ENGINEERING IMPLICATIONS OF NEWLY MAPPED WALLOOMSAC FORMATION IN LOWER MANHATTAN AND NEW YORK HARBOR

Previously mapped as the Manhattan Fm, the schistose rock in New York City is now separated into the Walloomsac (Ow), below the allochthonous Manhattan (Є-Om) and overlying Hartland (Є-Oh) formations (Merguerian 1983, 1986, 1994, 2008a, b & Merguerian and Sanders 1991). Imbricated during the Taconic Orogeny, each is in contact with the others along ductile faults. While generally schistose, the 3 units have differences that significantly affect the engineering properties of the rock.

The Walloomsac consists of a pyritic and graphitic schist that contains layers of calc-silicate and calcite-diopside marble. Recent construction confirms it is more prone to weathering and the calcareous portions occasionally contain voids. The Manhattan is more massive, gneissic and tends to be a harder rock, generally more resistant to weathering and difficult to excavate. The Hartland tends to be a well-foliated schistose rock with interlayers of granofels and amphibolite, often making it easier to excavate than the Manhattan, but more durable than the Walloomsac. Separated by ductile faults, the contact zones between the units are sometimes defined by highly sheared, micaceous mylonitic rock and/or slivers of serpentinite. This rock is often softer and more fissile than the main rock mass, and the serpentinite may naturally contain asbestos.

Recent borings made in New York Harbor and lower Manhattan enable the refinement of the current geologic maps of Northern NJ and NYC with respect to these units. While much of the NJ Manhattan is likely Hartland, in an area offshore of Bayonne, NJ it is actually Walloomsac, overlain by Stockton (TRs) to the west and in contact with Hartland eastward along the pier line. Layers of Walloomsac sheared together with Hartland were previously found in the vicinity of the World Trade Center site (Merguerian and Moss 2006). This mapping indicated that through folding +/or faulting the Walloomsac broke through “windows” in the overlying Hartland. Newer borings indicate that this mix continues northward along the shore, in a line between West and Greenwich Streets from Albany St. just south of the WTC up to around West 29th St., at which point the island shoreline shifts to a NNE trend. Within this narrow zone, the presence of additional slivers of mixed, sheared rock, with the associated properties, can be expected.