ESTABLISHMENT OF A MODERN 1:24,000-SCALE BEDROCK GEOLOGIC MAP FRAMEWORK FOR THE ADIRONDACK HIGHLANDS AND CHAMPLAIN VALLEY, NEW YORK AND VERMONT

The Mesoproterozoic Adirondack Highlands are a mountainous extension of the Grenville Province of Laurentia that is roughly the size of Vermont. The region is a domal uplift underlain by highly complex geology that contains little published, modern bedrock mapping compared to the rest of the northeastern United States. New bedrock geologic mapping improves our understanding of the Adirondack Highlands and the adjacent Champlain Valley and establishes a modern framework for 1:24,000-scale bedrock geologic mapping in the region. A new USGS Scientific Investigations Map of the Crown Point 7.5-minute quadrangle provides a modern context for historical iron and graphite mines in the Adirondacks and updates the stratigraphy of the Champlain Valley in New York and Vermont. Detailed mapping was integrated with lidar, airborne geophysics, geochemistry, petrology, geochronology, paleontology, geomorphology, and economic geology.

Granulite facies Mesoproterozoic paragneiss, marble, and amphibolite hosted the emplacement of granitic orthogneiss at approximately 1.18–1.15 Ga. Felsic magmatism of the Lyon Mountain Granite Gneiss spread by penetrative migration as semi-concordant alkali feldspar granite sheets at approximately 1.15 Ga. Extension and tectonic exhumation produced domes and basins, reactivation of the older foliation, partial melting, metamorphism, metasomatism, iron ore mobilization, and intrusion of magnetite-bearing pegmatite. Boudinage accompanied high-grade ductile shear zones and crosscutting pegmatite dikes. Kilometer-scale lineaments crosscut the basement rocks and are occupied by Ediacaran mafic dikes and Phanerozoic brittle faults. Paleozoic rocks deposited above the Great Unconformity as part of the Early Cambrian to Late Ordovician great American carbonate bank on the ancient margin of Laurentia record a 1-km-thick section that transitions from clastic rocks to passive margin carbonates, and finally foreland basin shale prior to the Taconic orogeny. The Champlain Valley is partly covered by thick glacial deposits locally marked by landslides. Holocene deposits include waste rock piles and tailings from historical mining and areas of artificial fill. The map is available online at https://pubs.er.usgs.gov/publication/sim3491.