A STUDY OF AN UNUSUAL GLACIAL ERRATIC OF PALISADES DIABASE IN STATEN ISLAND, NY

An unusual glacial erratic of Palisades diabase is present in a drainage basin in the Campus of the College of Staten Island. The base of this trapezoidal exposure measures 7 meters by 4.4 meters. The depth of this exposure has not been obtained. Attached to the eastern side of the diabase is a slab of hornfelsed Lockatong Argillite which exhibits mud cracks. The attached slab of Lockatong Argillite is vertically oriented.

Chemical analysis (in weight%) of the diabase is SiO₂, 53.39; Al₂O₃, 11.00; TiO₂, 2.87; MgO, 4.0; FeO,11.46; Fe₂O₃, 2.86; MnO, 0.2; CaO, 6.74; Na₂O, 3.3; K₂O, 1.83; LOI, 2.0. This author hypothesizes that this chemistry is indicative of the highly fractionated diabase of the upper Palisades Sill. Supporting evidence for this is the 20% by volume of interstitial remarkably unaltered granophyre composed of quartz and K-feldspar.

There are outcrops of Palisades Diabase on Staten Island, but this exposure does not appear to be one. The glacial origin of this exposure is supported by detailed subsurface geotechnical profiles that indicate bedrock of Staten Island Serpentinite at this locality. According to Lyttle and Epstein(1987) the Palisades diabase on Staten Island makes upper and lower contacts with the Lockatong Argillite. Therefore, this exposure could represent either a piece of the Palisades Sill containing of one of the contacts or a contact with a xenolith of Lockatong Argillite.

Furthermore, this exposure is excellent for teaching geoscience students.