Lake Winnipeg Beaches: Why is There so Much Sand?
When summer finally arrived here in late August this year, we took advantage of the remarkable weather and spent time relaxing at Victoria Beach, on the eastern side of Lake Winnipeg’s south basin. As I lay stretched on that beautiful beach, below sandy scarps and dunes, I started to wonder how all the quartz grains had arrived there, on the shore of a lake in the middle of North America. Victoria Beach is not just a small patch of sand; it is part of a very substantial deposit that also includes the famous perfect shores of Grand Beach.
I assumed that, given the number of people who vacation along the margins of Lake Winnipeg, there would be clear answers to this question readily available on the internet. Sure enough, I was able to find out several things quite quickly. A Manitoba Parks website and other references explained that Grand Beach is part of the Belair Moraine, a deposit 100 kilometres in length that rises 30-50 metres above the lake. This deposit was formed as high energy outwash water moved sediment from the glaciers, perhaps 11,000 years ago. The sediment was subsequently reworked by glacial Lake Agassiz, which was for a time the largest lake in the world.
The Belair Moraine includes a lot of other material besides sand. My friend Jim Teller of the University of Manitoba has had his students study the composition of the sediment in this area, and he tells me that in some of the beaches, much of the material consists of larger fragments of igneous or metamorphic rock. But even in those places, a considerable portion is well-rounded quartz sand. Other Quaternary deposits in eastern Manitoba, such as in the pits at Beausejour that once served a glassworks, consist almost entirely of beautifully sorted and rounded quartz.
From all of this, my long-view geological mind just had to ask: if there was that much sand to be shifted here, where did the glaciers and the lake find the sand? At this point I begin to move into the realm of speculation, since I have not been able to find a definitive reference on the sediment source, and I would be grateful if anyone reading this wishes to correct any errors or inaccuracies.
There is no question that a large glacier can produce great quantities of sediment as its weight grinds and pulverizes the surface of the bedrock below. But in the case of the Lake Winnipeg area, there is also a remarkable ready-made source of sand that I think must have been a major contributor to the beautiful beaches: the Winnipeg Formation. And if some of the beach sand was derived from the Winnipeg Formation, then that makes it a physical material of wondrous antiquity. If the Winnipeg Formation is the source, then the beach that we often take for granted includes grains that could have been sand for anything from 500 million to more than a billion years. Some of those grains may have been part of a sandy beach or dune before there was any complex animal life on this planet.
Before I travel further into hyperbole and reverie, perhaps I had better give a bit of background to this suggestion. The geology of Manitoba is defined by the Canadian Shield, which forms a broadly-domed turtle back that is gently blanketed toward its edges by near-horizontal younger sediments. The Shield rocks are of deep antiquity (though they keep a lot of this information to themselves); the rocks of the Superior Craton on the east side of Lake Winnipeg were formed during times of tectonic turmoil about 2.5 to 3 billion years ago.
We have little evidence for events in this region over the two billion years or so following the formation of the Shield rocks, but it is clear that there were immense periods in which the land was subject to erosion. Much of the Shield terrain must have been tall and mountainous in the early years, but over the endless millennia it was ground down, bit by bit and grain by grain, to a shape close to that of the low-lying terrain with which visitors to northern and eastern Manitoba are so familiar.
As the mountains were ground lower, those grains of sediment would have accumulated in vast bodies along their flanks. Finally, about 460 million years ago in the Late Ordovician Period, the sea intruded into the middle of what was then the ancient continent of Laurentia. The waves and currents played with the immense store of sediment, shaping it and laying it down into sandstones and shales. These would become the Winnipeg Formation, which blankets the eroded surfaces of the ancient Shield. Along the lake these rocks are not very thick, generally about 10 metres in total thickness, but they form very distinct deposits. They have been lightly touched by the forces of diagenesis, so lightly that you often do not need a hammer to collect them, but can scoop out the sand with a shovel or with your bare hands.
The Winnipeg Formation is variable stuff. In places it is an ugly, grey, muddy shale. In other locations, such as in parts of the silica sand pits on Black Island, it is a sparkling golden yellow sandstone. Much of the Winnipeg yields little to no evidence of ancient life, but elsewhere there are lamp shells, or conodonts, or evidence of animal burrows. Even though it is such a soft material, easily rendered to mushy soil and covered with vegetation, the Winnipeg can still be seen in several places around the lake for which it is named. In addition to the superb exposures on Black Island, it can also be observed near Seymourville and Hollow Water, at Grindstone Point, and at the north end of Hecla Island, among other locations.
