BEDROCK GEOLOGIC MAP OF THE PORT HENRY QUADRANGLE, ESSEX COUNTY, NEW YORK AND ADDISON COUNTY, VERMONT

The Port Henry 7.5-minute quadrangle lies on the eastern edge of the Adirondack Highlands and is comprised of deformed and metamorphosed Mesoproterozoic gneisses of the Adirondack Highlands unconformably overlain by weakly deformed lower Paleozoic sedimentary rocks of the Champlain Valley. Newly completed 1:24,000-scale bedrock mapping demonstrates paragneiss, marble, and amphibolite, were intruded by anorthosite-mangerite-charnockite-granite (AMCG) magmas, now exposed mostly as orthogneiss, during the mid-to-late Shawinigan Orogeny (~1160-1150 Ma). All rocks were subsequently metamorphosed to upper amphibolite to granulite facies conditions during the 1080-1050 Ma Ottawan Orogeny.

Mapping reveals four periods of deformation: D1 produced rarely preserved isoclinal folds in the paragneiss and marble; D2 produced the dominant gneissic fabric and deformed all the Proterozoic units in the map area; D3 involved upright folding, doming, and pegmatite intrusions; D4 is expressed by the development of late, kilometer-wide, SE-striking, transtensional shear zones, boudinage, metamorphism, pegmatite intrusions, and local reactivation of the S2 foliation. During D4 magnetite-bearing pegmatites and ore bodies were emplaced as Sn-parallel sills and cross-cutting dikes due to metasomatism. The development of the late-stage regional shear zones was likely due to the continuation of extensional doming and uplift from upper amphibolite facies conditions at the end of the Ottawan Orogeny. Kilometer-scale lineaments readily observed in lidar data are Ediacaran mafic dikes and Phanerozoic brittle faults.

The Paleozoic rocks are part of the Early Cambrian to Late Ordovician great American carbonate bank on the ancient margin of Laurentia. Cambrian-Ordovician stratigraphy records an approximately 1-kilometer-thick section and a transition from synrift clastics to passive margin peritidal carbonate buildups to gradually deeper water subtidal to shelf carbonates during foreland basin development associated with the Taconic orogeny.