TUNNEL GEOLOGY AS SEEN BY GEOLOGISTS: MANHATTAN, NEW YORK CITY

Current exploratory boring operations in and around Manhattan, New York City are providing geologists and geotechnical engineers with a plethora of new and interesting geological information, which has not been previously reported. The rocks encountered, mostly medium to high-grade metamorphic rocks, with both mafic and felsic intrusives, are highly variable in competency and mechanical durability. One of the most frequently encountered rock types is a garnetiferous-muscovite-biotite schist which grades into schistose gneiss and displays a wide variety of structural, compositional, and textural attributes. Metamorphic minerals showing the variable degree of metamorphism include graphite, talc, garnet, kyanite, tourmaline, emory, and occasionally sillimanite. The presence of magnetite-rich zones within the muscovite-garnet schist suggests a mechanism for the concentration of iron during metamorphism. Concentrations of garnet both in the schist and intrusive pegmatite is perhaps indicative of anatectic melting of the protolith. Marble is the dominant rock type east of CAMERON'S LINE and it varies from pure white calcitic to dolomitic coarsely crystalline marble, to siliceous calcitic to dolomitic marble. In places, highly pyrite-rich zones, perhaps suggesting hydrothermal alteration of the parent rock due to subsequent mineral-rich fluid flow, are observed. The timing of the sulfide-rich fluid-flow through the original bedrock is yet to be determined. Partial dissolution of marble at various depths has resulted in void (cave) formation and has posed a threat to the boring operations. Rocks of lower abundance include amphibolite, granodiorite, quartzite, serpentinite, and aplite. The overall structural fabric is controlled by the Taconic and Acadian Orogenic events and manifested in the development of characteristic foliation, joint patterns, intrusives, and the degree of metamorphism of the protoliths.