EARLY PLEISTOCENE AND LATE TERTIARY PALEOCHANNELS IN THE CANADIAN CORDILLERA AND ADJACENT PLAINS: DISCOVERY, RECONSTRUCTION AND ECONOMIC IMPLICATIONS

Fluvial paleochannels occur in a variety of geological settings in the Canadian Cordillera and adjacent plains. Due to widespread glaciation in these regions, drainage patterns have been repeatedly altered and paleochannel systems are often completely obscured by overlying glacial deposits. These include till, glaciolacustrine sediments and glaciofluvial deposits that locally are up to 100's of metres thick. In plateau areas within the mountains and in the plains east of the mountain front, the paleochannels often occur within flat to gently rolling terrain and commonly they have little or no surface expression.

A variety of tools have been used to detect these hidden channels including interpretation of satellite data, resistivity surveys, high-resolution aeromagnetics, seismic profiling, ground penetrating radar, and borehole evaluation. The advantages and disadvantages of these different methods will be discussed using case study data from several areas in British Columbia. The first step in identifying the channels is to locate the main paleovalleys by reconstructing the bedrock topography. These valleys are wide and deep and therefore more readily detected by remote sensing. Smaller tributary valleys are relatively difficult to map but often are more significant from economic perspectives, partly because their contained deposits occur closer to surface and have thinner overburden. Mapping of the smaller tributaries requires high resolution data and ultimately borehole data must be used.

Gravel deposits within these ancient paleochannels are relatively mature compared to modern fluvial systems and consequently they provide high quality aggregates and, in some gold provinces, they are excellent placer targets. They also are important aquifers and recently have become of interest for their hydrocarbon (shallow gas) potential. A number of paleochannels east of the Rocky Mountains are the focus of recent shallow gas plays. The channels are buried by thick sequences of Middle to Late Pleistocene, glaciolacustrine deposits and clay tills which act as cap rocks. Glaciers that advanced westward up the regional slope, dammed rivers draining eastward off the Rockies and eventually deposited a cap of clay-rich till derived from local shale bedrock.