GRAVITY AND MAGNETIC INVESTIGATION OF THE STRUCTURE OF THE CORTLANDT COMPLEX, NEW YORK

Abstract

This report is based on 185 gravity and magnetic stations established over the Cortlandt Complex and contiguous areas. The total area of the survey is approximately 75 square miles. The Complex consists of a group of basic igneous rocks near Peekskill, New York, in a region of granitic rocks and lower Paleozoic sediments. The gravity observations show a pronounced gravity anomaly of about 30 mgals, centered over the olivine pyroxenitic region on the east side of the Complex, and a smaller anomaly of about 15 mgals on the west side of the Complex in an area of augite norite and norite with prismatic hornblende. The larger anomaly corresponds in position to one of the foliation structures Balk mapped within the Complex. No separate anomaly occurs in the central area where a foliation structure exists with almost perfect zoning of rock types grading from norite with poikilitic hornblende to augite norite. The results of density determinations show the intrusive rocks to have an average density 0.4 gm/cc higher than the country rock. Calculations are presented to determine theoretical bodies that would satisfy the observed anomalies. The principal anomaly (30 mgals) is closely approximated by a vertical cylinder 2.4 miles in diameter and 4.7 miles thick, whereas the lesser anomaly of 15 mgals is approximated by a vertical cylinder 1.2 miles in diameter and 5.0 miles thick. These thicknesses are minimal values because infinitely long cylinders of the same cross sections have essentially the same gravity effect. The intermediate area between the cylinders is characterized by a residual gravity anomaly of 8 mgals suggesting that here the Complex is about 0.3 mile thick. The magnetic map shows no well-defined anomaly associated with the Complex as a whole. A central anomaly of 1200 gammas, occurring in the middle of the Complex, is ascribed to the rocks beneath the Complex and affords a basis for estimating the depth to the host rock, 0.3 mile. This verifies the thickness computed from the gravity data. In addition, four local anomalies of approximately 1000 gammas are developed near the boundaries of the Complex. Three of these are associated with known magnetite-bearing emery deposits.