INTERPRETATION OF GRAVITY AND MAGNETIC PROFILES ACROSS FAULTS IN THE TRENTON GROUP, TUG HILL PLATEAU

Normal faults are exposed in the Middle Ordovician Trenton Group along the western margin of the Tug Hill Plateau, 10 km south of Watertown, N.Y. Johnsen first recognized that faulting formed the topographic break at Talcott Falls, but mapped the down-side to the SW. We observe rotated blocks in this fault zone that demonstrate the fault down-dropping to the NE that suggest meter scale minimum throw. Normal faults are also revealed in a small horst structure in road cuts at Kellogg Hill along N.Y.S. Route 11 about one-half km south of the Falls. Faults there strike approximately N20W and dip 70 to 75 degrees. Total throw distributed across this fault set, as indicated by offset limestone and thin interbedded shales, is on the order of 10s of cm.

Two gravity and magnetic traverses were conducted across the proposed fault plane marked by the topographic break extending from Talcott Falls northeastward to Watertown Center. The gravity and magnetic measurements were made at 100 meter intervals along Fall and Dry Hill Roads which cross the fault at nearly right angles.

The gravity and magnetic traverses along Dry Hill Road indicate a gravity and magnetic high northwest of the fault with steep gradients separating gravity and magnetic lows on the southeast side of the fault. The gravity and magnetic traverses along Fall Road show similar profiles. These gravity and magnetic anomalies are interpreted as being due to the Precambrian Basement rocks lying beneath the Trenton-Black River formation. The coincidence of the anomalies with the proposed fault along the topographic ridge is interpreted as indicating the faults in the Trenton-Black River are controlled by faulting in the Precambrian basement.