Into the Interlake

The Interlake region of Manitoba not only offers spectacular scenery, lakes, and wildlife, but also some incredible geology. From a possible impact crater to ancient Paleozoic fossils to karst topography, the Interlake offers an array of interesting rocks and landscapes.

Devonian Delights

Tuesday was busy. A fellow geologist and myself spent the day roaming around the Interlake area, stopping to check out some important geological sites.

The day began with visiting the southernmost outcrop of Devonian rocks (the Devonian Period lasted from ~420 to 360 million years ago) in Manitoba. The site used to be an old quarry, but it hasn't been produced from for a long time. Adjacent to the road in the ditch the Devonian Elm Point Formation is exposed; the surface is very weathered, but grooves made when glaciers scraped over the top of the rock tens of thousands of years ago can still be seen.



Glacial grooves in the rock can be used to indicate
which direction the glaciers were moving in.



These Devonian rocks also contain fairly abundant fossils, the most obvious being brachiopods. Brachiopods are clam-like animals that were prolific during the Paleozoic (540 to 250 million years ago). However, there are still a few species of brachiopods that are currently alive living in deep and cold places in the ocean. I was able to find a couple of brachiopods in a quarry that were fairly well preserved. I also saw many exposed brachiopod beds and cross sections of shells.


 
 Brachiopods were extremely abundant during the Paleozoic. These animals are useful in understanding the environment in which they lived and from which these rocks were deposited in. Brachiopods have been used to preform regional correlations, to identify climatic and environmental conditions, to determine approximate water depths, etc. Although there are still a few living today, I wouldn't recommend eating them as they are toxic to humans.



In one of the quarries we visited, we checked out an unusual wavy structure in the rocks. These rocks were initially deposited in flat, horizontal layers, and it is unknown what caused this type of deformation to occur. It's also only one of a very few sites within the Interlake to possess this type of structure.

On the left side the photo, the horizontally-deposited carbonates have been folded into a wavy structure.

Interlake Impact Crater?



We kept travelling northward to Gypsumville and Lake St. Martin. The Lake St. Martin area is of particular interest as there may be an impact crater in the area. The geology of that area is anomalous; much of the Interlake is underlain by Paleozoic carbonates, like limestone. Around Gypsumville and Lake St. Martin there are Precambrian igneous rocks and Jurassic evaporates.

Our first stop in that area was to visit the Precambrian granite. It is a truly unique experience to stand on the middle of a hard rock outcrop in the middle of the Interlake. One theory to explain this anomalous outcrop is that an asteroid impacted this area sometime possibly in the Triassic or Permian (between ~200 and 300 million years ago). The force of the impact may have mobilized this granite upward to the surface, from its initial location several hundred meters below this spot.



The lichen-covered granitic outcrops and mixed forest makes it feel like you're standing in the Whiteshell, even though this location is some 300 kilometers northwest.
 



We stopped at a nearby quarry, where the Lake St. Martin granite is used for crushed stone. Some of the blasted rocks provided fresh surfaces for us to examine. The granite is a mixture of quartz, feldspars, and biotite, and sometimes contains very large crystals called pegmatites. The granite has a very distinctive pink colour.

 

We also examined an old gypsum quarry in the area as well. Historically, this area has seen a lot of gypsum production, and the quarry we visited was quite extensive. Much of the floor of the old quarry has been flooded, so the numerous outcrops around the edge of the quarry are largely inaccessible. Weathering and exposure to the elements have been wearing away at the very soft gypsum as well.

There are still a few places where the exposed gypsum is accessible and not covered by slumped material or vegetation. In the last post I discussed the gypsum deposits around the western edge of Lake Manitoba. Although it is believed that the gypsum around Lake Manitoba and Gypsumville were deposited at the same time and by the same body of water, the geology of the gypsum from both areas are somewhat different from each other.

 

The Gypsumville deposit of gypsum appears, on first examination, to lack silica concretions that have developed in the gypsum west of Lake Manitoba. There is also clearly defined bedding in the gypsum at Gypsumville; layers of gypsum are still oriented more of less horizontally and are easy to spot. It is thought that the possible impact crater in the area would have created a depression in the earth's crust that would have been filled in with ocean water during the Jurassic Period. The gypsum would have been deposited in this crater as the Jurassic seawater experienced increased rates of evaporation.

 

Horizontal bedding is visible towards the top of this section.

 

These are only a few of the many interesting sites around the Interlake. There is a wealth of ancient history contained within the rocks of this region, as well as a few mysteries that have yet to be fully understood. The next time you travel through the Interlake, take a moment and stop to examine an outcrop or bedrock pavement on the side of the road, you might be surprised by what you discover.

 

Written by Kathryn Lapenskie