GLACIAL STRATIGRAPHY AND IMPLICATIONS TO ENGINEERING GEOLOGY AND LAND-USE: AN EXAMPLE FROM FREDERICTON, NEW BRUNSWICK, CANADA

In North America, glaciation resulted in the deposition of thick valley-fill sediments that now underlie many urban centres in Canada and the northern United States. Studies at Fredericton, New Brunswick, demonstrate many features common to such cities, were growth and land-use are strongly linked to physiography and the style of local deglaciation during the late Wisconsinan. Here, deglaciation was accompanied by sediment deposition and valley infilling under changing glacio- marine, estuarine and lacustrine conditions. Glacigenic sediments are more than 80m thick in the centre of the valley and include variable clays (marine mud and laminated glaciolacustrine clay-silt) intercalated with glaciofluvial sand and gravel, underlain by a clayey lodgement till, and capped by a postglacial clay aquiclude and granular alluvium. Drilling logs indicate that the clay - silt unit surrounds a 45m thick glaciofluvial unit that forms the Fredericton aquifer, which supplies water for 98% of the city. Remaining water demands are supplied from wells in iron-rich bedrock that produce an aromatic Fe-Mn -rich water.

The deposits at Fredericton represent several conditions that demonstrate the need for detailed stratigraphic mapping and the implications for future land-use. For example, major parts of the high-density area of Fredericton are sited on a floodplain that is prone to periodic flooding. Until recently, the floodplain was considered to overly the clay aquiclude that protected the underlying aquifer from surface contamination. However, it has been shown that in some areas, postglacial fluvial erosion has cut through the aquiclude thus enabling migration of contaminants into the aquifer. A further consideration is the high sensitivity to disturbance and vibration of the underlying clay units. This sensitivity resulted in damage to walls and chimneys from ground motion associated with the 1929 Grand Banks earthquake, sited 830 km away, and more recently to building foundations at sites adjacent to construction activities.

Considerations for future development and land-use planning include; the potential for contamination of the aquifer, flood-prone urban areas, disturbance of sensitive clay by construction and its susceptibility to seismically-induced ground disturbance.