Overburden or Quaternary Stratigraphy, Firebag River; Northeastern Alberta: Preliminary Report

Requests received have shown a need for information on the overburden stratigraphy in the McMurray oil sand area. The Firebag River is the only place the stratigraphy is well exposed. This report describes the river and preliminary stratigraphic data collected during summer 1982.

The Firebag River flows westward from Saskatchewan through northeastern Alberta to join the Athabasca River north of Fort McMurray. The eastern quarter of the river flows through high relief, drumlinized terrain, the central portion through undulating moraine and the western quarter through undulating glaciofluvial terrain.

The Firebag River is about 160 km long and falls from about 550 m in Saskatchewan to about 225 m at the Athabasca River. Channel character is meandering except for a short central segment through wetlands where it is relatively straight. The incised alluvial plane increases downstream from 3 to 30 m deep and from 0.5 to 3 km wide.

About 27% of the precipitation leaves as surface runoff; annual extremes of discharge, measured near the mouth, range from 201 m³/sec dally maximum to 4.2 m³/sec daily minimum. Observations by the authors indicate river level varied from 2 to 4 m during spring of 1982 and 0.1 to 2 m during the summer. The river bed is armoured with cobbles and boulders; sand is present mainly at the junction with tributary streams and on point bars. During 1982, about 60% of the lower 2/3 of the river was choked with aquatic vegetation.

Within Alberta, the Firebag River flows first over subcrops of the Clearwater Formation and downstream of this subcrops of the McMurray Formation, the Waterways Formation and other undivided Devonian strata. The overburden stratigraphy is exposed on the lower two thirds of the river section and the central third was examined. Seven stratigraphic units have been recognized in the field on the basis of their texture, structure, bitumen content, colour and stratigraphic position.

The units and their most diagnostic properties are, starting from the base: Unit 1 - silty sand till, black in colour; Unit 2 - silty sand till with a strong bitumen odor and appearances; Unit 3 - silty sand till with no bituminous odor or appearances; Unit 4 - sand till with no bituminous odor or appearances; Unit 5 - medium to fine grained sands; Unit 6 - pink sediment; Unit 7 - sand with minor gravelly sand layers. Unit 2 is correlated with Unit B of Fenton and Dreimanis (1976) and the Firebag Till of McPherson and Kathol (1977). No other correlations are presently possible.