Geologic control on soil composition and properties, Lake Ojibway clay plain, Matagami, Quebec

Varved clays deposited in proglacial Lake Ojibway at Matagami, Quebec, were derived from two source areas: carbonate-deficient, Precambrian crystalline igneous rocks from New Quebec to the east and carbonate-rich Palaeozoic rocks from the James Bay lowlands in Ontario to the west. About 50% of the 12–15 m thick clay section consists of varved clays containing less than 3% carbonate, reflecting the New Quebec source. The remaining 50% of the section consists of two pulses of carbonate-enriched varves (15–30% carbonate) derived from rock flour in two glacial advances known as Cochrane I and Cochrane II, which advanced to within 20 km of Matagami.The clays are preconsolidated at the site by a combination of desiccation, groundwater level drawdown, and bonding, in such a way that σp′ is almost constant at 70 kPa at depths from 3.5 to 10 m.High in situ water contents correlate with thinner, low-carbonate, distal varves of high activity. The est correlations are obtained for relative activity, Ac(rel), defined as plasticity index ÷ specific surface area (Ip ÷ SS). Carbonate seems to have effectively neutralized smectite in the Cochrane proximal varves, rendering the distal varves (of New Quebec source) the most active clays at the site. Key words: varved clay, activity, carbonate, turbidity currents, glacial geology.