GRAVITY AND MAGNETIC SIGNATURE OF THE LOON LAKE SYNCLINE, ADIRONDACK HIGHLANDS, NEW YORK

In the Adirondack Highlands, a E-NE trending belt of metamorphosed supracrustal lithologies centered on Sabattis, NY extends for 25 km in the southern half of the Tupper Lake Quadrangle. Previously named the Loon Pond syncline by B. Potter, the belt was interpreted as cover sequence resting on gneissic basement rocks, cut by various granitic and syenitic intrusions. Metasedimentary lithologies include quartzite, calc-silicate, marble, and pelitic gneiss. Leucogranitic gneiss, amphibolite, and layered supracrustal units (layered gneisses) highly intruded by granite, charnockite, and/or mangerite also occur. Surrounding rocks displaying the regional horizontal E-W lineation trend while a shallowly SW-plunging lineation is preserved in the belt. The shape, extent, geologic context, structural features, and lithological character of the belt suggest a possible genetic connection to the Adirondack Lowlands. The goal of this initial geophysical study was to determine the three-dimensional thickness and extent of the supracrustal belt.

Detailed gravity and magnetic traverses were conducted across the structure and show an excellent correlation with geologic structure and lithology in the survey area. The three major rock types include: 1) quartzofeldspathic gneiss; 2) dolomitic and calcitic marble; and 3) highly layered and intruded metasedimentary rocks (layered gneisses). Quartzofeldspathic gneiss has an associated magnetic high and gravity low, suggesting it has a higher magnetic susceptibility and lower density than surrounding rocks. Layered gneisses produce a magnetic low and gravity high suggesting they are of greater density and lesser magnetic susceptibility than the quartzofeldspathic gneisses in contact wth them. Marble produces a gravity high and magnetic low in comparison with adjacent rocks. Gravity and magnetic profiles indicate faults between the lithologies are sharp and well defined.

Forward potential modeling of gravity anomalies across the traverse area indicates the belt of rocks is no more than 2 kilometers thick. While this is consistent with interpretations involving structural emplacement(e.g. klippe), it cannot rule out an unconformable relationship between highly metamorphosed basement gneisses and a cover sequence.