A DEEP PILOT BOREHOLE IN THE MANHATTAN PRONG- COMPLEX GEOLOGY IN NEW YORK CITY

Snug Harbor Pilot Borehole B1A, a New York City Department of Design and Construction Project, was completed in 2006 to study the feasibility of a Geothermal Standing Column Well (SCW) for a NYC facility at 1000 Richmond Terrace, Snug Harbor Cultural and Garden Center, Staten Island, New York. The borehole was drilled and cored to a depth of 869' (263m) below ground surface (bgs). The overburden consisted of unconsolidated sediment composed of sand, silt, gravel, cobbles, and boulders to a depth of 155' (46.9m). Serpentine bedrock was encountered at 155' (46.9m) and cored to 195' (59 m), highly weathered, fractured, and containing decomposed zones (saprolite) of greenish-bluish gray rock (Run 1 - Run 4), 40' (12 m) thick. A 3 inch diameter casing was set to 195' to keep the borehole open for rock coring. The following results from 195' to total depth: 1) 195' to 345' (Run 5 - Run 20) 150 (46.9 m)' thick, composed of Serpentinite, 2) 345' (104 m) to 353' (106.9 m)orange/red foliated serpentinite which was hydrothermally altered (Run 20), 8' thick, 3) 353' (106.9 m) to 363' (110 m) amphibolite bedrock (hornblende schist) (Run 21) 10' thick, 4) 363' (110 m) to 869' (263 m) pelitic schist (Waloomsac Formation), gray pyritized, garnet, biotite schist with calcite (Run 21 - 72) 506' (153 m) thick.

The borehole reflects a complex Manhattan Prong geology from Staten Island New York to the western side of the Hudson River to Hoboken New Jersey. The Waloomsac Formation crosses the Hudson River (eastside) to Manhattan at Leroy Street and north to West 57th street, where it is associated with serpentine. It additionally outcrops at the Bronx Botanical Gardens.

The bedrock at Snug Harbor is composed of Middle Ordovician Waloomsac Formation (Manhattan Schist Member A) which is over 500' (151m) thick and composed of pyritized, garnet biotite schist, and calcite. The serpentinite ophiolite bodies, almost 200' (60 m, were accreted to the Laurentian Continental Plate and the Waloomsac Formation during the Taconic Orogeny. The lithology of serpentinite has a complex history showing hydrothermal geochemical changes and mineral alterations.