow in colonies. The two major reef-building organisms in the Triassic were corals and sponges. Corals are genetically identical polyp animals with an external skeleton of calcium carbonate. Sponges are similar, with a perforated carbonate shell that allows nutrients and oxygen in the water to flow through and nourish the living organism inside.

Both anchor themselves to the sea floor and never move. Other reef residents at the time would have included molluscs, algae and foraminifera. Foraminifera are one-celled organisms that that have a hard shell of calcium carbonate. As new growth built on top of old, the reef would have started taking the shape of a dome-shaped mound. Modern healthy reefs can grow vertically 25 centimetres a year.

The Triassic limestone in the Alps is much more widespread and thicker than that found in the Yukon. Hoher Göll, a peak on the Germany-Austria border, has a well-developed reef over 1,200 metres thick. In North America, the reefs grew as small isolated build-ups of corals, shells and sponges, usually only tens of metres thick. These are known as patch reefs.

As well as the Yukon and the Alps, other places you can find Triassic reefs include Alaska, Vancouver Island, Nevada, the Andes and the Arabian Peninsula.

How were tropical reefs growing in a cold place like the Yukon? Thanks to plate tectonics, 200 million years ago, the Whitehorse area was at least 1,000 kilometres south of here. Offshore, away from the main part of Pangea, it was docked onto the continent of North America in a later tectonic event.

In the middle of the Triassic, a series of undersea volcanoes erupted in the Panthalassic Ocean. These formed a series of islands. They are called volcanic island arcs because they usually form in curves, like the Aleutians do today. The shallow, warm waters around these islands were ideal spots for a reef to start.

There is a very well-preserved and exposed Triassic reef, known as Lime Peak, on the north side of Thomas Lake, just east of Lake Laberge. R. Pamela Reid, a doctorate student from the University of Miami, spent a number of years in the early 1980s studying the reef’s formation.

The reef at Lime Peak is up to 150 metres thick and extends for an area of about three square kilometres. The reef is dominated by sponges, but also contains corals, molluscs and algae. Molluscs average two to three centimetres large. Foraminifera have also been identified. Most are less than one millimetre in size.

Parts of the reef appear as massive grey limestone with no obvious fossils. These would be parts of the reef that formed from accumulations of broken shell material that was worn down to silt and sand-sized pieces. They accumulated as a thick lime mud and sand on the seafloor, both in and outside the reef itself, and eventually solidified into stone.

It is fortunate Lime Peak is so well-exposed and preserved. The study of the Triassic reefs around the world provide scientists with valuable information on the past climate and changes in sea level. They can give us a window to the future.

At the end of the Triassic, most of the reefs died along with about 70 per cent of all marine life. This was known at the Triassic-Jurassic extinction. It was about this time that the breakup of Pangea really started to accelerate. The increased tectonic activity resulted in increased volcanic activity across the globe. Carbon dioxide levels increased in the atmosphere and seawater.

The increased carbon dioxide in the water formed carbonic acid and the oceans became acidified. This dissolved the coral, killing it. This is exactly what’s happening to the oceans and coral reefs today, as carbon dioxide levels increase.

Scientists have started looking at coral reefs that formed after the Triassic, in the Jurassic period, to see what we might expect in the future. It took about 20 million years into the Jurassic period before reefs began to thrive again.

When you look at most limestone outcrops, you see a nondescript grey rock. Geologists take what is known as acetate peels to examine what the limestone is made of. It is a process by which textures in the rock are transferred onto clear acetate film and then examined under a microscope.

The Triassic limestone in southern Yukon can be easily seen in several locations including northeast of Carmacks, near Braeburn, the east side of Lake Laberge, within Whitehorse city limits and just west of the Watson River toward Carcross. You will notice they all reflect their patch reef nature. They are relatively small, isolated low-relief mountains.

Limestone west of the Watson River, Carcross Road

The limestone you see south of Carcross and Tagish, down towards Atlin and White Mountain at Jakes Corner is different. It is 100 million years older than that in the Whitehorse area. The limestone that occurs from Watson Lake east and up the Dempster Highway is even older, formed up to 500 million years ago.

Next time you are out for a hike along “The May,” take some time to look at the limestone. See if you can find any fossils. You may get lucky and find some mollusc shells. At the very least, imagine what it would have been like to be at this spot when the reef was alive, the water was warm and the sun was shining.

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