THE CASE OF THE MISSING VALLEYS - BEFORE NIAGARA FALLS

The 5000 km² Niagara Peninsula 3-D Quaternary mapping project in Ontario, Canada will develop an interactive model and characterize the properties of sediment packages that form both regional and local aquifers and aquitards for groundwater and land use planning applications.

A 6828 station ground gravity survey was conducted along select roads and highways at a nominal spacing of 100 m. Calculated gravity residuals were used to identify buried-bedrock valleys and guide subsequent drilling. The survey was effective in identifying deeply incised valleys, but less definitive where valleys broaden and deep regional and shallow residual gravity components are not easily separated.

Five 4.5 to 21.5 km long shallow seismic reflection lines were acquired to determine whether multiple thalwegs exist within a centrally located buried channel, delineate the geometry of buried gravel beds and provide insight into late-glacial moraine systems. Downhole compressional and shear wave velocity logs were acquired to measure material velocities and verify the conversion of seismic time sections to depth sections. Apparent conductivity, natural gamma, and magnetic susceptibility logs were used to characterize lithological variations. High-resolution fluid temperature logs identified anomalies within boreholes and differences in regional groundwater temperature.

A total of 99 continuously cored boreholes with a combined length of 3192 m, 29 of which have been converted into monitoring wells, have been drilled for this project. The borehole data is augmented by 130 exposures, soil probe and hand-auger cores and extensive legacy datasets.

A high-quality bedrock topographic surface that forms the foundation for the overlying sediment model has been interpolated from points manually digitized onto the new and legacy borehole traces. By modelling the gravity results using representative density values for the Quaternary and bedrock layers, and integrating with interpretations from seismic studies in the area, it is possible to further constrain the shape of the bedrock surface. The resulting surface shows southward-dipping bedrock separated by the prominent features of the Niagara and Onondaga escarpments. Deep and narrow valleys bisect the resistant escarpments, becoming wider where they cross softer formations.