POSSIBLE IDENTIFICATION OF ANOTHER GRABEN IN THE CENTRAL ADIRONDACK MOUNTAINS THROUGH INTEGRATED STRUCTURAL AND MAGNETIC ANOMALY ANALYSIS

The Cedar River Flow (CRF) is a northeast trending shallow lake that occupies a broad flat valley on the western margin of the Snowy Mountain massif in the central Adirondack Mountains. Recent magnetic anomaly studies on Adirondack lakes (i.e. Piseco and Lewey Lakes), have revealed that they occur over graben with some containing Paleozoic sedimentary rocks. With a similar shape, dimension and structural attitude, the valley of the CRF is another Adirondack graben candidate. The low lying terrain that surrounds the flow has bedrock exposures, while the steep valley walls are underlain by charnockitic gneiss on the east and a mix of charnockitic and gabbroic gneisses on the west. A few rare outcrops within the valley are pelitic and calc-silicate gneiss, and marble, hence the valley was earlier portrayed as mixed metasedimentary rocks on the NYS geologic map. Metamorphic foliation dips shallowly westward on the east side of the valley, but strikes nearly east-west and is south dipping on the west side, and two subvertical fracture sets strike NNE (dominant) and WNW. In effect, the valley is parallel to the dominant fracture set and appears to cut the structural transition between the Snowy Mountain dome and the Moose River shear zone. A water-based magnetic mapping survey was completed using a Cs-magnetometer with GPS tracking. The CRF was systematically surveyed in a zig-zag traverse with a sampling rate of one reading per second. The total survey length was about 18 km long and was completed in one day. Diurnal variation was determined with crossing points in the survey in addition to repeatedly visiting one spot over the duration of the survey. Magnetic susceptibility measurements were collected for all lithologies. The derivative magnetic anomaly map shows a total range of 850 nT for the CRF. The northern region of the flow has a high anomaly (+400 to +450 nT), while most of the southern and western area has a low (-400 nT) magnetic anomaly. From the prominent low magnetic anomaly, it is apparent that the metagabbro of the Sturges Hills does not extend under the CRF, and the east-west trend is truncated at the valley wall. The strong low magnetic anomaly may be the result of local marble in the mixed metasedimentary unit, however, it may also be the result of Paleozoic carbonates, like those found in other Adirondack graben at Wells and Piseco.