The deep subsoil temps in Grand Bend are a relatively benign +8C, and the soil below the slab has both thermal mass and R value, so if you kept the temp at the basement ceiling at 21C, the slab temp would likely stay above 17C just on radiated warming alone, unless there is air leakage of much colder/denser air cooling the slab.
If you have an infra-red thermometer (even a $50 pistol-grip version from a box store is good enough) you could measure both ceiling and floor temps. My suspicion is that you will find a measurable variation in the ceiling to slab temp differences a half meter from the foundation, and it is likely to be a larger difference than in the middle of the basement. You may be able to identify spots that are colder at both ceiling & slab that may be pointing to a higher-leakage area. The cold spots may change with wind direction too. A blower-door test in combination with infra-red imaging would be the ideal way to ferret it out, but you can still do a lot with a cheap IR thermometer and some good observational sense. This kind of snooping is best done while it's still relatively cold outside, and its worth making a sketch of the floor plan and marking both ceiling & floor temps at a dozen or so locations to get a better sense of it.
Your binned hourly mean mid-winter outdoor temps for the 6 coldest weeks of winter is about -6C, which is ~ 13C colder than your deep subsoil temps, and 26-27C colder than your desired room temp.
http://weatherspark.com/#!dashboard;a=Canada/N0M/ON/Grand_Bend
With the lower R value of the wall as compared to the subsoil, and at twice the average temperature difference, average heat loss per unit of area through above-grade uninsulated foundation + finish-wall would be several times that of the heat loss through the slab and may be more than 10x, depending on the absolute thermal conductivity of the subsoil (which is affected by moisture content, soil type, etc.). If the wall is insulated the difference in average heat load per area will shrink, but it has to be a pretty well insulated wall to where slab losses at subsoil temps that modest begin to dominate the heat load number for the basement. A subsoil temp of +8C is nothing like perma-frost, and unless your water table is less than a meter below the slab you have substantial soil-insulation to cut the rate of heat loss.
But leakage of cold outdoor air (colder than the subsoil temps) the air leakage could easily exceed the conducted heat loss, and would be a strong contributor to room air stratification.