THE LEGEND OF LOUIS THE HERMIT: CONCEALED PALEOZOIC GRABEN IN THE CENTRAL ADIRONDACK MOUNTAINS, NEW YORK

Isolated half graben that contain Paleozoic strata are developed in Grenvillian basement rocks of the southern Adirondack Mountains, New York. These half graben have been associated with extensional deformation related to the uplift of the Adirondack massif through the Mesozoic, and they contain remnants of the Potsdam sandstone and carbonates of the Black River and Trenton Groups. As well, they occur along NE striking faults with east-side-down offset, and are typically less than 10 km long and a few km wide. A number of valleys with similar geometry and possible fault scarp borders are possible candidates for concealed half graben in the central Adirondacks. Lewey Lake, named for the hermit Louis Seymour, occurs in a northeastern trending valley about 5 km long, 1 km at the mid-point and tapers at the ends. The northwestern valley wall is straight and marked by the steep local topography, while the southeastern side of the valley has a gradual gradient. Although Lewey Lake covers the northeastern half of the valley, the southwestern region is concealed by wetland that is divided by the Miami River. With the use of water based magnetic gradiometry, the valley that contains Lewey Lake was surveyed to assess magnetic anomalies to reveal the basement geometry and valley fill materials. The magnetic anomalies are overall negative relative to the total field with a range of about 500 nT. A high magnetic anomaly spans the width of the lake with a well-defined NW trend that is parallel to local fracture zones. A second magnetic high anomaly with a diffuse transition has a trend parallel to the northeastern trend of the valley. Magnetic susceptibility measurements of charnockitic gneiss that outcrops on either side of the lake cannot account for the overall negative magnetic anomalies in the valley. Detailed mapping has uncovered a substantial zone of fault gouge that is parallel to the steep western valley border, in addition to numerous carbonate veins within the basement rocks on the southeast side of the valley. Herein, we propose that Lewey Lake and the southern extension of the valley are most likely underlain by Paleozoic carbonates to account for the low magnetic anomalies, and these strata may occur in several structural blocks to explain the anomaly distribution. Future work will involve magnetic anomaly mapping over the SW wetland.