INTEGRATED BEDROCK GEOLOGIC MAPPING IN THE ADIRONDACK HIGHLANDS OF NEW YORK

The Adirondack Highlands are underlain by complexly deformed Mesoproterozoic crystalline rocks, presenting significant challenges for adequate assessment of natural and environmental resources. The vast ~30,000 km² area contains the fewest published modern bedrock maps in the northeastern United States. Despite over a century of topical research, no peer-reviewed 1:24,000-scale bedrock geologic maps exist for the Adirondack Highlands. Our new detailed 1:24,000-scale bedrock geologic mapping is the first of its kind in the Adirondacks, and integrates LiDAR, geophysics, geochemistry, petrology, geochronology, and economic geology to improve our understanding of the framework geology of the Adirondacks.

New mapping in the Crown Point and Ticonderoga area illustrates the topics addressed. Granulite facies Mesoproterozoic paragneiss, marble, and amphibolite hosted the emplacement of anorthosite-mangerite-charnockite-granite (AMCG) plutonic rocks at ~1.18-1.15 Ga. The earliest of four phases of deformation (D1) is characterized by gneissosity, rarely preserved F1 isoclinal folds, and migmatite in both AMCG and host rocks. Subsequent D2 deformation produced a composite penetrative gneissosity, migmatite, and large isoclinal F2 folds. Towards the end of D2, felsic magmatism including the regionally extensive Lyon Mountain Granite Gneiss (LMG), spread semi-concordant alkali feldspar granite sheets by penetrative migration, sub-parallel to S2 and into previously deformed lithologies. The LMG crystallized at ~1.15 Ga and displays syn-kinematic F2 folds thus constraining the time of D2 deformation. Exhumation during D3 produced F3 folds exhibited in regional domes and basins, local re-activation of the S2 foliation, partial melting, metamorphism, metasomatism, iron ore re-mobilization, and intrusion of magnetite-bearing pegmatite both as layer-parallel sills and cross-cutting dikes. D4 created NE- and NW-trending boudinage, local high-grade ductile shear zones, and cross-cutting granitic pegmatite dikes. Km-scale lineaments readily seen in LiDAR data are Ediacaran mafic dikes and Phanerozoic brittle faults. These findings provide a framework for ongoing investigations into historically significant iron and graphite deposits in the Adirondacks.