INVESTIGATION OF GEOLOGIC RELATIONS IN DRILL CORE FROM THE SYLVIA LAKE SYNCLINE, ADIRONDACK LOWLANDS

Diamond drill core (>100m) from the overturned limb of the Sylvia Lake Syncline penetrating the upper portion of the Popple Hill Gneiss and its contact with the Upper Marble in the Adirondack Lowlands has been logged and examined in thin-section. Stratigraphically upwards, lithologies range from mg to fg, plagioclase-quartz-biotite (PQB) gneiss to nearly pure quartzite to highly altered and schistose units (± garnet, ± sillimanite) largely replaced by secondary minerals. Included within the uppermost schists are tourmaline-rich interlayers (~ 1m aggregate thickness) and a calcitic marble layer (~40 cm). The upper section of the core shows an intense schistosity with pervasive alteration to serpentine, chlorite, and sericite. The lithologies in the core are cut by several small (5-10 cm), phyllosilicate-rich, mylonitic fault zones, numerous cm-scale microcline-bearing pegmatites and deformed leucosomes. The PQB gneiss is compositionally variable and varies in grain-size; sillimanite and garnet are rare. Numerous sand- and silt-sized rounded zircons occur throughout. Coarser-grained leucosomes (mm- and cm-scale) and resulting melansome modify compositional layering and enhance foliation. A dcm thick unit with hornblende porphyroblasts and zircon may represent a volcanic interlayer of intermediate composition. Notably absent are amphibolitic layers seen in outcrops of the Popple Hill Gneiss elsewhere. Sparse, but nearly pure quartzites with numerous detrital zircons, are found in thin (5-10 cm) layers throughout the core and become more prevalent and much thicker (up to 10m) upwards. Despite subsequent modification due to upper amphibolite facies metamorphism and porphyroblast growth, intrusion of leucosomal and pegmatitic melts, accompanying deformation, and widespread alteration, the core displays the transition from deposition dominated by immature, plagioclase-rich wackes to quartz arenites to Al-rich shales. The shales were likely deposited in a shallow quiet water basin (lagoon?), punctuated by deposition of quartz arenite, evaporates and chert, and finally, basal limestone of the Upper Marble. The core provides a detailed record of sedimentation during the transition between deposition of predominantly clastic to predominantly carbonate rocks in the Lowlands portion of the Trans-Adirondack Basin.