Late Precambrian metamorphic rocks below the Allegheny Plateau of central New York State, USA, revealed by geothermal exploration drilling

The spatial extents of buried Precambrian basement terranes and tectonic features are poorly known below the Allegheny Plateau of New York and Pennsylvania, USA. Geological features around the plateau periphery and geophysical variations within the plateau imply the occurrence of both Mesoproterozoic Grenville province terranes and Neoproterozoic basins associated with Iapetan rifting of the Laurentian margin. Cornell University explored geothermal energy reservoirs below Ithaca, New York, by drilling borehole ESH 1 to 2978 m depth, ending 119 m below the Cambrian-basement unconformity. The substantial dataset for the upper part of the basement enables characterization of the lithologic sequence, which is interpreted to be the fill of a Neoproterozoic basin, metamorphosed to greenschist grade. Petrophysical properties differ among four intervals 18−44 m thick, further divided into meter-scale intervals of repetitively variable properties. Wireline logs and drill cuttings for all four intervals demonstrate high abundance of minerals containing structural water (e.g., phyllosilicates) and potassium (muscovite, biotite, K-feldspar). Cuttings and six sidewall cores reveal fine-grained metamorphic rocks derived from both sedimentary and volcanic protoliths. K-feldspar dominates the upper petrophysical zone while phyllosilicates and quartz dominate the lower three intervals, consistent with alteration of the shallowest basement. Downhole patterns of density and gamma variations are similar to those of sedimentary fills of documented rift basins elsewhere and distinctly different than patterns in high-grade metamorphic and plutonic basement. Applying to other regional wells the criteria developed here may improve mapping of basement boundaries and facilitate identifying Iapetan basins below the Allegheny Plateau.