Seismic survey of the pre-glacial outlet, Saint John River, Saint John, N.B.

Abstract

The retreat of the Pleistocene ice from southern New Brunswick 13,000 years ago left thick deposits of bouldery till throughout the Saint John region. One result of this deposition was the divertion of the Saint John River from its old, pre-glacial outlet to its present one, several kilometers further to the east. Seismic refraction surveys conducted by the author while working for the Seismic Section of the Geological Survey of Canada during the summer of 1976 show clear evidence for the pre-glacial outlet. A deep north-south channel in the bedrock was found to start from South Bay, along the present-day bank of the river, and to terminate with multiple outlets at the present shoreline of the Bay of Fundy. The geographical and geological evidence of surficial geology as well as borehole data support this interpretation. Three instruments were used to obtain the seismic refraction data. The instruments were the Huntec FS-3, a single channel hammer seismograph, the S.I.E. RS-4, a twelve channel seismograph used with an explosive energy source, and the Nimbus-Oyo ES 1200, a twelve channel signal enhancement seismograph. A comparison of the various advantages and disadvantages of the three instruments is given. The average compressional velocity of the bedrock is approximately 4,500 to 5,000 metres per second, a velocity that agrees with the accepted values for the Precambrian argillites, quartzites, and carbonates that underlie the area. Overburden thicknesses vary from zero to the edges of the pre-glacial channel to over 100 metres at its centre.