The Winnipeg Formation sediments are so soft and friable that they are almost never exposed except in places where they are capped with a much harder rock, the mottled Dog Head Member of the Red River Formation. Often, this cap is thin, only a few metres thick, but don’t be fooled by appearances! For this snippet of the Dog Head is itself just the introductory paragraph to the novel-length story of Ordovician limestone seas that was to follow. Along the lakeshore the overlying parts of the Dog Head have been eroded away, but in other places it has not been eroded, and in the subsurface (under ground) it is in the range of 30 metres (100 feet) thick. The Dog Head is itself overlain by three subsequent members of the Red River Formation, including the famously fossil-rich Selkirk Member (Tyndall Stone). Above these, the four members of the Stony Mountain Formation and the lower part of the Stonewall Formation were also deposited before the Ordovician story was complete. But the Winnipeg Formation is older than any of these, and its sandstones were already in place even before the Tyndall Stone cephalopods and Churchill horseshoe crabs were alive.
A few years ago, a detailed scientific study of Lake Winnipeg was carried out, and some remarkable discoveries were made. One of the most significant (from my perspective) is that there is virtually no Ordovician bedrock under the eastern part of the lake. Rather, the lake bed in that area consists of the ancient Precambrian rock, with Lake Agassiz sediments above it. If the Winnipeg Formation has been almost completely stripped out of the eastern part of the lakebed (where it almost certainly occurred in the distant past), then an utterly mammoth quantity of sand, silt, and clay has been shifted by the combined efforts of water and ice.
The total area of Lake Winnipeg is 24,514 square kilometers. Even if we assume that the Winnipeg has been removed from only 10% of that area (and I’m sure it would have been more than that), and if the formation is about 10 metres thick (as it is along parts of the lake), then that is still about 25 cubic kilometres of sediment. Since quartz is remarkably durable stuff, the sand grains have tended to outlast the clay and silt, and the overlying carbonates.
This conversion of the antique Winnipeg Formation sand into modern sandy beaches is almost certainly happening still, because the Winnipeg is very close to the beach level in the eastern beaches area. Just east of the pump house at Victoria Beach, blocks of Dog Head rest high on the beach, but on a day when conditions are right, you can find bits of greenish, clayey sandstone exposed at mid beach level. On a really lucky day, you will find beautiful little Ordovician fossils in that sand: brachiopods (lamp shells), bryozoans (“moss animals”), crinoids (“sea lilies”), even trilobite components. That greenish sand is part of a thin layer, called the Hecla Beds, lying horizontally between the Winnipeg Formation and the Dog Head. So the Winnipeg itself is just below your feet. Along the shore, where pelicans glide above the windsurfers, grains of Winnipeg sand are gently washing out into the lake, beginning the next phase of their billion-year progress through the sediment cycle.
Even though this is far from being a scientific treatment of this matter, I ended up consulting quite a few references as I compiled the above piece. Some of these include:
Bannatyne, B.B. 1988. Dolomite resources of southern Manitoba. Manitoba Energy and Mines, Economic Geology Report ER85-1, 39 p.
Burt, A.K., T.A. Brennand, G.L.D. Matile, G. Keller, and H.L. Thorleifson. 2002. Reinterpretation of the Belair Moraine, southeastern Manitoba, Canada, based on a regional digital elevation model and new geological data. Geological Society of America, North-Central and Southeastern sections Joint Annual Meeting, Abstracts, Paper 29-0.
Elias, R.J. 1981. Solitary rugose corals of the Selkirk Member, Red River Formation (late Middle or Upper Ordovician), southern Manitoba. Geological Survey of Canada, Bulletin 344, 53 p.
McCabe, H.R. 1971. Stratigraphy of Manitoba, an introduction and review. In Turnock, A.C. (ed.), Geoscience Studies in Manitoba: p. 167-187. Geological Association of Canada, Special Paper 9.
Young, G.A., R.J. Elias, S. Wong, and E.P. Dobrzanski. 2008. Upper Ordovician Rocks and Fossils in Southern Manitoba. Canadian Paleontology Conference Field Trip Guidebook No. 13, Geological Association of Canada – Paleontology Division and The Manitoba Museum, 97 p.