The Champlain Sea: here yesterday, gone tomorrow

Wednesday, March 1, 2006

One of the surprising things of the "Ice Age" is that about 10,000 to 12,000 years ago, a large embayment of the Atlantic Ocean extended up the St. Lawrence Valley into southern Quebec and eastern Ontario. The seashore extended across from the Lake Ontario basin near Kingston northeastward near Carleton Place to the Ottawa Valley as far as Pembroke. All of Ontario east of that shoreline, including Ottawa, and much of the St. Lawrence Valley of Quebec was flooded by sea water to depths as great as 100m (Fig. 1).

Lake ChamplainFigure 1 (left): Lake Champlain including areas where vertebrate remains have been found (white dots).

It has long been debated as to whether the sea extended into the Lake Ontario basin after glacial Lake Iroquois drained down to a level far below present Lake Ontario level. The latest information suggests Ontario basin water got as low as sea level but the narrow connecting strait at Kingston and runoff from the land kept the water fresh in the Ontario basin. These conjoined waters only lasted a few centuries before uplift of the Kingston sill raised Lake Ontario above sea level.

The great weight of glacial ice that covered Canada pressed the Earth's crust downward so that part of it was below sea level. As the ice sheet gradually thinned and melted away, the St. Lawrence and Ottawa valleys became open to the sea and residual glacial lakes mixed with sea water and became salty. The slower process of isostatic rebound as the ice disappeared raised the area over a 2000-year period and drained off the sea. As a result, the seashore was slowly moved eastward to where it is today. At Covey Hill, south of Montreal, these sea beaches are well displayed down the slope.

We know the waters of the Champlain Sea were marine (salt water) because the fossil shells in the beaches and deep-water clays are species found in the ocean today. We know the water was very cold, as you would expect from the retreating glacier, because many of the species are found today in the arctic. Analysis of the porewater in the clay also shows that the water in which the sediments were deposited was salty; the low permeability of the clay has preserved some of the salty water in the clay.

Variety of shells with penny for size comparisonFigure 2a (left). L. Hiatella arctica, Portneuf. Que. C.Portlandia arctica, Ste. Genevieve, Que.* R. Macoma balthica, Ste. Genevieve, Que. *Originally Leda glacialis, then Yoldia arctica

The kinds of fossils found in the sediments of the Champlain Sea include molluscs (snails and clams: Fig. 2, most common and obvious), barnacles, urchins, sponges, foraminifera, ostracodes, and vertebrates (sorry, no corals-too cold). Many species of molluscs have been identified from the Champlain Sea, although only a dozen or so are common. Barnacles are common, fastened onto rocks, boulders, or molluscs. At some places fossil molluscs are so abundant hundreds or thousands can be collected in a short time.

Two shells with penny for size comparisonFigure 2b (right)L. Mya truncata, Ste Genevieve, Que. R. Neptunea despecta tornata, Ste Genevieve, Que.

The vertebrate fossils of the Champlain Sea include several species of whales and seals. Their bones are usually found in gravel pits in beach deposits along the old sea shore, suggesting they beached themselves as they sometimes do today, or were carried by waves onto shore after death. Figure 1 shows where Champlain Sea vertebrate remains have been found. Near Ottawa, carbonate concretions in the clay contain fossils (fish, molluscs, even a bird feather!). These have been dated as 10,000 years old, i.e. from near the end of the Champlain Sea.

Figure 3 (below). Fish in a carbonate concretion from the Ottawa area.

Fish in a carbonate concretion

The clay of the Champlain Sea (referred to by engineers as Leda clay from a characteristic fossil) is "sensitive" so that when disturbed by construction, earthquakes, or stream erosion it liquefies and causes spectacular earthflow landslides. Areas along the South Nation River in eastern Ontario; and many others in Quebec, show the ribbed or chaotic landslide topography that results, particularly evident on air photos, sometimes with loss of life and often much property damage.

Figure 5 (below). At about 1815 December 11, 1963, a large landslide took place at Saint-Joachim-de-Tourelle, Quebec. The earthflow involved sensitive marine clays (otherwise known as Leda Clay) which flow like a liquid when in the remolded state.

large landslide; some houses near a mountain

Marine invasions in other parts of Canada
Other former seas of similar origin have also been named, such as the Goldthwait Sea east of Quebec City, and the Laflamme Sea along the Saguenay Valley. The area flooded by the sea inland from Hudson Bay is called the Tyrrell Sea. Over 100,000 years ago the Bell Sea occupied the same area at the close of the second last glaciation - geologists know well that history repeats itself. Other areas submerged by the sea because of crustal downwarp from glaciation include southwest British Columbia (Victoria, Vancouver) and large areas in the arctic islands of northern Canada.

Figure 4 (below) Earthflow


For further reference: Gadd, N.R. (ed.), 1988. The Late Quaternary development of the Champlain Sea basin. Geological Association of Canada Special Paper 35, 312p.

Paul F. Karrow PFK16

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