MAGNETIC SURVEY ALONG THE ST. LAWRENCE RIVER FROM MASSENA, NEW YORK TO LAKE ONTARIO

A detailed magnetic survey of the St. Lawrence River from Massena, New York to Lake Ontario was conducted to investigate a possible major geologic structure extending beneath the St. Lawrence River into Lake Ontario. Some researchers have suggested a possible geologic alignment or fault connecting the seismically active northeastern St. Lawrence Valley in Canada and the seismic activity in the Attica, New York area. Over 3500 magnetic measurements of the total intensity of the Earth's magnetic field along 506 traverses across the river were made. These measurements were plotted on a base map of 1:30000 scale and contoured at intervals of 100 and 200 gammas. The magnetic contours trend continuously across the river indicating no large offsets in the Precambrian basement complex beneath the river. Also, the magnetic anomalies indicate a diabase dike swarm at the Gananoque, Ontario area extends across the river through Grindstone Island and into the Clayton Quadrangle, New York without offset indicating no lateral strike slip movement.

Although there is no evidence from the magnetic anomalies of a major fault extending along the river, there are several interesting anomalies. An elongate elliptical shaped magnetic high of 1800 gammas extends 8 km from Nevins Point to Ogdensburg. North of Cardinal the anomalies have low gradients due to greater depth of basement while south of Cardinal to Lake Ontario the numerous circular and elliptical shaped anomalies show high gradients due to the shallow Precambrian basement. A magnetic trough extends across the river at Cardinal separating the high magnetic gradients on the south from low gradient anomalies in the north. Circular shaped magnetic highs occur just south of Prescott and Cardinal in the river. Most of the magnetic anomalies on land adjacent to the river can be traced into the River.